Determine full Moon position into sky from Sun position Soon a lunar eclipse will take place soon.

I'm not sure if I will be able to see the moon during the eclipse (from 9:30 PM to 11:00 PM), because, I have some building, mountains... in my field of view.

So, I wonder:

On lunar eclipes, will the Moon position at 9:30 PM be the same than the Sun position at 9:30 AM ?

The Sun's and Moon's hour angles at those times will be similar (both about 48$^\circ$ east of the meridian), but their declinations will be opposite (Sun 19$^\circ$ north of the equator, Moon 19$^\circ$ south).
The Sun in the morning will be over 30$^\circ$ higher than the Moon in the evening.

As the Moon rises and sets about 50 minutes later each day, a better predictor for the Moon's position at 21:30 on July 27 is the Moon's position at 20:40 on July 26, about 1$^\circ$ lower than the eclipse position if observing from a mid-northern latitude.


Which planets have abundant amounts of photoluminiscent matter? Which planets have abundant amounts of photoluminiscent matter? Photoluminiscent or chemoluminiscent whatever the glow-in-the-dark stuff it may be. Planets or bodies which have good enough the photoluminiscence(light absorb and then release) property.

Sorry, but none of the planets have substantial amounts of phosphorescent chemicals.

Perhaps the closest to what you are thinking is the aurora, which can be seen on all the planets with atmospheres (to a greater or lesser degree) notably on the Gas Giants and on Earth. However, while the mechanism of aurora is similar to florescence, it is not phosphorescence.

No planets outside of the solar system have been directly imaged beyond a dim dot, so there is no information about exoplanets.


Is Neowise viewable from my position? I have had an impossible time trying to locate this comet. My lat/long is 38.3/-82.4. My elevation is 705ft.
The application that I'm using shows me exactly where Neowise is supposed to be (North East, best viewed before sunrise around 4:00am, etc.), but I've managed to not locate it once in the last three days at that time. Stayed out for an hour straight each time, starting at 4:00am.
Is it not viewable from my position?

From that location, the easiest time in the morning to see the comet is between 4:50 and 5:10 AM EDT.
I've seen it at similar times relative to nautical dawn at my location.
Much earlier than that, the comet may be too low, especially if nearby trees obscure the horizon.
Some software mishandles daylight saving time.
As the comet shifts eastward, nautical dusk around 10:00 PM will be a good time in the evening to look for it.
This Sky & Telescope article provides both morning and evening finder charts.


How can a black hole ever form? We know that an object falling to the black hole, as observed from an observer far away, will freeze as it gets closer and closer to the event horizon. So my question is how can a black hole form in the first place? I mean we usually say thing a star collapses into a blackhole. But considering the effect of "time-freezing", how could that be possible in the first place?

After a supernovae explosion from stars whose mass is more than the Chandrasekhar limit, the remaining neutron star is so dense that it collapses into a one single singularity( A point where the density in infinite and is compressed into a point).the only way we can detect a black hole is by the entropy given by the black hole. so at the point of formation the black hole is not taking any matter into it.After the black hole is formed we will see any object going into it as frozen in spacetime


How could visible light be in pitch-black? Can it be? How could visible light be in pitch-black? Can it be?

Black is not a color; it is the absence of light.

I think you interpret the sentence as "the light is black", but it's just a way of saying "there is no light (at optical wavelengths, but there is at radio wavelengths)".

In my opinion this is not a good way of explaining the CMB. The sky is not black at any wavelengths, but has a background radiation at all wavelengths. In the optical, this background originates (primarily) from the integrated light from stars; in the microwave it originates (primarily) from the afterglow of Big Bang; in the infrared from dust, etc.

See the figure in this answer to a question about the number density of CMB photons, to see how large the background is in various wavelength regions.


What type of energy is escaping from black-hole's poles? Following this article, it is stated that:

  As a star drifts too close to a supermassive black hole, intense tidal
  stresses rip the star to shreds. As this happens, the shredded
  material will be dragged into the black hole’s accretion disk — a hot
  disk of gas that is gradually pulled into the black hole’s event
  horizon, bulking up the black hole’s mass or blasted as energetic
  jets from its poles.

Questions are,

What is the energy type that escapes from the poles?
What is the magnitude of the power?
Can this energy be potentially harnessed?
How far into space do these energy jets blow?

Also, if you feel like discussing this: if mass m goes into the black hole and comes out as energy E from the poles, assuming the speed of light c has no play in this transaction of going in or out, what happened to $c^2$ from ? Could a black hole be hypothetically faster than light? Or maybe the reverse direction of light? (I even got confused from this thought...)

So, if those jets are not pure energy, but are energized matter,

What type/kind of matter it would be? (gas? solid? liquid?)
Is all the matter that comes in, also goes out? I am thinking if the black hole works like a car's engine - does it have conversion efficiency?
Can this matter be cultivated, harnessed, or converted back to what it was or something new all together?

.

This question is presented here after it was asked on the space stackexchange. I want to leave it there because it has some relevant discussion to this question overall.

The article you've read is not quite accurate/correct. A more correct pictue is as follows:

A star may approach a super-massive black hole (SMBH) so closely that the tidal forces of the SMBH tear it appart. The distance to the SMBH at which this happens is often referred to as the tidal radius. For a (non-rotating) SMBH with a mass in excess of about $10^8$M$_\odot$, the tidal radius is within the event horizon and hence this process is invisible.
About half (depending on the initial orbit of the star) of the stellar material is caught by the SMBH onto a elliptic orbits (the other half escapes on hyperbolic orbits), but due to gas interactions and dissipation (shocks transfer orbital energy into heat which is then radiated away) it soon circularises to form a (or contribute to an existing) accretion disc. (Note that feeding from stars is not thought to be the main mechanism for the formation of accretion discs; a more likely process is the direct infall of gas clouds).
Material in the accretion disc orbits the BH on nearly circular orbits. It cannot simply fall into the SMBH because its angular momentum is conserved. However, gas at adjacent radii orbits at different velocities (differential rotation), i.e. the disc is shearing. Viscosity in the gas then causes angular momentum to be transported outwards, mass being transported inwards, and the material to heat up. The material in the disc is a hot (presumably) magnetized plasma, and the viscosity is thought to be provided by the so-called magneto rotational instability. The time scale for this process is, however, very long and only becomes efficient ($\sim10^6$yr) rather close ($\sim100$AU) to the SMBH.
The temperature of the disc material is extremely high, such that it radiates in the X-ray and UV, visible as quasars. This radiation is the main energy output from the accreting BH and constitutes about 10% of the gravitational energy gain $mc^2$ ($m$ being the mass accreted). This radiation interacts with any surrounding material (including the outer accretion disc itself, in particular if it is warped), generating an outflowing wind
(by its radiation pressure) and possibly preventing further accretion (this corresponds to the Eddington limit).
The rotating magnetized disc may also generate collimated bi-polar outflows, commonly referred to as jets. However, IMHO, only a small fraction of the energy can be emitted in these jets, already because the jets have low entropy (they are dominated by ordered motion), while the hot disc has high entropy.


Storm on Earth vs. storm on Jupiter? Why is the storm on Jupiter much stronger than storms on Earth and why don't the storms on Earth get as big as Jupiter's great red spot

Basically because Jupiter is a gas giant and thousands of times more massive than the Earth. Everything on Jupiter is on a gigantic scale. The Great Red Spot for instance could accommodate several Earths, so that explains why you couldn't have a storm on Earth which came anywhere near that size. As well as heat from the sun, which is what drives the weather on Earth, Jupiter has a massive internal source of heat.


Could images of (very) nearby targets be recovered from Event Horizon Telescope data? Reading this question I was struck by the fact that the "pointing" of an interferometer is effectively done in the correlator, at least within the common field of view of all the telescopes, which is many thousands of times wider than the imaged area. Could the same data be reprocessed to reveal images of other "tiles" from this common field of view? Could it image the accretion disk, for example, or the base of the jets? Are there any targets that are close enough and bright enough to be worth doing this?

The question asks about "targets" but doesn't specify black holes. However the other "targets" are related to infall of black holes.

Nonetheless, I think the answer is a solid "yes" either way.

Data is recorded separately at each site. Hours and hours of digitization of one or more 2 GHz wide data channels around 230 GHz, down converted to baseband and synched using a local atomic clock. See this answer and this answer

Any signal reaching the feed horn of each radio telescope is going to be recorded in its data stream. So if there is another source nearby that is covered by the radiation pattern of all of the dishes in the EHT collection, then it certainly could be imaged to some (indeterminate) level of resolution, with (at least) the following caveats:

You have to run the numerical correlations again, with a new range of time-offsets for each telescope. If the source is much brighter, you wouldn't have to use the whole dataset. But if it's just as dim or dimmer, you will have to correlate lots of data.
If the source is far away from the imaged event horizon, there may be several different types of calibrations for each dish and site that have been optimized, and may not work as well for a substantially removed object. The existing data may result in slightly lower resolution far from the target, depending on how calibrations were done. In addition to signals of the black hole, signals from other sources were recorded to assist in calibration of the whole array.

I need to read further on that, I'll update the part about calibration tomorrow.


What will be the impacts on the planets' orbits from Gliese 710? In 1.5 million years, the star Gliese 710 is expected to pass through the Oort cloud. Now obviously this will shower the inner solar system with comets, but the Wikipedia page doesn't say anything about the star's impact on the orbits of the planets.

Will the star not throw the Earth and other planets into highly elliptical orbits, destroying all life? Or, simply, do we not know because this would involve solving the 11-body problem?

It'll have basically no effect on the planet's orbits, except perhaps on the theoretical planet 9 and even then, not very much.  As close as it's expected to get Gilese will till be too far away to affect the 8 planets much.

Gilese 710's mass is about 60% the mass of our Sun.

I've seen two estimates, of 13,000 Astronomical Units (AU) and 13,365 or about 77 light days or 0.21 light years at closest pass, and some estimates put a +/- margin of error at as much as 20% of that, but lets use 13,365 AU as an estimate.   For a star, that's exceptionally close but it's enormously distant for a planet.

Neptune, the most distant planet, averages about 30.1 AU from the Sun, so Gilese will be about 440 times further away from Neptune than Neptune is from the Sun.   That will give it a negligible gravitational effect of less than 1 part in 300,000 on Neptune's orbit.  That's, by crude calculation, a bit less than the effect Saturn has on Earth's orbit.  That's enough for a small perturbation, but you'd need some pretty high tech equipment to even detect it.   It's about 50 times less than the gravitational effect Neptune had on Uranus when it was noticed that Uranus' orbit was behaving oddly and that observation lead to the discovery of the planet Neptune.

So, basically, for the 8 known planets, the outer planets will get small pushes, noticeable by very meticulous observation only and the inner planets, essentially no effect.

It's effect on Planet 9 would be greater but still small.  The estimate for planet 9 at Aphelion is about 1,200 AU or 1/11th the closest pass of Gilese.  That's close enough for a noticable push.  Not enough for any major changes like pushing it's eccentricity to where it passes anywhere close to Neptune, or ejecting it from the solar system, but Planet 9 could be jostled somewhat, perhaps as much as a 1% change to it's orbital period and/or eccentricity, which for a planet's orbit, 1% is a large change over a single orbital period.

The bigger effect, as you mentioned, will be on more distant objects, several thousands to tens of thousands of AU from the Sun, or several times more distant than the theoretical Planet 9 at it's most distant.   Objects that distant have never been observed, but are theorized to be out there in vast numbers based on a handful of observed objects with very high eccentricity.  Oort cloud objects at those distances will get cast every which way, some cast out of the solar-system and a small percentage, pushed to higher elliptical orbits where they could pass through the inner solar system.   Our sun will do the same thing to Gilese 710, casting it's Oort cloud objects in every direction and each star will steal a number of orbiting objects from the other.

The initial effect, the inner solar system might pass through the outer edge of Gilese 710's outer oort cloud.  I don't think anyone knows how extensive it's Oort cloud is at 13,000 AU, but any initial increase in objects passing through the inner solar system will be from Gilese 710's Oort cloud.

Any of our Solar system's Oort cloud objects that Gilese 710 sends towards the inner solar system will take (roughly) between 50,000 and 400,000 years to reach the inner solar-system, so Gilese 710 will be perhaps still visible to the naked eye when the proposed rain of comets begins, but it will be long past being the brightest star in the sky if/when any objects it sends towards the inner solar system make it here.   The reason for this is that it moves much faster than any objects it's likely to send our way.   Gilese 710 will pass through the solar system at about 50,000 kph and any objects it sends towards the inner solar system, because they're so spread out and most are likely to just get a small push from the passing star, they will leisurely drift towards the inner solar system at roughly between 500 and 1,500 kph, requiring about 50,000 years for the first ones to begin to reach the inner solar system and become comets or perhaps crash into the Earth or Moon or other planets.

Also, fun fact.   At closest pass, Gilese 710 should be a hair brighter than Jupiter at Jupiter's brightest, nearly -2.8 apparent magnitude.  It will be the brightest star in the sky for ballpark 15,000 - 20,000 years, over that time-frame it will move across the fixed stars a good percentage of the 180 degrees of horizon.   In a human lifetime it might move a bit over 1 degree of arc at closest pass, so it won't be very noticeable that it's moving, but that's very rapid proper motion for a star.

(Too long?)

I've excluded the mathematical formulas cause I don't do the that well and it would get a little ugly.  I can't do the math of a 3 body system, so this is an approximation, but I my approximation should be in the ballpark.


Do liquids exist in the interstellar medium? Am not asking about planetary/meteorite surfaces and am restricting myself to the interstellar medium

edit:
by interstellar medium I mean, the following:

  "Interstellar medium (ISM) is the matter that exists in the space between the star systems in a galaxy. This matter includes gas in ionic, atomic, and molecular form, as well as dust and cosmic rays. It fills interstellar space and blends smoothly into the surrounding intergalactic space."

(copied verbatim from a google search).

Basically, No. for liquid as we commonly know them on earth. If you trow a water bucket in space, it gets quickly vaporized. From this interesting ref:

  having a pressure vacuum will cause the water to boil almost instantly

There is however an exception I've been thinking of. Since glass has an amorphous structure, it can also be regarded as a liquid... And if some advanced form of life or intelligence managed to send some into the interstellar medium, then yes there is.

It's also possible that some stellar explosion send away magma of some form with an amorphous structure similar to that of glass (e.g. obsidian)


Are larger stars rounder? The Earth is a very smooth sphere, and the Sun even more so, with only minor fluctuations. I am wondering: are larger stars even rounder? Intuitively, that seems self evident, but I am not so sure. For instance, the hydrostatic equilibrium causes larger stars to be much less dense than red dwarfs. So what is the most important factor for how round a star is, a higher mass, or less activity? The most prominent cause of irregularities is of course the rotation rate of the star, which is pretty much independent of size. Ignoring that, do larger stars have smaller a deviations from the ellipsoid relative to their size?

Edit

As it seems like the "other than rotation rate" criterion is not really meaningfull, I now terminate it.

In terms of mean angular velocity, the distribution of rotation rates among main sequence stars is well known. Allen (1963) compiled data on mass, radius, and equatorial velocity, which was then expanded upon by McNally (1965), who focused on angular velocity and angular momentum. It became clear that angular velocity increases from low rates for spectral types of G and below before rising to a peak around type A stars and then slowly decreasing.
Equatorial velocity continues increasing to mid-B type stars, before slowly decreasing, but because of the increased radii of O and B type main sequence stars, the peak in angular velocity occurs before this. As part of Jean-Louis Tassoul's Stellar Rotation notes, many O type stars have rotational periods similar to that of the G-type stars like the Sun!
The distribution is not smooth and uniform (McNally noticed a strange discontinuity in angular momentum per unit mass right for A0 and A5 stars; see his Figure 2); Barnes (2003) observed two distinct populations in open clusters, consisting of slower rotators (the I sequence) and faster rotators (the C sequence). Stars may migrate from one sequence to another as they evolve. Interestingly enough, stars on the I sequence lose angular momentum $J$ faster than stars on the C sequence:
$$\frac{\mathrm{d}J}{\mathrm{d}t}\propto-\omega^n,\quad\text{where}\begin{cases}
    n=3\text{ on the I sequence}\\
    n=1\text{ on the C sequence}\\
  \end{cases}$$
Here, of course, $\omega$ is angular velocity. These results obey Skumanich's law.
Oblateness can be determined from mass, radius, and angular velocity as
$$f=\frac{5\omega^2R^3}{4GM}$$
Using this and McNally's data, some quick calculations get me the following table:
|Spectral type|$f/f(O5)$|
|--|-------|
|O5 | 1    |
|B0 | 1.28 |
|B5 | 1.84 |
|A0 | 1.67 |
|A5 | 1.35 |
|F0 | 0.482|
|F5 | 0.0387|
|G0 | 0.000314|


galaxies in a specific radius in different shapes of universe Let the distribution of galaxies be a number like $n$. we want to find the total number of galaxies within the radius $r$ from a specified point. in which shape of the universe (flat, open, closed) the number we get is higher? ($n,r$ are the same in this universes and only the area (volume) in each universe is different).

It seems clear to me that this is a homework question. I believe what the question is after is to look at the equations for distances in the universe as a function of curvature and see how this curvature affects things. Read, for example, sources like this one or this one.

As with most of things in cosmology, there's lots of equations and many ways to write them. A relevant equation would be the following angular diameter distance:

$$D_M = (1+z)^{-1}
\begin{cases}
\frac{1}{\sqrt{\Omega_k}}\sinh\left(\sqrt{\Omega_k}D_c/D_H\right), & k = -1 \\
D_c, & k = 0 \\
\frac{1}{\sqrt{\left|\Omega_k\right|}}\sin\left(\sqrt{\left|\Omega_k\right|}D_c/D_H\right), & k = +1 \\
\end{cases}
$$

In this case, you can see the three forms it takes for a closed universe ($k=-1$), a flat universe ($k=0$), and an open universe ($k=+1$). It should be pretty clear that if you chose some volume with some radius calculated by $D_A$, you'd get different volume sizes and thus different amounts of galaxies in each volume for the three universe types.

By looking at the equations and playing around with them, you should be able to convince yourself which universe type results in the largest volume.


Locating a star cluster on a Hertzsprung-Russell diagram with other color index Recently I took pictures of the NGC 884 cluster in the B and I bands and selected some 30-what stars from them. I've reduced the data to a colour-magnitude diagram (absolute magnitude in the I band $M_I$ vs. (B-I) colour index). I am now asked to designate where the stars I've selected from this cluster are located on the Hertzsprung-Russell diagram and the approximate age of the stars.

My problem is the following: any HR diagram that I can find is either expressed in effective temperature instead of colour index, or is in the colour index (B-V). Also I can only seem to find empirical formulas for the effective temperature that use the (B-V) colour index, not the (B-I) one.

Does anyone know where to find an HR diagram that uses the (B-I) colour index?

With an absolute magnitude of zero and a colour (any colour) of zero, your stars are around spectral type A0 (by definition). The only thing that can confuse this is reddening/extinction.

If there is appreciable reddening/extinction, then your stars could be more luminous and hotter (O/B stars), but there isn't any way to tell from your data.


What would happen if a Sun-like star were to consume a Jupiter-like planet? A substantial number of discovered exoplanet systems are likely contain more than one gas giant with distances far closer to the star than in our solar system.

It is known that in solar system evolution gravitational effects between objects can lead to some of them migrating outwards while others migrate inwards.

Has there been any study of what happens when a gas giant approaches its star close enough to start losing mass to it? In this case could the planet end up completely in the star?

For the purposes of this question let's choose a solar-mass G-type star and a gas giant such as Jupiter, since we have so much first-hand knowledge of these objects.

Jupiter has plenty of hydrogen and this would represent the addition of more nucleosynthetic fuel.

Would this be enough to cause the Sun to become the red giant in this case?

There are scenarios where the effects of planet accretion on a sun-like star would be very significant indeed, at least in the short term. Whilst the amount of mass accreted by the star would be a tiny perturbation, the amount of accreted energy and/or angular momentum may not be.

Scenario 1: The scenario where a hot Jupiter just drops into the star from a distant radius would certainly have drastic short-term effects. But short-term here means compared with the lifetime of the star.

The kinetic energy of say Jupiter falling to the star's surface from a distant (more than a few solar radii) would be of order $GM_{\odot}M_\mathrm{Jup}/R_{\odot} \sim 4\times 10^{38}$ joules.

For comparison, the solar luminosity is $3.83 \times 10^{26}\ \mathrm{J/s}$.

The addition of this much energy (if it is allowed to thermalise) would potentially affect the luminosity of the Sun for timescales of tens of thousands of years. The exact effects will depend on where the energy is deposited. Compared with the binding energy of the star, the additional energy is negligible, but if the energy is dissipated in the convection zone then the kinetic energy would do work and lift the convective envelope. In other words, the star would both increase in luminosity and in radius. If the effects were just limited to the convective envelope, then this has a mass of around $0.02 M_{\odot}$ and so could be "lifted" by $\sim 4\times 10^{38} R_{\odot}^2/GM_{\odot}M_{\rm conv} \sim 0.05 R_{\odot}$.

So in this scenario, the star would both expand and become more luminous. The relevant timescale is the Kelvin-Helmholtz timescale of the convective envelope, which is of order $GM_{\odot}M_{\rm conv}/R_{\odot} L_{\odot} \sim $few $10^5$ years.

If the planet somehow survived and punched its way to the centre of the star, then much less energy would be deposited in the convection zone and the effects would be lessened.

On longer timescales the star would settle back down to the main sequence, with a radius and luminosity only slightly bigger than it was before, commensurate with its tiny (0.1%) increase in mass.

This all assumes that the planet can remain intact as it falls. It certainly wouldn't "evaporate" in this direct infall scenario, but would it get tidally shredded before it can disappear below the surface? The Roche limit is of order $R_{\odot} (\rho_{\odot}/\rho_{\rm Jup})^{1/3}$. But the average densities of the star and (gas giant) planet are almost identical. So it seems likely that the planet would be starting to be tidally ripped apart, but as it is travelling towards the star at a few hundred km/s at this point, tidal breakup could not be achieved before it had disappeared below the surface.

So my conclusion is that dropping a Jupiter into a Sun-like star in this scenario would be like dropping a depth charge, with a lag of order $10^{5}$ years before the full effects became apparent.

Scenario 2: A hot Jupiter arrives at the Roche limit (just above the stellar surface) having lost a large amount of angular momentum. In this case the effects may be experienced on human timescales.

In this case what will happen is the planet will be (quickly) shredded by the tidal field, possibly leaving a substantial core. At an orbital radius of $2 R_{\odot}$, the orbital period will be about 8 hours, the orbital speed about  $300\ \mathrm{km/s}$ and the orbital angular momentum about $10^{42}\ \mathrm{kg\ m^2\ s^{-1}}$. Assuming total destruction, much of the material will form an accretion disc around the star, since it must lose some of its angular momentum before it can be accreted.

How much of the star's light is blocked is uncertain. It mainly depends on how the material is distributed in the disk, especially the disk scale height. This in turn depends on the balance of the heating and cooling mechanisms and hence the temperature of the disk.

Some sort of minimal estimate could be to assume the disk is planar and spread evenly between the solar surface and $2R_{\odot}$ and that it gets close to the solar photospheric temperature at $\sim 5000\ \mathrm K$. In which case the disk area is $3 \pi R_{\odot}^2$, with an "areal density" of $\sigma \sim M_{\rm Jup}/3\pi R_{\odot}^2$.

In hydrostatic equilibrium, the scale height will be $\sim kT/g m_\mathrm H$, where $g$ is the gravitational field and $m_\mathrm H$ the mass of a hydrogen atom. The gravity (of a plane) will be $g \sim 4\pi G \sigma$. Putting in $T \sim 5000\ \mathrm K$, we get a scale height of $\sim 0.1 R_{\odot}$.

How long the accretion disk would remain, I am unsure how to calculate. It depends on the assumed viscosity and temperature structure and how much mass is lost through evaporation/winds. The accreted material will have radiated away a large fraction of its gravitational potential energy, so the energetic effects will be much less severe than Scenario 1. However, the star will accrete $\sim 10^{42}\ \mathrm{kg\ m^2\ s^{-1}}$ of angular momentum, which is comparable to its current angular momentum of the Sun.

The accretion of an exoplanet in this way is therefore sufficient to increase the angular momentum of a star like the Sun by a significant amount. In the long term this will have a drastic effect on the magnetic activity of a Sun-like star – increasing it by a factor of a few to an order of magnitude.


Why isn't the constellation of that month's zodiac prominent or even visible during that month? For instance, the month of October is called the month of Leo, but you will mainly see Aries, Taurus and the Southern skies during October, Leo isn't very prominent. So how is the zodiac decided?

The Zodiac sign of a month is decided by the month when the Sun is "in" that sign from the point of view of Earth.
(See Mike G's comment below - because of the precession of the Earth, Zodiac signs are now 30 degrees "off" from their original markings/associations with the months of the year.)
I believe that August is actually the month of the Leo, not October - more specifically, July 23 to August 23. (see here for more date info) If you look at the diagram, you'll see that the Sun is right in front of Leo from the point of view of Earth during the month of most of August, and thus we call that the month of the Leo. Some more examples - during October, the month of Libra, the Sun is directly in front of Libra, and during March, the month of Pisces, the Sun is directly in front of Pisces.


Equating critical energy density to matter density I'm asked first to calculate the critical energy density $\rho_{crit}$ of the universe given certain constants. The value I got was $5500\,\mathrm{MeV}\,\mathrm{m}^{-3}$.

Then I'm asked to calculate various number densities of particles assuming the universe is made up of a single type. To do this, I'm told I need to set the universe's matter density equal to $\rho_{crit}$.

I'm not sure how to do this since they are in different units. My expression to find $\rho_{crit}$ had a $c^2$ in it, and if I take that out I would have units of matter density. Is that what I'm supposed to do?

By the mass-energy equivalence:

$E = mc^2$

If $E = 5500$ $\mathrm{MeV}$, then:

$m = \frac{E}{c^2} = 9.8 \times 10^{-27}$ $\mathrm{kg}$.


Angular diameter result confirmation Not being a math-minded person at all, however basic it might appear to be to all of you in this particular question, I would like to get the following calculation confirmed.
I'm trying to calculate the angular diameter of two natural satellites orbiting a planet, as seen from the planet.

Satellite A is located 568350 km from the planet and has a diameter of 4400 km.
Satellite B is located 357094 km from the planet and has a diameter of 1900 km.

For Satellite A, we would therefore get:
$$\arctan\left(\frac{4400}{568350}\right)=0.44^\text{o}$$
For Satellite B, we would therefore get :
$$\arctan\left(\frac{1900}{357094}\right)=0.3^\text{o}$$
Similarly, for our moon, the same calculation goes as follows:
$$\arctan\left(\frac{3476}{384402}\right)=0.51^\text{o}$$

To find the angular diameter of a satellite you need to find
$$\arctan\left(\frac{\text{diameter of satellite}}{\text{distance to satellite}}\right)$$
As you have been using the diameter of the planet, your formulae are wrong.
You should also make use of the small-angle approximations $\arctan(x)\approx x$ for small x in radians. So to get the value in degrees you can do
$$\frac{180^\circ\times\text{diameter of satellite}}{\pi\times\text{distance to satellite}}$$
And you should round your values. You don't need 15 significant figures of accuracy!


How long will it take to reach the closest earth like planet Using the engines today how long will take humanity to reach earth like planets?

HD 219134b is apparently closest "super earth"; 21 light years away.
New Horizons, our speediest yet, is currently running at about 15 km/sec relative to the sun.
At 9.46e12 km/lightyear HD 219134b is 2e14 km away
At 3.16e7 seconds per year, New Horizons will travel about 4.74e8 km/year.
2e14/4.74e8 = 422000 years.

HD 219134b isn't even a very good earth; gravity is too high.


How much do starlight and artificial light illuminate the Moon? The Moon receives most of its light from the Sun, either directly through sunlight or indirectly through Earthshine. However, there are many other dimmer sources of illumination.

Starlight. This is probably the most important one. Under exceptional conditions, it creates shadows here on Earth.
Artificial light. Mostly city lights and gas flares.
Meteorological activity. Lightning storms on Earth and other planets are visible from space, and here on Earth, they can trigger other sources of light such as firestorms.
Bioluminescence. Light-emitting microorganisms are bright enough and numerous enough to be seen from space.
Transient bright lights. Events such as supernovas and asteroid impacts are rare, but when they happen, they are a significant source of illumination. Over long timescales, for example a billion years, this may be the biggest contributor of non-solar light.

How much do these light sources illuminate the Moon? If there was no Sun, could we still see the Moon with these lights? And of the three Earth-based sources (artificial light, meteorological activity, bioluminescence), which one illuminates the best?

To reverse the question: if you were on the Moon, what sources of light would you see?

The Sun would be the major source of light.
Although the Moon's albedo is only about 12%, reflected light from the ground would be the second major source.
The Earth would be in a poor 3rd place. You could see the general shape of the continents, but not storms or any other light-emitting meteorological or biological activity (it's there, but much too faint to be seen from the Moon)
The other planets would be poor 4th place.
Starlight would be in a poor 5th place.

It would be similar to standing on Earth, except:

The Moon has no atmosphere, so:

the Sun would be brighter
there'd be no light diffusion, so you could see the Earth/planets/stars in the daytime
Meteors would not leave a fiery trail -- they would simply hit the moon silently. Asteroid impacts on the Moon would also produce no sound.

There wouldn't be a moon in the sky, but there would be an Earth instead.

You add a twist to the question by asking about past and future light:

In about 7.6 billion years, the Sun will expand to (probably) engulf the Earth and Moon, which will probably be the most light the Moon will see before it's reduced to nothingness.
The Moon possibly saw a lot of light when it was created through planetary collision. Not sure this counts, because the Moon didn't really exist until sometime after this.
Any other major source of light (such as supernovas) would be visible from Earth as well. That doesn't mean there haven't been any super-bright supernovas, but if they were bright enough and lasted long enough to generate more light than the Sun has during its lifetime, they would probably have a major impact on Earth (who knows, maybe that's why the dinosaurs died ), and there's really no evidence anything that liked happened.


How was Jupiter formed Two days ago I went to a mathematics conference where there was a paper presented on Jupiter's formation via the disk-instability model.

I know that there are two different theories for the formation of the planets. One of them is the Core accretion model and the other one is the disk-instability model. I also tried to read this paper on planetary formation in which they said 161 planets were put on to test. 90% followed Core accretion model and rest of them followed the later model. Did not say anything explicit about Jupiter.

Jupiter is too close to the Sun to follow the Disk-Instability model (I am not sure completely).

Now I want to know which model Jupiter follows for its formation.
(I asked this question on physics stack-exchange too but unfortunately there were no answers)

Maxwell's Smith Prize Essay on the formation of the Rings of Saturn is relevant. Using structural stability as his criterion, he deduced that the 'rings' could only consist of a single almost infinite mass of very small particulates each orbiting in accord with Newtonian dynamics--the correct result as verified by NASA/ Cassini.
 Applying the same approach to a forming  solar system this analysis shows that rings of particulates are a structurally stable attractor for the dynamics, provided the central star is much larger than the mean particulate size.
I am now  well outside my comfort zone so will leave further inferences to others :).
Maxwell's essay on rings of saturn


How long have the TRAPPIST-1 planets been in a habitable zone? The planets orbiting TRAPPIST-1 are in orbits around their star much closer than Earth orbits Sol. However, since TRAPPIST-1 is a cool brown dwarf, some of the planets are in an "inhabitable zone", where the temperature would not be too extreme for life as we know it.

What I wonder is, how long have those planets been in an inhabitable zone? The wikipedia page says that TRAPPIST-1 is older than 1 Gyr. But, how much of that time has it been a cool brown dwarf?

In other words, since the abiogenesis of life and the evolution of living organisms has dependency on time, how much time has life had a chance to develop in the star's habitable zone?

For this question, I'm not concerned about the chemistry or other characteristics of the planets themselves.

The luminosity of Trappist-1 is estimated to be $5.25\times 10^{-4}\ L_{\odot}$, but it has not always been like this.

The luminosity of a brown dwarf decreases with time and it this measured luminosity (along with the spectral type) that allows an estimate of the mass and a lower limit to the age using stellar evolutionary models.

If I look at the Baraffe et al. (2015) low-mass evolutionary models and look at the locus of luminosity versus time for a $0.08\ M_{\odot}$ star like Trappist-1, you can see that the current luminosity implies an age of $\sim 500$ million years. But if you go back in time, the star was more luminous and for that reason, planets that are currently in the habitable zone (said to be planets e,f,g) were not so in the past.

The details of a habitable zone (HZ) calculation can be complex, but basically the radius of the habitable zone scales as the square root of the luminosity. If planets d and h are not currently in the HZ then we can use these as a conservative definition of the HZ boundary.

From this (and using the published orbital radii of the planets), I can see that if the luminosity is increased by a factor of 9, then none of the planets b-g are in the HZ, it is larger than all of their orbits. Trappist-1 had a luminosity that was 9 times larger when it was younger than 27 million years old. On the other hand, if I want to move the HZ just outside the orbit of planet e (and simultaneously include planet h inside the HZ), then this would occur when Trappist-1 was at an age of 206 million years. As a final thought, you can see from this particular model that Trappist-1 may fade by a further factor of two as it gets older. This decreases the HZ radius by a factor of 1.41 and would mean that g (and possibly f) would fall outside the HZ, whilst d (and possibly c) would be brought into the HZ.

It should be noted however that: different models give slightly different results, these loci are mass dependent and the mass is not known, it is inferred from the same models using a temperature estimate (which is also uncertain). So whilst my qualitative conclusions about the past location of the HZ are likely to be correct (though the detailed age numbers are model-dependent), the future behaviour of the HZ is more uncertain because Trappist-1 may be slightly more massive than assumed and already reached its minimum luminosity.

The luminosity evolution of Trappist-1, assuming a mass of $0.08M_{\odot}$ and the models of Baraffe et al. (2015). The horizontal dashed line marks the best estimate of its current luminosity, for which planets e-g are said to be in the HZ. If we go back in time, the rightmost vertical dashed line marks the age below which the luminosity increases to the point that e becomes too hot to be habitable. Then further back, the leftmost vertical dashed line marks the point where all the currently known planets (b-h) become uninhabitable.

So the answer to your question is quite uncertain and depends critically on the age of Trappist-1 now and of course whether the planets were always at the orbital radii they are now$^{*}$. As you can see from the plot above (note the logarithmic scale on the x-axis), the luminosity evolution referred to above takes place early-on. If Trappist-1 could be as young as 500 million years, then life on planet e may only have been possible for 300 million years. However, if the star is slightly more massive and is 10 billion years old, then life has had 9.8 billion years to get going.

If you are talking about planet f, then it has had slightly longer ($\sim +100$ million years) inside the HZ, and planet g slightly longer ($\sim +70$ million years) again. Planet h will have spent comparatively little time (in the past), less than a few hundred million years, inside the HZ.

$^{*}$ The abstract of the discovery paper by Gillon et al. (2017) briefly discusses the possibility that the planets migrated inwards after formation, through a process of "disc-driven migration". If so, then this will not alter the discussion above. Disc around very low-mass stars may be longer-lived than those around higher mass stars, but have essentially dispersed after $\sim 10-20$ million years (Kennedy & Kenyon 2009; Dawson et al. 2013; Binks & Jeffries 2017), and the planetary configuration would have to be settled where it is now by the time the disc has gone.


How is space-time bent by black holes? I just read When a star becomes a black hole, does its gravitational power become stronger? and it confused me a bit;

I always thought black holes are supermassive and that is why they bend space-time. But it is said therein that the black hole is less massive than the star that created it (and it seems logical if we think about that - indeed, the star exploded and released a lot of matter).

Thus my question: How can a black hole bend space-time more than the star which created it?

It doesn't bend it more. It is just that the matter of the star is now concentrated in a very compact region of space. So, previously you could get only to the surface of the star but now as the star is very compact, you can get much nearer and thus you will experience a greater gravitational force.

I had a similar discussion here. Check it out and maybe your question will be answered.


Could there be a closer star to Earth than the Alpha Centauri triple star system, excluding the Sun? Do we know for certain that the nearest star to Earth, excluding the Sun and the theorized Nemesis, is the Alpha Centauri multiple star system, at +/-4.4 light-years away? Have we been able to identify every star that is <5 light-years from Earth or is there a possibilty a faint star could be nearer?  Are there stars that we haven't looked at yet that could be closer?

It's begining to look like there aren't as many brown dwarfs as was once thought:

  NASA's Wide-field Infrared Survey Explorer, or WISE, has been turning up a new crowd of stars close to home:

  ...

  Previous estimates had predicted as many brown dwarfs as typical
  stars, but the new initial tally from WISE shows just one brown dwarf
  for every six stars. It's the cosmic equivalent to finally being able
  to see down a mysterious, gated block and finding only a few homes.

Reasonably current data puts the current closest brown dwarf at 6.5LY (2 parsec) and less than 0.08 solar masses.

A nearer one could show up, but it's not looking likely.


Deriving Dark Matter; specifically looking for a table of stellar speed versus distance from center of galaxy to derive dark matter I'm trying to find some data that I can use to derive dark matter (in a loosey-goosey sort of way, I won't be too rigorous). I'm helping a friend out with a final project for his astronomy course and I'm tasked with doing the maths. Any idea where I can find a table of various stars distance from the center of our galaxy and their speed (how fast they are going around the center of the galaxy)? This does not need to be comprehensive but would ideally contain several data sources from various representative radii. Also some sort of formatted text would be SUPER cool so I could just tell my computer to do the calculations!

It is very difficult to measure Galactic orbital speeds of stars in our galaxy for several reasons.  We are moving with them and do not have a reliable measurement yet of even our own orbital motion. We only get one component of motion, the radial component (although Gaia is about to change that).  Dust in the galactic plane prevents us from seeing very far in our own galaxy.  So, when we measure Dark Matter in galaxies, it usually done in other galaxies where we can see both sides of the galaxy and, except for inclination angle, directly measure the orbital motion.


Obtaining deltaT for use in software I'm currently developing a javascript application in which I want to calculate the approximate position of the sun. This works quite fine but requires the value for deltaT (TT-UT) to be set depending on the year for which I want to calculate the solar position.

Currently, I'm using a default value of 67 for my calculation. However, since I want to calculate the solar position for several years I'm looking for a convenient way to obtain the deltaT value for each year.

To all of you, that have some experience with programming:
Is there any interface (API) that provides me with the desired values?
Of course, it would also be sufficient to get the universal and terrestrial time so that I can calculate deltaT on my own.

See Where can I find/visualize planets/stars/moons/etc positions? for an extremely general answer on how to compute positions, but the files you're looking for specifically are the "leap second kernels" at http://naif.jpl.nasa.gov/pub/naif/generic_kernels/lsk/

There was a mild kerfuffle on spice-discussion lists when NASA failed to update the kernels after the end-of-2016 leap second was announced, but they have updated it now.


The origins of Dark Matter/Dark Energy Layman here with an affinity for astronomy, space and everything associated with it. I'm a big fan of the space oriented programs on the Discovery and Science channels and I was watching an episode recently that got me to thinking about the origins of Dark Matter (DM) and Dark Energy (DE). Do any of you know if any thought has been given to the creation of DM and DE as byproducts of space being created? As I understand it, as the universe expands it essentially creates space where nothing was before. To be honest, I know nothing about what that even means: creating space where nothing was before.

Your question is quite complex, since dark matter and dark energy are two different things, that only have in common that they are called "dark" and are hypothetical quantities. They are hypothetical, because we only defined them as mathematical quantities to make physical laws, we invented, comply with the observations.

what is the origin of dark matter? In 1930 Fritz Zwicky observed the coma cluster. The coma cluster is a cluster of galaxies that move amongst each other based on their gravitational pull to one another. He found to his surprise that the galaxies in this cluster were moving faster than the gravitational pull based on their visible mass would predict. Since the visible mass of the galaxies couldn't explain the movement of the galaxies, he suggested there was another, yet unseen, matter in that cluster that contributed to the gravitational pull. He called it "Dunkle Materie" (he was Swiss) - Dark Matter. Later studies on the rotation of galaxies also showed that the rotational speed of the galaxies couldn't be explained by the visible matter in those galaxies, so another dark matter needed to be present to explain for the anomaly.

after adding the hypothetical dark matter in the laws of nature many things could be explained on the theoretical level. The forming of galaxies clusters in the current observable universe and the fluctuations of the cosmic microwave background could be explained with dark matter. The theory here brings the existing physical laws in accordance with the current observation. The problem is that dark matter has not yet been proven to exist in experiments. It has not been observed and many theories exist on what that dark matter could be.

Dark Energy has a different history, leading to a similar thing. When Einstein laid out his theory of general relativity he included in his field equations a cosmological constant, lambda, to be able to come to a static universe based on his equations. He assumed the constant to be zero and therefore come to a static universe. A negative constant would mean the universe would shrink and a positive constant would mean the universe would expand.

Unfortunately, Hubble showed that the universe was not static, but expanding and in the 1990's measurements showed this expansion was accelerating. Now the cosmological constant was not zero and an explanation needed to be found why the universe was expanding ever faster. For this expansion an energy is needed that counters the gravitational pull of the (dark) matter in the universe and increases with each expansion of this universe. Later observations from a supernova in 1998 and the Boomerang and Maxima experiments on the cosmic microwave background gave strong evidence of the existence of dark energy. This is, when we believe the standard model of cosmology is right.

The problem with dark energy is that in the most cases the laws of cosmology are correct and explain the universe. This means that dark energy must be real, or the cosmological laws must be altered. Currently, the search for dark energy is going on. The definition of dark energy in the models explains the expansion in of the universe and also the fast expansion during the early years of the universe. However, dark energy is a hypothetical quantity and has not been observed yet.

I hope this gave you a brief history how dark matter and dark energy have come into astrophysics. Don't mix it up with the dark force. That's not even hypothetical and only exists in certain movies.


Understanding the JPL Ephemerides I'm working on an animation that replicates our Solar System. I know this has been done before, but I'm working on something that requires me to make one myself. With that being said, I'm having a tough time deciphering the JPL Ephemerides. Particularly for the Moon because I have already figured out how to work the VSOP Ephemerides, which gives me the positions for the planets.

Does anyone know of a source I could use to learn how to utilize the ASCII JPL Epehemerides file that are found here? It would be a huge help and I think a step by step walk through would probably help others in the future.

Thanks!

Link to Epheremerides files

There is a python package called Skyfield that loads, reads and interpolates the binary forms of the JPL Development Ephemerides or DEs for you, and does everything else you need to get the absolute best results possible from them. If you can use even a tiny bit of python then this would be the way to go rather than trying to figure out how to interpolate JPL's "special flavor" of Chebyshev polynomials.

See these in Space Exploration SE:

What is the exact format of the JPL ephemeris files?
How is the Chebyshev method used by JPL?
Is there a way to extract the Chebyshev coefficients for a body from a SPICE kernel?
How to obtain Chebyshev coefficients directly from the JPL Horizon's interface?
I've almost learned to spell Chebyshev, why has JPL switched to Hermite interpolation for DE438?

and also see how easy it is to do things like this with Skyfield in this answer to Sunset on [Mountain] from [Viewing Point]

Also this answer to How to calculate the planets and moons beyond Newtons's gravitational force? shows how I used JPL's Horizons interface output (which also interpolates the DEs using Spice) to compare my numerical orbit integrator results to the ephemerides.

And just for fun see How to pronounce “Ephemerides”? in English SE.


How do astronomers determine the texture of an exoplanet? So I was wondering how astronomers are able to get the texture/detail of an exoplanet or how they know a planet has water or not.  I know how they discover planets by using the light of a star and seeing if it gets faint or by seeing if the star is getting gravitationaly pulled back and forth,  to determine the planet size. But I just don't understand how they are able to get the texture of a planet. Thanks for the help!

This is known as spectroscopy.  Every molecule and atom in the universe emits and absorbs light at specific frequencies.  This is a result of the quantization of the energy levels (for electrons) in an atom.  Although there are lots of complicating factors, such as redshift, to account for, the patterns are usually so distinctive that the complications can be accounted for, and do not prevent scientists from figuring out chemical compositions (and even, to some extent, how abundant each chemical is).

For example, here are the emission spectra of several common atoms at rest (with respect to the observer):

Absorption spectra, on the other hand, appear as darker regions in the band.  Here's an example of an absorption spectra, which identities several elements by knowing that it is distinguished by absorbing light at specific wavelengths that others do not:

For exoplanets with significant atmospheres, we can only expect to see the spectra for its upper atmosphere.  All other signals will be muted out by the rest of the atmosphere.  We will not be able to tell what the surface looks like, or what it is made of.


When we see the Sun, do we actually see its past? The Sun's rays hit our eyes around 8 minutes after they are emitted from the Sun.
Does this mean that the Sun that we see is always the Sun as it was some 8 minutes before? I strongly think this must be happening; is it really a fact? Do we always see the Sun's past?

Yes, you are right. We don't only see the Sun 8 minutes in the past, we actually see the past of everything in space. We even see our closest companion, the Moon, 1 second in the past.

The further an object is from us the longer its light takes to reach us since the speed of light is finite and distance in space are really big.


Is there a way to know how the magnetic field was at the very beginning of our universe? I recently read that

  Gamma-ray bursts are catastrophic events, related to the explosion of massive stars 50 times the size of our sun. If you ranked all the explosions in the universe based on their power, gamma-ray bursts would be right behind the Big Bang,

I'm asking this question in the tentative to compare the Big Bang to a gamma-ray burst. I keep in mind that a key difference between both is that Big Bang was Not an explosion, but want to know similarities to point out.

Near the formation of a black-hole they tell in this same article that matter and magnetic field play a key role the gamma-ray bursts.

I know magnetic fields are induced by charge displacement. Is it possible there was a kind magnetic field before protons and electrons formed, or during the first expansion phase?

Here are pictures I modified from Wikipedia's chronology of the universe to illustrate:

Do we have a way to know how magnetic fields were at that time?

Close-up (image borrowed from donsnotes ):

Any way we could have information on magnetism at this point of universe creation?

In the Electroweak Era?
During the Great Unification Theory (GUT) Era?
And then in the Plank Era?

Is there a way to know how the magnetic field was at the very beginning of our universe?

I don't know of one. But then again I think there was no magnetic field at all at the very beginning of the universe. I can't point to any material that proves this beyond a doubt. But I can say that a quiescent black hole "has no hair", so it has no magnetic field, and the evolution of the universe is said to be something like pulling away from a black hole. See Hawking saying the universe is like a time reversed black hole here.

  I recently read that "Gamma-ray bursts are catastrophic events, related to the explosion of massive stars 50 times the size of our sun. If you ranked all the explosions in the universe based on their power, gamma-ray bursts would be right behind the Big Bang". I'm asking this question in the tentative to compare the Big Bang to a gamma-ray burst. I keep in mind that a key difference between both is that Big Bang was Not an explosion, but want to know similarities to point out.

There aren't many similarities. A gamma-ray burst is where some of the infalling matter is directed by magnetic fields generated by infalling matter and converted into gamma rays. The Big Bang is when the space of the universe started expanding.

  Near the formation of a black-hole they tell in this same article that matter and magnetic field play a key role the gamma-ray bursts.

True enough. But as far as I know matter and magnetic fields played no role in the Big Bang.

  I know magnetic fields are induced by charge displacement. Is it possible there was a kind magnetic field before protons and electrons formed, or during the first expansion phase?

It's possible I suppose. A magnetic field is associated with curl aka "rot" which is short for rotor. The early universe could have had an overall magnetic field, or gravitomagnetic field, if it was somehow rotating. See Mark Hadley's Galaxy sized twist in time pulls violating particles back into line. Also see the universe may have been born spinning. But I think it's rather speculative stuff myself.

  Do we have a way to know how magnetic fields were at that time?

I don't think so. The early universe was said to be maelstrom. Hence I think information about the initial state got scrambled.


Why does a right-handed circular polarized wave get lagged when going through ionized plasma? I have a question related to Faraday rotation. In Abigail Polin's Faraday rotation video I have seen that a linearly polarized wave can be decomposed in a left-handed circular wave and a right-handed circular wave and that when passing through ionized-magnetized plasma one of them gets lagged due to a difference on their refraction index.
Moreover, the video shows that the wave that gets lagged most is the right-hand circularly polarized.
Why is not the left-hand circularly polarized? What is the mathematical difference between their refraction indices?

What is the mathematical difference between their refraction indices?

The conventions on the signs of the direction of magnetic field vectors and of charges.

The tutorial specifies several things that can have a sign;

direction of the light propagation
direction of the magnetic field
sign of the particle's charge (electrons have negative charge)

Let's look at WIkipedia's Faraday effect; Interstellar medium where an equation for $RM$ or rotation measure is given:
$$RM = \frac{e^3}{2 \pi m^2 c^4} \int_0^d n_e(s) B_{\parallel}(s) ds$$
This expression includes $e^3$ and $B_{\parallel}$. Notice that the power of $e$ is odd. The assumption is that the interstellar medium is negative electrons, so they've replaced $q^3$ with $e^3$ but if it were positrons they'd have to write
$$q^3=-e^3$$.
Also remember that $B_{\parallel}$ can be positive or negative since the field could point either way relative to the direction of propagation.
The tutorial shows some of the intermediate steps where we get the idea that both the sign of the charge and the directions of the field will be important.

Why right handed circular polarization gets lagged when going through ionized plasma?

It could be either the RCP or LCP that is delayed more than the other, depending on the direction of the magnetic field relative to the direction of propagation, and on the sign of the lightest charged particles in the plasma that can respond most quickly to the electromagnetic wave.
In this example, the field just happened to be drawn parallel to the propagation, and normal plasma with negative electrons. In this case RCP is delated more than LHP. Had either one of those been opposite in the example, the result would have been oposite.


Shouldn't the estimate of the universe's age be higher, not lower, after the attractive strength of gravity is taken into account? From 'Fundamentals: Ten Keys to Reality', by Frank Wilczek :
"Running the movie of cosmic history backward in our minds, we found the galaxies all coming together to meet at a definite time.  When did it happen?  To calculate how long ago, we simply divide the distance a galaxy must travel by the speed at which it's moving.  (Since a galaxy's speed is proportional to its distance according to Hubble's observations, we'll find the same result consistently whichever galaxy we choose.)  Doing that, we estimate that galaxies were all smushed together about 20 billion years ago.  More accurate calculations, which include how the velocities change over time due to gravity, yield a somewhat smaller result.  Today's best estimate is that 13.8 billion years have elapsed since the big bang."
Since gravity is an attractive force, partially offsetting the expansion of the universe, shouldn't the estimated age be higher than the initial 20 billion years, not lower?
For instance, if astronomers, somehow, discovered that there was even more matter in the universe than estimated, wouldn't that cause them to revise the estimated age of the universe upwards, not downwards?
In other words, with gravity partially resisting and slowing down the expansion, wouldn't it have taken that much longer for the universe to reach its current size?

Attractive gravity means a shorter time since the Big Bang, for any given Hubble value. Here's a picture (from lecture notes by Sean Carroll):

The averaged distances and speeds of nearby galaxies give us the slope of the curve at the horizontal position marked "now". If you assume they've always moved at that speed, you get the scale factor indicated by the dotted line, while if you take into account that they're slowing down, you get the solid curve, which reaches $0$ (intersects the $t$ axis) at a more recent time.
Wilczek is wrong when he says that linear extrapolation gives an age of 20 billion years. It actually gives about 14 billion years, close to the correct value. This is more or less an accident: it's only true during the transition from matter-dominated decelerating expansion to dark-energy-dominated accelerating expansion, which happens to be happening now. In old matter-dominated cosmological models (predating the discovery of dark energy), linear extrapolation gives you about $1.5$ times the correct age, so it's possible that Wilczek got 20 billion by multiplying a modern estimate of the age by a ratio from an obsolete model.


Source of high energy cosmic particles Earth receives shower of both solar and cosmic radiations every seconds. The Sun put up a heliosphere around the solar system and thus foreign high energy charged particles could not penetrate easily. Is there any other source most probably that lies within our solar system?

Note:

Very high energy cosmic particle beyond 5.7 x 10^19 eV were observed
by Pierre Auger Observatory in Argentina. (rare but the GZK limit
seems to rule out distance sources)
GZK limit roughly capped the upper energy level of cosmic ray as
they are slowed due to CMB over vast distance

Normally, the two main sources of cosmic rays are considered to be the Sun (solar energetic particles) and galactic or extragalactic sources, although extragalactic sources are more common. Surprisingly, though, there is a third source, a family of particles known as anomalous cosmic rays (see Hovestadt et al. (1973) and Garcia-Munoz et al. (1973)). These are particles - generally ionized nuclei of lighter elements - primarily originating from interstellar space with energies on the order of $\sim10$ to $\sim10^2\text{ MeV}$.

Anomalous cosmic rays are generally accelerated at the termination shock of the heliosphere, moving into the heliosheath. Some may then escape into the inner Solar System. Here's an image from Stone et al. (2005), which shows them as a distinct population from galactic cosmic rays:

Now, these particles are technically of interstellar origin, even though they are accelerated to great speeds only at the boundary of the Solar System. However, a related population may have been found in the Kuiper Belt (see Schwadron et al. (2002)), formed from grains there after collisions between objects. While these are not necessarily - or even likely - the dominant source of anomalous cosmic rays, they do exist, and thus constitute another source of cosmic rays inside the Solar System.


Does the gravity of the planets affect the orbit of other planets in our solar system? When one planet passes near another during its trip around the sun, does their gravitational pull is strong enough to disrupt noticeably each other's orbit ?

It does - although the term 'disrupt' may be a bit too strong to describe the effect; personally, I think 'influence' would fit better.

An interesting consequence of such iterations is something called orbital resonance; after long periods of time - and remember that the current estimate for our planet's existence is 4.54 billion years - the ebb and flow of tiny gravitational pulls cause nearby celestial bodies to develop an interlocked behavior. It's a double-edged sword, though; it may de-estabilize a system, or lock it into stability.

Quoting the Wikipedia entry,

  Orbital resonances greatly enhance the mutual gravitational influence
  of the bodies, i.e., their ability to alter or constrain each other's
  orbits.

Another gravity-related effect (although, as pointed out by Dieudonné, present only on our solar system between bodies that have very close orbits like the Earth-Moon and Sun-Mercury systems) is known as Tidal locking, or captured rotation.

More about orbital resonance on this ASP Conference Series paper: Renu Malhotra, Orbital Resonances and Chaos in the Solar System.


Maximum distance for earth eclipse First of all sorry if this question was already asked, but I made a small search here and on google and I couldn't find any answer, so.. Here I am.

I was reading NASA's webpage about the Webb telescope, and I noticed that they are saying that one of the sides will be very hot.

I thought that one of the reasons to put the telescope on the L2 point was to avoid "seeing" the Sun, but apparently the L2 point is too far to have Earth fully cover the Sun.

So.. What is the maximum distance from Earth you have to be in order to have our planet block the Sun? Of course neglecting the effect of the atmosphere, which creates a sort of "halo" even if the Sun is fully covered.

Moreover what will the telescope see towards the sun? Just the star with a very tiny dot in the middle, or a very big black dot (optionally with the moon moving around it), or... What?

Thank you

Since the Earth is approximately 109 times smaller than the Sun (regarding radius), and the distance between them is about 150 million km, the eclipse will be visible up to about 1.39 million km from the Earth. This distance will vary (1.36 - 1.41 mln km) because the distance Earth-Sun varies during the year. This is about 3.6 times more than the Moon-Earth distance.

At this position, the apparent size of the eclipsed Sun will be only 1.08% smaller than the one on the Earth. The Moon has the radius about 1/4 of Earth's, and it will be visible as such on average. However
when in front of the Earth, it will be apparently almost 2 times bigger (regarding radius) than when behind.

We could orbit Earth at such distance, because Lagrangian points L1 and L2 are a bit farther (1.5 mln km). This means also that at L2 we could not see full Solar eclipse, but only annular with magnitude 0.93.


Why can I see the Andromeda Galaxy despite Milky Way's rotation? Okay ... so, I woke up from a dream last night wherein I was (apparently) attempting to devise a time-keeping system for interstellar travel (similar to the stardate system used in Star Trek). In the dream, I was attempting to correlate our current calendar, based on Earth's movements relative to the sun, with this fictitious calendar using the positions of intergalactic stellar bodies in comparison to Milky Way. (i.e., What if we could tell time based on the way other galaxies moved in relation to our own galaxy?)

So, in the dream, I was understanding that Milky Way is a spiral galaxy which rotates around a galactic center. That Earth is toward the middle-end of one arm of Milky Way's spirals. That almost every star we see in the night sky is inside Milky Way with us, and that the galaxy Andromeda is a great distance beyond Milky Way and can be seen in the night sky over Earth in the same location (between constellations Cassiopeia and Pegasus) every night since (at least) 10th century C.E.

Assuming Milky Way was graphed on a flat sheet of paper with the Galactic Center plotted at X=0,Y=0, and the arm which contains Earth was located at X=0,Y=-δ, and also assuming the galaxy Andromeda can be seen in earth's sky from this position ... why then, when Milky Way rotates around it's galactic center so that the arm which contains Earth is located at X=+δ,Y=0 (a rotation of 90°), or even X=0,Y=+δ (a rotation of 180°) is the galaxy Andromeda still visible in the same location in the sky?

Unless the galaxy Andromeda is in some sort of galactic-synchronous orbit with Milky Way (which seems unlikely since most of the posts referencing Andromeda here deal with the collision of the two galaxies), then, in the in either of the second rotational positions listed above, Andromeda should no longer be visible.

What am I missing? Why does Andromeda, which is not a part of Milky Way, only move in the night sky in relation to Earth's seasons and not in relation to any other galactic movement?

As barrycarter already pointed out, it is due to the huge time it takes for the milky way to rotate.

For once, as he pointed out, the stars in an orbit similar like the sun take about 250 million years for one full revolution around the center of the galaxy. If you put that in a relation to an earth year and take our civilization of roughy 6000 years of keeping records in comparison, we exist roughy 12 minutes of our galactic year. If you only take the last 150 years of the modern era, it's down to 18 seconds.

So if you compare that, how much does Earth move around the sun in 18 seconds? That is about how much Andromeda has apparently moved relative to the constellations in the night sky since we can capture visual images on film.


After what time interval do the closest approaches of Mercury to the Earth repeat? The sidereal period of Mercury's revolution is 88 days and the synodic period — 116d.
my solution, but in the question featured "the greatest rapprochement." And this is no longer so easy. Because the orbit of Mercury is noticeably elongated and ... in turn, it experiences rotation around the Sun. The effect is called "precession of the perihelion of the orbit of Mercury". We must take it into account. The closest approach of Mercury to Earth will be when it is at this point, and it, in turn, is on the Sun-Earth line.

The time of Mercury's revolution around the Sun is 88 days,
Earth around the Sun -365 days,
Let's say we are on Mercury and today there is an Earth opposition,
the next one will be when Mercury returns to its original place
(having made a circle in its orbit) it is 88 days, but these 88 days
the Earth does not stand still, it has shifted in its orbit by 88/365
by about a quarter => Mercury will have to "catch up" with the Earth
for 22 days, but during these 22 days the Earth will still shift in
orbit by 22/365 approximately one sixteenth => another +5 days. Total
88 days + 22 days + 5 days = 115 days
Answer: oppositions of the Earth for an observer with Mercury are
repeated with a frequency of 115 days

I found the answer 7.26 years but how do get it?

@Glorfindel's explanation is very clear. I wondered about how big of an effect "everything else" would have, but I couldn't figure out how to add an image to a comment. Here is a graph from 2018 to 2023 of the distance between Mercury and Earth. You can see every 3rd minimum is a bit deeper than the others just like Glorfindel said. The details of each local minimum change in a somewhat irregular way (about 10% of the closest distance variation), but the broad pattern fits the simple explanation well.

Here is python code to generate such a graph yourself for a different time period. It uses skyfield to access JPL's data on the planetary positions and matplotlib to graph it.
from skyfield.api import load
import numpy as np
import matplotlib.pyplot as plt

planets = load("de421.bsp")
ts = load.timescale()

times = ts.utc(2018,1,np.linspace(0,365*5,10000))

distances = (planets["Mercury"] - planets["Earth"]).at(times).distance()

plt.plot(times.utc_datetime(), distances.km);
plt.title("Distance from Mercury to Earth in km versus time");

plt.savefig("Mercury-Earth-2018-2023.png");
plt.close()


Why is every year the same number of days despite the gravity in the solar system? I was just watching a Geoff Marcy talk on YouTube showing how they infer the presence of planets transiting distant stars. The supposed periodicity wasn't always quite regular from what I could see, and so I wondered if there might be something tugging at the planets as they orbit their star. Then I thought of the earth: Surely there are some years when we are aligned with Jupiter so much so that it would exert some sort of perceptible influence on the planet, thus lengthening the year, even if only by some hours or days, considering that the sun is much more massive. And what would happen every few thousand (?) million (?) years when all the planets and even some of the larger asteroids, Kuiper Belt Objects - everything but the kitchen sink - come into alignment? Wouldn't the earth be drawn outwards to some extent, thus making a year perhaps 370 days long (or whatever)? And maybe temperatures would drop with the planet a fraction of an AU farther from the sun? I know that this is a website for the already enlightened, but I would appreciate an answer that even Josephine Bloggs would understand.

Derived from this paper the interval between March equinoxes for 1989/1990 has been 4 minutes 28 seconds longer than the corresponding interval for 1987/1988.

So the length of a year can vary several minutes, e.g. by gravitational effects of other planets. For calendars a mean year is used. Atomic clocks run far more precise than the more or less periodic motion of Earth.

All planets aligned don't cause anything grave. A similar kind of sysygy has been 1982.


Can we measure comets in a similar way to exoplanets? A 2km comet at 40AU's can dim a sun sized star at 100 light years by 3%, and a 10km comet can cause an eclipse (is that right?) So would 2,3,10 km objects be easy to measure via star dimming if we had enough telescopic data from space?

Can we measure lines of stars that are eclipsed/dimmed in sequence by transiting comets?

Given 1000 frames of a small patch of space, where all the images are recorded into a 3D image stack (a 3D array, a voxel) X,Y and Time as dimensions, then an Intel processor can compare 20 million voxels per second of that collection of images to search for vector information of aligned patterns of star dimming, i.e. searching for vectors of comets.

If that process is repeated for many patches of space, perhaps we can detect many comets, and I don't know the real maths involved, if a satellite of that design would realistically only find 1 comet, or 5000?

It's for a theoretical comet apprehension system which uses 2/3 identical satellites  each comprised 100Tb of SSD and a 20Mflops processors to search for vector information in the images.

Some kind of fairy idea going around in my head, because of Oumuamua and because I work on voxels and I know that image collections from space can be stacked into 3D arrays and be searched for vector patterns at 2million voxels per second on a 200W PC, an idea that I am confusedly astrally-projecting as a basis to find comets.

edit- A quantitative answer would be cool, say if the voids between stars on the milky way arm are 8 times the width of the comet, the comet would on average hit 2 stars every 8x8 frames.

The thing about detecting exoplanets by eclipses is that the eclipses are repeatable.  You have the same star, whose light curve dims in the same way with each eclipse, which gives you information about the planet's orbital period and its apparent size relative to the star.  The Kepler mission observed many fluctuations in stellar brightness that were not associated with exoplanets, but instead with changes in the instrument, fluctuations in the actual brightness of the star,  probably some other explanations that I can't think of off the top of my head, and actual mysteries.

Basically, if you can only observe it once, then you can't claim to explain it.

Occultations of distant stars by planets or comets don't give you a light curve: the star goes from "on" to "off". So a single occultation doesn't give you any information about the size, speed, or direction of the object doing the occulting.  And the occultations are sufficiently rare that I think you'd be extremely unlikely to find a "line" of them by chance --- though that would make a fun data-mining project for an astronomy student.

What you can do, if you have an object that you already know about, is to make many observations of the same occulatation.  That's the mission of the International Occultation Timing Association.  If you click through to their web page you'll see this image:

Each line here shows the observed brightness of a particular star observed from a particular location at the same time.  You can see clearly that some observers saw the star disappear very briefly, some for longer, and some (who might have been predicted to be in the very center of the path) saw no occultation at all.  Assembling all these individual observations, properly synchronized, shows clearly the shadows of the two parts of a binary asteroid, each of order 50 km in diameter.

Note that 90 Antiope is a main-belt asteroid.  The angular diameter decreases linearly as the distance increases, so an object the same physical size orbiting out by Neptune (perhaps fifteen times farther) would need fifteen times the density of observing locations to create an image with the same detail.


UFO or something else? It's 5.50 am now in Dhaka, Bangladesh.  I just saw a moving star around 5 am. It was a starry sky and that star was moving in a straight line. It was very fast, faster than regular planes (though I didn't hear any sound) and disappeared in the Eastern sky.  It was a little brighter and a little closer than the other stars. Can anyone tell, what was it?

According to this (https://spotthestation.nasa.gov/sightings/view.cfm?country=Bangladesh&region=None&city=Dhaka#.WcATdWdSz0o) the ISS was around at that time of morning. The sunrise was 5:45, so it could have been illuminated by the sun before it rose.


Green objects in Hubble Ultra Deep Field I was wondering what are the green objects that you see scattered in the Hubble Ultra Deep Field image (Warning possibly large/slow download : http://imgsrc.hubblesite.org/hu/db/images/hs-2004-07-a-full_jpg.jpg)

A crop of the image reduced in size :

what are the green objects that you see scattered in the Hubble Ultra Deep Field image

The Hubble Ultra Deep Field (HUDF) has approximately 10,000 objects in it. Between 25 and 50 of them are identified as extremely faint stars. That means the remaining ~10,000 objects are galaxies. To answer your question literally then, the green objects are galaxies.

However, I believe what you probably really mean to ask is, why are these galaxies green? To answer that, there are actually a number of points to address.

You seem to be under the belief that the concept of redshifting causes objects to appear red. While this is certainly a true statement, it is not exclusively true. Redshifting simply makes wavelengths longer (and thus shifted red-ward, towards the red end of the spectrum). It is certainly possible that an object which was once blue could be redshifted into the red and thus appear more red than it naturally is. However, an object that emitted most strongly in the UV could redshift into the green and thus appear more green than it naturally is. Some such effect may be going on in this image, but in actuality, I think there's other stuff going on.
You have to account for image processing. It's very difficult to take a "real color" image of a galaxy. Very often (as was the case with the HUDF), images are taken in a few different color bands and combined together with some sort of image processing program. Its up to the discretion of the image processor to balance the colors correctly, get the right saturation, hues, luminosities, etc. There are actually a variety of different forms of the HUDF that include different bands and different forms of image processing where the images that appear green in your image don't appear green in those. An example is shown below of a rendition of the HUDF, taken directly from the same NASA/STScI Hubble website you found your image on. The coloration is significantly different than yours and many of the greenish galaxies from your image appear more blue.

I'm not saying this is the case with the galaxies in the HUDF, but some galaxies actually are green! A few examples include the Pea Galaxy or the Green Bean Galaxy. When you plot up galaxies based on color (note that this is the astronomical definition of color which doesn't necessarily have a 1-1 correspondence with visual color) you get a Color-Magnitude diagram (CMD) wherein most galaxies are clumped into a red portion or a blue portion. However, there are a small group of galaxies in what is known as the Green Valley which do appear green for one reason or another. What actually causes their greenness, I think, is beyond the scope of this question and would be better suited for a new question.


Why are there no green stars? There are red stars, and orange stars, and yellow stars, and blue stars, and they are all understandable save the fact that there is a 'gap': There are no green stars.

Is this because of hydrogen's chemical properties (e.g. the emission spectrum) or some other reason? Or are there just green stars that I have no idea of? If so, I need some pictures; they must look awesome.

Human color vision is based on three types of "cones" in the eye that respond differently to different wavelengths of light. Thus, not counting overall brightness, the human color space has two degrees of freedom. In contrast, the spectra of stars are very close to a black body, which depends only on effective temperature. As one varies the temperature, the color of a star should make a one-dimensional curve in this color space. Thus, unless some perverse shenanigans are going on, it is intuitive that we necessarily miss most of colors, i.e. there will be no stars of those colors.

Our Sun actually has a peak at about $500\,\mathrm{nm}$, which is a green. However, that's just the peak: since the Sun also radiates lots of light with shorter wavelength (bluer) and also longer wavelengths (redder), the resulting mixture doesn't look green to human eyes.

An image from wikipedia on the color of a blackbody of a given temperature:

Note that the colors at near edges of this color space aren't accurate, because the sRGB standard used in computers only covers a fairly small triangle portion of it. Still, that's a complication that's not very important here.


Near-Earth supernova There are 51 stars within 17 light years of the Earth (source). If one of these stars was to become a supernova, how would they effect the Earth?

None of those stars can go supernova, so the question is rather moot.  If you look at the classifications, the most luminous is Sirius A (an A sequence star even) you can get an idea of its mass. If you look at your source page, and link to the explanation you see that A stars range from 1.4 to 2.1 stellar masses. In order to go supernova though, you need about nine solar masses.  The nearest supernova candidate is IK Pegasi (HR 8210), located at a distance of 150 light-years (and that's a Type Ia). The closest Type II candidate that I can think of is Spica, which is 260 light years away. Although this list doesn't include Spica, and has the closest Type II as Betelgeuse, at 640 light years away.

In his book, Death From The Skies, Dr. Phil Plait (a professional astronomer, writer, lecturer, etc.) covers exactly what would happen to earth if a star nearby did go supernova, and it isn't pretty. Basically, it would strip us of our ozone layer and turn the upper layer of our atmosphere into a dirty brown smog layer, let in a lot of UV radiation that would devastate plankton and plant life, and be very bad for all life down on the surface. However, for a Type II, they need to be around 25 light years or closer to affect us.  Dr Plait's writing style is very much geared for a layman reader, and explains things in a very conversational tone, so I highly suggest that you get the book and read it. It will tell you exactly what to expect, and also tell you exactly why it's nothing to worry about.

All that said, there are two things in his book that you may want to consider as more likely than a supernova. A GRB from Eta Carinae, or an asteroid impact. The latter we may actually have the technology to eliminate as a concern though, so we're not doing too badly!


Given that gravity bends the path of light, how confident can we be that galaxies are where they seem to be? The widely known phenomenon termed gravitational lensing is believed to curve space, thereby affecting the path followed by light and other electromagnetic waves.

This effect is associated principally with galaxies and galaxy clusters, as it requires considerable mass. But in theory any star or star cluster, or black hole, can have this effect to some degree.

What can we tell for sure about the position of external galaxies, if we are uncertain that their light is actually coming to us along a straight path: i.e. if light follows the curvature of space, and that curvature is not uniform, how can we be sure we are not observing, for example, the light from a single source appearing to us as more than one object.

Can a galaxy appear to be in more than one place in the sky?

The widely known phenomenon termed gravitational lensing is believed
  to curve space, thereby affecting the path followed by light and other
  electromagnetic waves.

and gravitational waves! In fact, the paths followed by light and other massless particles (for the lack of a better word) are called null geodesics and they have in common that $$\mathrm ds^2 = 0, $$ i.e. the 4-distance they experience is zero.

  This effect is associated principally with galaxies and galaxy
  clusters, as it requires considerable mass. But in theory any star or
  star cluster, or black hole, can have this effect to some degree.

One could argue that the Shapiro delay is of the same nature, only temporal rather than spatial. You can observe the Shapiro delay from the sun using radar probes!

  What can we tell for sure about the position of external galaxies, if
  we are uncertain that their light is actually coming to us along a
  straight path:

It is by definition! The path that light takes is straight, even if it seems bent to us in 3-dimensional space. Therefore, galaxies appear where they are because that's where they are. It's just that we project our idea of Euclidean (i.e. unbent) space onto the sky!

I know what you want to ask though: How can we be sure that the sky isn't curved in some insane way somewhere and we're actually looking somewhere else? We do assume that on the whole, space is Euclidean.

Well, on the small scale this might be the case: We might be looking at stuff whilst it is "really" somewhere else (note that it is really there, the "really somewhere else part" is just us imposing our coordinates on the sky!!).

But we know beyond reasonable doubt that the Universe, as a whole, is more or less Euclidean. At least up to redshift z~1000, i.e. about 300,000 years after the Big Bang. We can observe the Cosmic Microwave Background, and it tells us something about the curvature in the sky. We know that there's structures there which have a certain angular size, as we know that they had 300,000 years to form and we know the speed at which they formed. So we can predict their typical size, and it turns out that the angle under which they appear would be bigger if the universe were oblate/hyperbolic and smaller if the universe were closed. They are just right for a flat universe. So we know that on big scales, space is indeed Euclidean. It might not be the case for individual stars, galaxies, etc. but on average, it works. And cosmology / gravitational lensing usually works with averages and statistics.

  Can a galaxy appear to be in more than one place in the sky?

Totally. There's a plethora of cases of multi image lens-source systems. You can usually sort them out by looking at their spectra and checking their time-variability.


Would it be possible for life found on moon of rogue planet I have heard that there are at least three Basic requirement for life exist in the universe, they are:

Source of Energy
Complex chemistry (including solvent/medium for chemical reaction)
Protection for external threat (e.g. high energy electromagnetic wave, asteroid etc.)

In earth, our major source of energy come from our sun, which provide enough heat to initial chemical reaction in our ocean.

One day, when I was searching the internet, I came upon the ideas of rogue planet, brown dwarf, tidal heating. Which keep me thinking of the possibility of life on the moon of a rogue planet (probability a large gas giant/ brown dwarf).

I have a naive thought on the situation , where the rogue planet radiation and its mass (effect of tidal heating), provide sufficient energy of the moon (hot enough to maintain solvent say water/methane) . And somehow the moon like Titan, which have a dense atmosphere, to protect the surface of the moon.

As I digging deeper, I find some other sources saying that, these kind of situation is not possible because of tidal locking (somehow related to small radius of orbit to keep the moon warm? not quite remember) or the moon atmosphere will be evaporated by the large rogue planet.

So, my question is would the moon of a rogue planet ever have chance like my naive situation exist?(What would be the ideal size of both planet and moon, and their orbital radius?) Or the situation is simply impossible, if so would there be other cases, where the moon of a rogue planet can support life to exist. (may be life within thick ice sheet? or not in the moon but in the planet?)

No one knows for sure, but the idea is plausible. There would not be enough radiation from a rogue planet to provide much help, but we know from the moons of Jupiter and Saturn that tidal heating can keep water liquid. There might have to be multiple moons for it to work. The outer moon keeps making sure the inner moon has an elliptical orbit, and as the inner moon gets farther and closer to the main planet, the tidal forces get weaker and stronger, causing a "squeezing and unsqueezing" that warms the moon.

Of course, at the distance of Jupiter the Sun still provides some warmth, so it would take a lot of tidal heating. But I don't think it's out of the question.


Earth as a grain of sand If Earth was a grain of sand in Buenos Aires

in what point in this observable universe map would be LA be?

The key to this kind of comparisons is to get a ratio. How much bigger is one number than the other?

From Buenos Aires to Los Angeles, there are nearly 10,000km. The size of a grain of sand varies a lot, but let's say it's a millimetre in diameter.

That's 4 orders of magnitude in the numbers themselves, then three orders of magnitude from there being a thousand metres in a kilometre, and three more from there being a thousand millimetres in a metre.

$=10^{10}$

This is how much larger the LA - Buenos Aires distance is than the diameter of that grain of sand. It is also how much larger the distance out in the universe is compared to the diameter of the Earth.

The Earth's diameter is a bit more than ten thousand kilometres, so after multiplication, we will end up with about $\approx 10^{14}km$. A light year is about $10^{13}km$, so we get out to the handful of closest stars.


Do gravitational waves really travel in "nothing"? How does a Gravitational wave create ripple in the fabric of space, if space itself is an empty and it is made-up of nothing. Through out my academic life in school, I was told that space is made-up of nothing.

If so, then in which medium waves travel?

If they do indeed travel in "nothing", how is that possible?

Well, space isn't really nothing.

Empty space (properly spacetime), also called the vacuum, is pretty complicated in modern physics. All of the fields, like the electromagnetic field and the electron/Dirac field, are there whether or not there are any particles (photons or electrons). The particles are vibrations of the fields – or to put it another way, they're vibrations of the vacuum.

Although people tend to say that gravity is different from everything else, it isn't really very different. The vacuum has a "shape", and it can vibrate in various ways. Some of the vibrational modes are gravitational waves. Others correspond to electrons, photons, etc. The details are complicated but the idea is simple.

The vacuum is different from an ordinary wave medium in some ways. Most importantly, it doesn't have a state of motion: you can't detect a "vacuum wind" or "vacuum current" resulting from moving through the vacuum. But if you keep that imporant caveat in mind, then the analogies of a "fabric of spacetime" or of gravitational waves as waves on the surface of the ocean are not so inaccurate.


Struggling to understand the phases of the moon Source

Typically this diagram is how the phases of the moon are explained but the issue is this, namely, the phases seen on the right half are not possible. All the phases should appear on the left half during the night when the moon is typically most visible.

So how on earth are the phases even generated?

The most remarkable phases are waning moon phases because the moon is lit up on the face away from the sun light source.

What am I missing?

I’ve edited the question by adding two additional photos showing the progression of the moon in the night sky over the course of a given time interval and several days, as well as a synodic month which would hopefully indicate based on the position of the moon the sequence of days and based on the height the time of day/night.

Source

Source

Here is a link to the explanation of how she generated her photo and over what time span but you must have a Facebook account to read it. It is easier to follow the Italian than the English but both are available

Explanation

As noted in a comment, you should imagine the picture is looking down on the Earth, and remember that the actual moon is much further from the Earth, relative to their sizes.

Imagine a person standing on the left side of the Earth (in the diagram). The time would be midnight for them. They could see the moon when it is full, and they could see the moon when it is a gibbous moon (but the gibbous moon would be lower in the sky)  They could even see the moon at first quarter, but it would be on the horizon.  You can't see a crescent moon at midnight.

Now imagine a person standing in the top left on the Earth (in the diagram). The time for them would be about 9pm. (and night time) That person could see the half moon and even a crescent moon (but the crescent would be on the horizon)

It is only possible to see a crescent moon at dawn or dusk, you never see a crescent moon at midnight. At 6pm (at dusk) you can see the crescent moon. If you look to the moon, you see mostly the side in shadow, but a little of the light shining around the side, so it looks like a crescent.

Now you are also confused about the illumination side shown in the waning phases, This is because the picture shows the view of the moon that a person would see if they were standing on the Earth. Because that person is upside down (from our point of view) they see the moon reversed. In other words, from our position above the North Pole, we see the right side of the moon illuminated.  From their position, it is the left.

If the diagram still confuses, discard it and think in terms of angles.

From the perspective of the Earth, the sun and moon go around in circles at different rates, so the angle made between the sun and the moon changes. When the angle between the sun and the moon is about 90 degrees, then half of the near side of the moon is lit by then sun, and a half moon is visible.

When the angle is close to 180 degrees, then nearly all the moon is lit by the sun, and the moon is full.

When the angle is close to 0 degrees, then none of the near side will be lit, and the moon won't be visible.

When the angle is small, ie the moon is close to the sun, then a crescent moon will be visible. A crescent moon can only be seen if it is near to the sun, so the sun cannot be far below the horizon. This means that the a crescent moon cannot be seen late at night when the sun is far below the horizon, but at some times and locations the sun might not set until late in the evening, so the setting time of the moon might be quite late.

Nevertheless it is the angle between the sun and the moon that determines the phase of the moon, and nothing else.