What other hints of possible life in Venusian atmosphere have we dismissed? Now that phosphine has probably been discovered in relatively large quantities in the Venusian atmosphere, I heard that there have been other unexplained phenomena (i.e. dark patches) seen. Does anyone here know of reliable sources with such information?

Now that phosphine has probably been discovered in relatively large
quantities in the Venusian atmosphere

It is not large, it's only 20 ppb.

I heard that there have been other unexplained phenomena (i.e. dark
patches) seen.

Yes, dark patches has been observed on Venus from quite a time. Scientist aren't able to figure it out and are speculating to be clouds that are absorbing UV light thus making it look dark. Some suggest uneven mixing of clouds creating dark and light spots or some unknown chemicals present or even a sign of microbial growth.
These "dark patches" are later named "unknown absorbers" because they absorb large amounts of solar radiation.

What makes the patches dark? Why is it dark?

The dark patches are made up of concentrated sulfuric acid and some unknown chemicals that is absorbing the UV radiations making it appear dark. Scientists have claimed the chemicals to be some forms of sulfur, disulfur dioxide or ferric chloride.
These dark patches seems to affect Venus's climate by affecting its albedo. The clouds absorb solar radiation, which causes temperature changes that affect wind patterns and this affects albedo (how much energy is reflected back to space). If we talk about numbers then, Venus’ albedo had been observed to diminish by about half between 2006 and 2017, before then returning to normal. This affected the upper atmosphere, including variations in the “super-rotation,” which is driven by winds exceeding 200 miles per hour (322 kph).

It is also further noted that around 30 miles (48 km) above the planet's surface is a lower cloud layer with temperatures around 60° C (140° F) and pressures similar to that of Earth and it is hypothesized that there is some sort of microbial life or algal bloom that is absorbing light and radiation and causing the dark color.
References

Sanjay S. Limaye, Rakesh Mogul, David J. Smith, Arif H. Ansari, Grzegorz P. Słowik, and Parag Vaishampayan, Astrobiology, Sep 2018, 1181-1198. DOI: 10.1089/ast.2017.1783
Lee et.al. Long-term Variations of Venus's 365 nm Albedo Observed by Venus Express, Akatsuki, MESSENGER, and the Hubble Space Telescope, The Astronomical Journal, Volume 158, Number 3, 26 August 2019. DOI: 10.3847/1538-3881/ab3120


Does the sun have a protective shield Does the sun have a protective layer that at least diminishes its radiation. I know the earth's atmosphere protects us from harmful rays. But does the sun have a sort of shield as well that if removed would radiate more heat.

You seriously cannot expect the sun to have a layer that would contain some of its harmful radiation. Sun consists of a plasma and is not solid. Learn more about its structure here. Magnetic field is a kind of layer you might think of apart from all gases and energy it radiates. But that is harmful and not protective for us. It is highly unlike earth which has a solid crust. Any layer around sun wouldn't actually protect us as it would have to contain a huge amount of energy and that would make it more unstable.

Earth just happens to be at the right distance from the sun that was conducive enough to harbor a suitable atmosphere with just the right combination of the right elements and compounds (especially oxygen for breathing, ozone for protection and nitrogen for maintaining atmospheric balance). This is why life has been possible here.
Earth also happens to have a magnetic field strong enough to protect us from the solar wind, as described in details here. It just diverts the harmful energy away from us. Containment of huge amount of energy is the major concern here and nobody is doing that.

So life has been possible only because the way earth has been created, well of course all energy comes from the sun. But the sun just provides energy and doesn't save us from itself.


What is the force that changes a satellite’s inclination when orbiting a planet with an equatorial bulge I tried looking this up without success and I imagine there's a formula for it, with some ridiculous numbers like it drops off to the 5th power of the distance or maybe even more.

Imagine a planet like Jupiter, large equatorial bulge and imagine an artificial satellite orbiting in a polar orbit, not an equatorial orbit.    Jupiter's enormous equatorial bulge should disrupt the satellite orbit and in time (perhaps a very long time), turn it into an equatorial orbit.

Is that correct or incorrect thinking on my part and is there a formula that applies?    I imagine that some kind of measure for equatorial bulge would be needed as well and I'm not sure how that would be measured in a gravitational field from orbit.

I can explain it for rings.

The phenomenon you're looking for is called nodal precession. Meaning, when the planet has an equatorial bulge, all non-equatorial orbits will precess, will wobble around the spin axis.

https://en.wikipedia.org/wiki/Nodal_precession

  Because of the bulge the gravitational force on the satellite is not
  directly toward the center of the central body, but is offset toward
  the equator. Whichever hemisphere the satellite is in it is
  preferentially pulled slightly toward the equator. This creates a
  torque on the orbit. This torque does not reduce the inclination;
  rather, it causes a torque-induced gyroscopic precession, which causes
  the orbital nodes to drift with time.

For rings, any such precessing orbits would be unstable - ring particles would collide with each other. The only non-precessing, non-colliding orbits are the equatorial ones. So that's where the rings persist.

As for satellites, I would venture a guess that their orbits actually do not migrate towards the equatorial plane, they stay where they are, just slowly precessing. Someone please correct me if your orbital mechanics is in better shape than mine.

EDIT: There's a ton of perturbations from an oblate planet, and some do apply to the inclination of the orbit. So it's not just precession. Some terms are periodic. Google "inclination of orbit oblate" without the quotes. It's... complex. I'll edit this answer tomorrow if I find something relevant.

EDIT2: This is far more complex than I thought. A thorough treatment is given in Kozai, Y. The motion of a close earth satellite:

http://adsabs.harvard.edu/abs/1959AJ.....64..367K

Chapter 11.15 in J.M.A. Danby, Fundamentals Of Celestial Mechanics, is entirely dedicated to this topic. He talks about critical values for initial parameters where his results are not applicable, so obviously there isn't a simple conclusion that applies in all cases.

I am not going to interpret the math, sorry. That's all I had to say on this topic.


If two binary brown dwarfs began to exchange gas with each other, would they both ignite into stars? If two brown dwarfs got close enough to exchange gas with each other, could it trigger nuclear fusion within their cores? Is it possible for two brown dwarfs to orbit so closely and become a rocheworld in the first place?

It is possible for two brown dwarfs to form a contact binary. This would not, of itself, cause core temperatures in either to rise.

If the two brown dwarfs were to merge, the mass of the resulting body could be enough to generate sufficient heat and pressure in the core for nuclear reactions to start. If both brown dwarfs had a mass of about 60 Jupiters, the combined size of 120 Jupiters would certainly be big enough to form a red dwarf.


Problem with Tychonian model of geocentric universe? Just like other proposed models of universe in history, Since Tycho's aren't being used, there MUST be any flaws in compare with widely agreed today understanding
Is there any detailed reference criticizing his concept? Moreover in recent years? I've been searching for awhile by now but couldn't find one
If universe is both isotropic & homogeneous, shouldn't Tycho model be just fine for now? We simply take current model, but using earth as perspective. The rest still maintain the same

Or no?
edit: I think i missed Tycho's whole idea. I thought all the time he was referring to rotating earth, but after @James answer and some reading it doesn't seems so. My central question is on the assumption that his system have rotating earth. If it does, what likely be the problem?

Tycho was concerned that the Earth must be too "heavy" and "sluggish" to move.  So his system solves this problem. It turns out not to be a problem at all.  The problem it solves is "we know that the Earth can't move, so how can we describe the motions of the planets".  As the Earth can move, this isn't a problem. So the whole purpose of Tycho's system is gone.
It is a kinematic, not a dynamic model.  It describes the motion of the planets but does not give any reason for that motion.  This is also the case for Kepler's and Copernicus model.  But Newton's model, on the other hand, describes planetary motion in terms of forces.
Now you can do Newtonian mechanics in any coordinate system.  It is just more convenient to do it in an inertial system.  When you do Newtonian mechanics in a barycentre based inertial frame you derive Kepler's orbits.  But if you do Newton's mechanics in an Earth based non-inertial coordinate system, you get Tycho's model. (Or something very like it, but with elliptical orbits)
There are other problems:

It treats the Earth as "special". This treatment is unjustified. So it isn't isotropic and homgenous:  The Earth is still, everything else moves about it.
It requires the stars to orbit the Earth once a day, this would require them to move faster than light.
It does not predict things like Foucault pendulum, which demonstrates a rotating Earth.
It does not predict stellar parallax.

If you start from the axiom "The Earth cannot move" then you must get Tycho's system, or something like it.  But if that axiom is rejected then you can get a much simpler model with an inertial coordinate system in which the sun is almost stationary.
Some of these can be "fixed", for example by allowing for a rotating central Earth.  But remember the whole point of Tycho's model is to have a stationary Earth.  If you can have a rotating Earth, then why not a moving Earth too?


Increase in Luminosity of the star The Luminosity of a star is proportional to the 4 th power of its temperature and square of its radius. Also the mass-luminosity relationship says that the luminosity of star is proportional to approximately M^3.5.
Consider a star in main sequence. It is in hydrostatic equilibrium so there is no collapse or expansion. Thus the Radius does not change much. Also the star is thermal equilibrium so the energy it generates equals the energy it radiates away. Thus the temperature must not change significantly.
Moreover the star is losing mass as energy due to fusion. Now if we consider the mass luminosity relationship here, then why is there a slight increase in luminosity in star during its stay in the main sequence. If it's losing mass then it should lose its luminosity as well, right?
Please tell me where I am going wrong in my reasoning.

The luminosity mass relation is inexact. The luminosity also depends on the composition of the star, particularly in and around the nuclear burning regions.
The composition changes during the main sequence lifetime as hydrogen gets turned into helium. The average mass per particle goes up and the number of electrons per mass unit goes down.
The former means the temperature increases to maintain a similar pressure, thus increasing the fusion rate. The latter decreases the opacity of the gas leading to a shallower temperature gradient and so a bigger star. A star like the Sun gets about twice as luminous over the course of its main sequence life.
The loss of mass due to radiation from the star's surface is negligible for the Sun, but more important in higher mass stars.
For the Sun, the mass loss rate due to this process is about $6\times 10^{-14}$ of its mass per year, so only about 0.6% of its mass is lost in this way over its main sequence life. The mass lost via the solar wind is probably a bit more important but the total is insufficient to counteract the compositional change effect. The radiation mass loss rate goes up as $M^{3.5}$ for higher mass stars, but of course their lives are also shorter by $M^{-2.5}$. The net effect is that the fraction of mass lost is proportional to the mass, so about 6% for a ten solar mass star. On its own this would only reduce the luminosity by about 20%; the compositional change effect is much stronger.


Name and language of symbol in astronomy book I've been working on a programming project involving Peter Duffett-Smith's Practical Astronomy With Your Calculator (3rd Ed.), which has a large number of mathematical/astronomical formulas in it. There's a part at the end of the book for symbols and abbreviations, which contains this:

After doing some Google-fu, I was unable to find the name of the symbol. Would anyone smarter than me (which should be quite a few of you... heck, quite a lot) know what this is? Hopefully there's Unicode for it...

This is a variant of pi, or "pomega".
In LaTeX, you can get it by using
                                          \varpi

$$\varpi$$
TeX has a question on the 'var' prefix, and Wikibooks has something relevant.

The various forms of pi are present in Unicode as:

U+03A0 Π greek capital letter pi (HTML Π · Π)
U+03C0 π greek small letter pi (HTML π · π)
U+03D6 ϖ greek pi symbol (HTML ϖ · ϖ)
U+220F ∏ n-ary product (HTML ∏ · ∏)
U+1D28 ᴨ greek letter small capital pi (HTML ᴨ)
U+1D70B  mathematical italic small pi (HTML )
U+1D6D1  mathematical bold small pi (HTML ).

It is used to denote the longitude of the periapsis, as in your case.


What is the density of the solar system per cubic light second outside the Jupiter ecliptic? It would seem logical that there is some amount of material falling from the Oort cloud toward the Sun and other mass concentrations in the Jupiter Ecliptic disc defined by Jupiter's gravitational field.
Is there any research to determine the distribution of this material related to the the Massive objects in the the system ?

I assume you want density expressed in units of average mass in kg per cubic light second.
An ecliptic plane is infinitely thin, so we might assume this is irrelevant to the question.  Perhaps you meant "not including matter within a Jupiter radii of the Jupiter ecliptic," but this isn't very clear from the question.
The Solar System is defined as all objects gravitationally bound to the Sun, so it doesn't have a volume per se.  For example, when Oumuamua cruised by the Sun, it was never part of the Solar System because it always had greater than escape velocity.  But the interplanetary medium is not a perfect vacuum, but is rather about 5 particles per cubic centimeter. Even the interstellar medium isn't quite a vacuum.
If we assume the edge of the Solar System is at the edge of the Oort cloud at 50,000 AU, which is given as an answer to Where does the Solar System end? Then the volume of the Solar System is $4/3\pi r^3$ or $5.24\times 10^{14}$ cubic AU, or $6.5\times 10^{22}$ cubic light seconds.  The mass of the Solar System is well approximated by the mass of the Sun, or about $2\times 10^{30}$ kg.  So the Solar System average density is about $3.07\times 10^{7}$ kg per cubic light second.


Is Jupiter a failed star? The elemental make-up of Jupiter is about entirely hydrogen and helium, along with a very small fraction of the atmosphere being made up of compounds such as ammonia, sulfur, methane, and water vapor. These elements are dominant in stars, so how likely is it that Jupiter was a companion to our sun, but failed to ignite?

No.

Besides the 13 Jupiter-masses required to ignite deuterium burning, and make Jupiter into a Brown Dwarf, there is a clear difference between the formation pathways of Brown Dwarves and Gas Giants.
Gas Giants are planets, that form via processes in their parent protoplanetary disc. Contrasting this, Brown Dwarves form via direct fragmentaion of the parent giant molecular cloud, possibly as binary fragments of a more massive companion star. The distinction between those two processes is supported by the finding and characterisation of the Brown Dwarf desert in i.e. Grether & Lineweaver (2006).

The 'Brown Dwarf' desert is a sharp break in the number distributions between low-mass stars and high-mass planets. This is commonly interpreted as those two classes of object being distinct in origin. Furthermore we understand distinct theoretical pathways to form each of those object classes, as mentioned above.

Note that if you're talking about the other elements, or as astronomers would call them, metals, then there is data on those as well: The sun, and any star possesses a certain amount of metals, see i.e. Asplund et al. (2009). For example the ratio of Oxygen to Hydrogen in the solar atmosphere is about $n(O)/n(H)\sim 5\times 10^{-4}$, which is the most abundant 'other' element in the sun.
The values of heavier elements found in the gaseous planets are elevated compared to the solar values. This is thought to represent a contribution of solid material that planets need to form according to current theories. If the giant planets would have formed through direct collapse from the same cloud material as the sun did, the abundances of metals in the giant planets would not be elevated compared to the solar values.


Milky Way stellar number density : is the stated equation in this paper incorrect? The paper is :
http://www.astro.washington.edu/users/ivezic/Publications/tomographyI.pdf

The equation is equation #23 in the paper. It's a model for the density of stars in the Milky Way's disk. It has an exponential dependence on both $R$ and $Z$. $R$ is the distance from the center of the galaxy, and $Z$ is the distance above/below the plane of the disk.

The relation is roughly this:

$$\varrho=\text{constant}\times e^{-\frac{R}{L}-\frac{Z+Z_0}{H}}$$

The problem I'm seeing is the $Z$-dependence of the formula. $R$ and $Z$ here are standard cylindrical coordinates. Thus, $Z$ can be positive or negative. The problem is that the formula blows up when $Z$ is negative. This is unphysical, since the number density must generally decrease going further away from the disk.

Am I missing something? Or should the equation really have an absolute magnitude of $|Z + Z_0|$?

[Added Later:]
I finished making a 3D demo of the stellar number density of the Milky Way. Note that your browser needs to support WebGL.

It may make more sense to convert the exponential into the product of the individual exponential terms:
$$
\exp(-\frac{R}{L}) \exp(-\frac{Z}{H}) \exp(\frac{Z_{\odot}}{H})
$$
This makes it clearer that the $Z_{\odot}$ term is a constant, which is just part of the overall normalization (similar to the $\exp(R_{\odot}/L)$ term earlier in the original equation).

And, yes, $Z$ is assumed to be always positive, so $|Z|$ would be slightly more correct.


What is the acceleration of the stars' speed in the Universe? Positive or negative? I have read quite different opinions about that, so can someone tell me what is the sign of the acceleration of the stars in the Universe if we accept that the center of the Universe is the beginning of the coordinate system? At least what is considered as correct finally :)

This is the classic misconception about the expansion of the Universe: there is no center of the Universe. The whole universe expands as one, not from a single point nowhere.

Knowing this it makes no sense speaking of the direction of an object due to the Universe. You could compare the directions of two objects in it though.

Therefore star's direction depend on where and how they where created and also the interaction with other stars and their home galaxy.


What is the relative time difference between us and a star system in outer layer of our galaxy? Just curious to know what will be the time difference between someone living on the outer layer of our galaxy and us, considering the known facts that

Time is relative to gravity
Our sun goes around at 483,000 miles/hour(792,000 km/hr) to complete its galactic year
Star systems at outer layer travel faster than ours due to Dark Matter and Dark Energy - couldn't find speed of those star systems though!

We can assume that both (Solar system and outer layer of star system) are on the same side of the galaxy.

EDIT:
In order to re-phrase this question more clearly, as per Joan.bdm, it is related to time dilation - what is the aging rate difference if we left one of two twins on the Earth and the other one on a planet at outskirts of the Milky Way.
Conversely, what is aging rate difference between closer to center of our galaxy and us?

Your third question has a very well known answer here:

What means, that although a big time difference had to exist, there is a much smaller one, and in the opposite direction. This is considered the effect of the dark matter.

Our Sun is around 30000 ly from the galactic center, the edge of the galaxy is around 50000 ly, so we can read on the diagram, that the actual speed difference is around 15 km/s, which is 1/20000 of the speed of light. This causes a (not really big) time dilation due to the special relativity.

There is another source of the time dilation, which is caused by the gravity of the galaxy, which differs in the case of us and in the case of somebody living on the outer edge. On such weak gravitation (i.e. for from any black hole, neutron star, etc) it practically depends on the newtonian gravitational acceleration.

But this acceleration is very small - it takes a half galactic year (some hundred millions of year) to revert the orbital speed of the Sun around the galactic core! So, we can consider that actually negligible.

Thus only the special relativistic effect remains. 15 km/s is around 1/20000 of the speed of the light. In cases of speed which are much smaller as the light speed, we can use the speed dilation formula 1/(2*(v/c)^2). Substituting 1/20000 into this formula, we get 1:800 000 000 .

For laymans, we can say, it takes around 25 years for the clocks of this people living on the outer edge of our galaxy to dilate 1 second.


What does this information about "upper limit column densities" and "upper limit relative abundances" mean? I was reading the abstract of a paper regarding attempts at detection of a molecular precursor to the amino acid glycine, and I came across concepts (and notation) that I found confusing. If someone could describe in layman's terms (I've taken an astrophysics class so I'm not totally unaware) that would be great.

In the question I refer to the following section of a paper's abstract:

  "we derive upper limit column densities of NH$_2$OH of $\leq 1.4 \times 10^{13} \mathrm{cm}^{-2}$
  and $\leq 1.5 \times 10^{13} \mathrm{cm}^{-2}$ toward the B1 and B2 shocks, respectively, and
  upper limit relative abundances of $N_\mathrm{NH_2OH}/N_\mathrm{H_2}$
  $\leq 1.4 \times 10^{-8}$ and $\leq 1.5 \times 10^{-8}$, respectively."

The paper in question is called CSO and CARMA Observations of L1157. I. A Deep Search for Hydroxylamine (NH$_2$OH), and located here.

A column density is just the integral of a (number) density along the line of sight.

Often, you would get the same observational absorption or emission signature from a number density of $n$ molecules cm$^{-3}$, spread along a line of sight through a cloud of diameter $x$, as you would with a density of $0.1n$ spread over $10x$.

In other words, all we can infer is the product or, more accurately, the integral of $n\ dx$, and this is known as the column density.

In the paper you refer to, I doubt they know the thickness of the shock they are investigating, so a column density is all that can be determined.

The relative abundance is more straighforward; it is just the ratio of the number densities of the two labelled species.


Does the dust around SMBH's protect habitable planets from jets? The supermassive black hole in the Milky Way is covered by dust as seen from here. Is it common that SMBH's are covered in dust in their galactic disks? Would that dust absorb and disperse a jet from an active galactic nucleus, so that the atmospheres of Earth like planets would be protected?

AGN jets build bubbles perpendicular to the galactic plane, but the jet could be oriented in any direction, right? The bubbles are shaped by general stellar winds from the disk, redirecting the jet streamed material towards the galactic poles, if I get that rigt.

Most galaxies have enough dust to hide their cores from investigation in visible light. But the jet from an AGN is composed of large amounts of relativistic particles, and is easily powerful enough to clear any dust out of its path. We see in M87 a jet punch 5000 light years from an active nucleus.

The dust that hides Sagittarius A* is not localised around the black hole, it is spread through the bulge, bar and disk. You would not want to be anywhere near an active galactic nucleus. Large amounts of radiation are produced by the accretion disc surrounding the black hole. Distance and dust may provide some protection to a star orbiting an active galaxy at the distance of the sun from Sag A*, but not if the star is directly in the jet.


Why is there free magnesium in the ecliptic (if there is) and why would some be orbiting within the ecliptic plane but retrograde? This answer to the question What direct or indirect observations of dust can one make by eye or with binoculars? mentions Brian May's thesis and it's connection to Zodical Light:

Brian May (CBE):

In 2007, Brian May, lead guitarist with the band Queen, completed his thesis, A Survey of Radial Velocities in the Zodiacal Dust Cloud, thirty-six years after abandoning it to pursue a career in music. He was able to submit it only because of the minimal amount of research on the topic undertaken during the intervening years. May describes the subject as being one that became "trendy" again in the 2000s.

Brian May's Thesis: A survey of radial velocities in the zodiacal dust cloud
And for good measure, Brian May's announcement of IAU's naming of the asteroid 17473 Freddiemercury YouTube: https://youtu.be/o2vo6VR51eA

I believe that MgI stands for neutral, atomic magnesium, and since this is optical (not X-rays) this would have to be free atoms, or perhaps molecular, but not bound magnesium in compounds or metals.
Question: Why is there free magnesium in the ecliptic (if there is) and why would some be orbiting within the ecliptic plane but retrograde?

Here is the abstract of the thesis in full:

This thesis documents the building of a pressure-scanned Fabry-Perot Spectrometer, equipped with a photomultiplier and pulse-counting electronics, and its deployment at the Observatorio del Teide at Izaña in Tenerife, at an altitude of 7,700 feet (2567 m), for the purpose of recording high-resolution spectra of the Zodiacal Light. The aim was to achieve the first systematic mapping of the MgI absorption line in the Night Sky, as a function of position in heliocentric coordinates, covering especially the plane of the ecliptic, for a wide variety of elongations from the Sun. More than 250 scans of both morning and evening Zodiacal Light were obtained, in two observing periods – September-October 1971, and April 1972. The scans, as expected, showed profiles modified by components variously Doppler-shifted with respect to the unshifted shape seen in daylight. Unexpectedly, MgI emission was also discovered. These observations covered for the first time a span of elongations from 25º East, through 180º (the Gegenschein), to 27º West, and recorded average shifts of up to six tenths of an angstrom, corresponding to a maximum radial velocity relative to the Earth of about 40 km/s. The set of spectra obtained is in this thesis compared with predictions made from a number of different models of a dust cloud, assuming various distributions of dust density as a function of position and particle size, and differing assumptions about their speed and direction. The observations fit predominantly prograde models fairly well, but show a morning-evening asymmetry, different in the two observing periods. Models are investigated containing various components, including prograde and retrograde orbiting dust around the Sun, a drift of interstellar material though the Solar System, and light from distant emitting matter. The implications for possible asymmetries of the Zodiacal Cloud are discussed. Other researches on the Zodiacal Dust Cloud, before, during, and after my observations, are reviewed, including recent insights into its structure, orientation, and evolution, up to the present day, and my observations are evaluated in this context. Period of study, 1970-2007.

Source

Mg I does indeed mean neutral magnesium atoms, but May and his collaborators weren't observing magnesium atoms orbiting in the inner Solar System -- they were seeing absorption by magnesium atoms in the Sun's atmosphere.

In the optical and near-infrared, the zodiacal light is sunlight scattered/reflected by interplanetary dust particles. Not surprisingly, the spectrum is largely the same as that seen coming directly from the Sun, complete with all the Fraunhofer absorption lines. Their approach was to focus on a narrow region of the solar spectrum containing a strong Mg I absorption line, and measure how the the dust particles redshifted or blueshifted the reflected solar spectrum, since that would tell them the velocities of the dust particles relative to the Earth.

(They also sometimes detected Mg emission partly filling in the absorption line; since this was always at the rest wavelength of that transition and thus not Doppler shifted, they deduced that it was probably emission from Mg atoms in the Earth's upper atmosphere, likely deposited by vaporized meteoroids.)

As for possible retrograde motions: the zodiacal dust is thought to come from three main sources: occasional collisions between asteroids, sublimation from comets passing through the inner Solar System, and a possible contribution from interstellar dust grains as the Sun moves relative to the local interstellar medium. The second source -- comets -- can give you retrograde orbits because many long-period comets (coming originally from the Oort Cloud) have retrograde orbits.


How many planetary systems exist in our galaxy? For many years scientists have studied our own solar system and modern technology allows them to look deeper and deeper into space. Knowing the Sun is only one of a few billion stars in the Milky Way galaxy, I can imagine scientists also started studying other planetary systems… at least enough to know how many planetary systems (like our solar system) reside in our galaxy.

Is there any information available that would be able to (at least "indicate") how many planetary systems reside in our galaxy?

A good, but hard question to answer - as you stated, only an indication could be provided.  But, there is some information available from the number of exoplanetary systems found 'locally', according to the NASA web page How many solar systems are in our galaxy?:

  So far, astronomers have found more than 500 solar systems and are discovering new ones every year. Given how many they have found in our own neighborhood of the Milky Way galaxy, scientists estimate that there may be tens of billions of solar systems in our galaxy, perhaps even as many as 100 billion.

Another estimate from the article 160 Billion Alien Planets May Exist in Our Milky Way Galaxy (Hall, 2012), particularly with their quote from a scientist:

  "This statistical study tells us that planets around stars are the rule, rather than the exception," said study lead author Arnaud Cassan of the Paris Institute of Astrophysics. "From now on, we should see our galaxy populated not only with billions of bright stars, but imagine them surrounded by as many hidden extrasolar worlds."

An important caveat from the NASA site is that exoplanetary observations are still in its infancy - so over time, this estimate is likely to improve.


Get visible galactic longitudes in astropy I would like to know which part of the Milky Way is currently up. Preferably using astropy. The ingredients I have are the current time, and my current position on earth. How would I use these to get the minimal and maximal galactic longitude currently up?

(I know how to convert from Alt-Az to Galactic coordinates, but I don't know at which azimuth the galactic plane crosses the horizon at altitude=0.)

The following works for me; it uses the rootfinder from scipy to find the zero.

from astropy.coordinates import EarthLocation, AltAz, Galactic
from astropy.time import Time
from scipy.optimize import newton
import astropy.units as u

dwl = EarthLocation(lat=6.39688*u.deg, lon=52.6275*u.deg)
mytime = Time('2017-09-16T21:00:00')

def altitude_of_longitude(longitude):
    ''' Give the altitude (in deg) of a galactic longitude (in deg) '''
    return Galactic(l=longitude*u.deg, b=0*u.deg).transform_to(
                    AltAz(location=dwl, obstime=mytime)).alt.value

Now use the rootfinder from scipy, with two different initial guesses, hoping that they end up at the two different solutions where the galactic plane crosses the horizon. This gives:

newton(altitude_of_longitude, 0)
>>> 16.231753986033333
newton(altitude_of_longitude, 180)
>>> 196.23547201834657

The only remaining part is to check which part is visible:

altaz = Galactic(l=0*u.deg, b=0*u.deg).transform_to(AltAz(location=dwl, obstime=mytime))
altaz.alt
>>> −9°08′51.8623′′

Apparently the longitude at 0° is under (negative altitude), so the visible part is between 16° and 196° galactic longitude.


Can you see both Jupiter and Saturn (and their moons) in detail during their upcoming conjunction? In December of this year, Jupiter and Saturn are going to be in a conjunction. Are they going to be close enough that you could resolve both as more than just dots and see their respective satellites in the same FOV with a telescope?

At their closest on the 21th of December (in the UK), they will be very close, only about 6 arc minutes.  That is in the field of view of most telescopes (about 1/5 of the diameter of the moon.
It will look like this (simulated)


Did I see another planet? A couple days ago, I zoomed in with my 30x optical zoom camera, and after some exposure adjustments, a bright star in the night sky turned into this:

Are those other planets or other stars?  Or is that a lens effect?

EDIT: The bright object in question was ~60 degrees above the horizon, and ESE of me (East-south-east). I took the picture on 3-25-16 from Madison, Wisconsin.

EDIT: Question answered, more clear picture added FYI.

You don't say what time you were looking. Here is a screenshot from Stellarium at 10pm Wisconsin time on 25th March 2016. Jupiter is in the ESE, but the altitude is a bit lower than 60 degrees. Seems fairly conclusive. You were seeing Ganymede and a Europa/Io combination.


what is the net effect of nodal and apsidal lunar precession? In apsidal lunar precession, the moon's elliptic major axis precesses eastward and completes one revolution eastward in 8.85 years.  Because the orbit is inclined relative to the ecliptic, basic geometry dictates that the ascending and descending nodes must also rotate about the ecliptic at the same rate (all else constant.)  But then there's nodal precession, rotating the nodes in the opposite (west) direction, one revolution every 18.6 years.  So what is the net effect of these two precessions on the positions of the nodes? Subtract the two?

(Hope I’m not too late, two and a half years later… Just saw this post now—and I wasn’t a member of Astronomy SE back then…)
The short answer is no. These two effects don’t occur in the same plane. Imagine the lunar orbit as an elongated ellipse—much more elongated than it is in real life. Imagine the Earth as one of the foci, closer to one tip than to the other. Imagine the plane of the ecliptic as, if you wish, parallel to your floor or your desk. Now, the Moon’s orbit is tilted—by a small amount, about 5.14°—to the ecliptic (NOT to the Earth’s equatorial plane, like other planetary satellites, but that’s a different story). So imagine your first ellipse tilted with respect to your floor or desk.
The first movement, the precession of the perigee, makes it so the ellipse pivots in is own plane around the Earth. So its plane doesn’t change at that point.
The nodal precession, though, changes the orientation of the plane. The movement is still around the Earth, but parallel to the plane of the floor or the desk.
The closest everyday parallel I can find is if you own a ventilator (fan) that pivots on itself to aerate in different directions. The movement of the fan blades would represent the apsidal precession (the first you mention), while the swiveling of the fan head left and right would represent the nodal precession.
Hope this helps!


Is there enough light to grow plants in the ocean of Europa? What is the underwater temperature of Europa?

I know Jupiter emits infrared light but does it emit any light able to support plant life of any type on near by Moons? Can under water volcanic activity of Europa produce enough light to support plant life?

The surface of Europa gets some sunlight direct from the sun and some long wave radiation from Jupiter so it's not in darkness.  It's also in Jupiter's radiation belt so it gets hit with high energy charged particles and the surface of Europa, despite being bombarded by particles is still a vacuum.  The surface of Europa is a terrible place to look for life, with the exception being that the liquid ocean of Europa explodes onto it's surface regularly, so evidence of life, if it exists, may well be found on the surface, but it's unlikely that anything can live there for long.  It's very inhospitable.

The inside of Europa is more interesting.  That's why Neil deGrass Tyson says he wants to go "ice fishing on Europa" to see what "licks the camera" and if we find life there we'll "Call them Europeans" . . . funny comments aside, the inside of Europa is probably dark, outside of some infra-red rays due to temperature.   Photosynthesis doesn't work well in darkness and with just long wave light, though maybe it's possible, it wouldn't nearly as much growth due to far lower energy levels.

As noted in the answer to your other question, if there is life in Europa's ocean, it's more likely to be something like chemotrophs.

What makes Europa interesting is the energy it gets from tidal forces being stretched between Jupiter and Ganymede, so it gets a regular influx of heat and perhaps internal volcanism, so it may have undersea vents similar to our oceans.   The energy influx and possibility of underwater volcanoes makes it more interesting than most other moons or dwarf planets.

If life is found in Europa's oceans, even if it's very primitive life by most standards, it would still be a very cool discovery because it could tell us something about how life starts and that's a big unknown right now.  The discovery of life outside the Earth would be a very big story.

We'll probably have to wait till 2030 before we begin to get some answers.  Sorry for the long wait, but it is what it is.


Does the Earth orbiting around the Earth-Moon barycenter cause a measurable centrifugal force? I just realized the Earth is not a stationary object with the moon orbiting around it.  As shown in this minimalist animation from Wikipedia, the Earth actually orbits a common barycenter with the moon.  If this was a carnival ride, people sitting in the moon and on the far side of the earth would certainly feel the centrifugal force acting on them.

However, this is a very large system in reality, and the Earth also has quite a strong gravitational force felt on the surface anyway.  So when it comes to accurately measuring gravity or doing experiments that rely on gravity, how big of a factor is this centrifugal force?  Does the apearant force of gravity fluctuate from 9.9 m/s^2 to 9.7 or is it more along the lines of 9.800001 to 9.799999 (assuming the average is exactly 9.8, which is a simplification).  Or is there something I'm missing that means the force is non-existent?

That the Earth and Moon orbit about their center of mass from the perspective of an inertial frame of reference is a bit irrelevant. One thing that is quite relevant is that gravitational force is undetectable by a local measuring device. For example, people standing still on the surface of the Earth do not feel gravity. They instead feel the normal force pushing them up, away from the center of the Earth. The gravitational force on astronauts in the International Space Station is about 90% of what they experience on the Earth's surface, but they feel none of that.

Another relevant factor is that the Earth as a whole, along with objects on the surface of the Earth, accelerate toward the Moon (and the Sun, and Jupiter, and Venus, and ...) gravitationally. The gravitational acceleration of those surface-bound objects toward those other bodies is not exactly the same as is that of the Earth as a whole.

The difference between these accelerations results in a force that can be measured. This is the tidal acceleration. An extremely sensitive scale will show that you weigh slightly more when the Moon is on the horizon than when it is directly overhead. For a 61 kg person, this difference in weight between the Moon being on the horizon vs directly overhead is about 10-4 newtons.

Compared to the ~600 newton weight of that 61 kg person, this is a very small effect. This very small effect, along with an even smaller effect from the Sun (roughly half), are however responsible for the tides in the oceans.


What exactly is 'eastward movement' I am reading a book on spherical astronomy. Surprisingly, my problem is not with the mathematics. The author mentions several times words like 'eastward movement' and 'westward movement'. For example, the Earth revolves around the Sun in an eastward direction. When I look at the diagram from the perspective of someone above the north pole of the Sun, the Earth seems to go around the Sun in an anticlockwise manner. My problem is I don't understand what is meant by eastward movement ? Can someone explain what this means without ambiguity ? Is there some sort of a right-hand rule that one can follow such as Fleming's right-hand rule in electricity ?

From the point of view of an observer hovering above the Earth's surface, the rotation of the Earth is eastward. That is, points on the surface appear to move in an easterly direction as the Earth spins.

Extending this to the orbital motion, there are at least two possible interpretations. One is that the spin angular momentum and orbital angular momentum are in the same direction. This is roughly true (and certainly closer to true than the opposite). Another interpretation would be that the Earth moves around in its orbit such that it heads in an easterly direction when referred to a longitude on the Sun. This is also approximately true.

If you are asking what is a general definition of the north pole, then I suppose it could be the direction of the angular momentum vector. Then yes indeed, the right hand grip rule tells you the spin is eastward as it relates to north.


Is it coincident that all planets with tight orbit are tidally locked to their parent body? I know mercury is tidal locked to our Sun like our Moon is to Earth, is it merely coincident or there is some sciences behind it? Some man-made communication satellites are also tidal locked to Earth! I think it is intentional so that there is minimum interruption for signal transmission, they used thrusters and gyro to help them to always point toward Earth.

This is a physical result:

  The change in rotation rate necessary to tidally lock a body B to a larger body A is caused by the torque applied by A's gravity on bulges it has induced on B by tidal forces.

Stellar bodies get tidally locked with time. You even get graphs like there.

On how long it generally takes, from Wikipedia

$$t_{\text{lock}} \approx \frac{\omega a^6 I Q}{3 G m_p^2 k_2 R^5} $$

Where $Q$ and $k_2$ depend on the constituents and age of the satellite, which themselves evolve with time. Difficult to estimate. Moreover, you rarely know the age of your system.

But it explains that for long-time established system of relatively close bodies, tidal lock is generally the case. The more tide there is, the quicker it gets.


Are galaxies expanding away from us at more than the speed of light? If you go to the following video, at this certain time:

https://youtu.be/Iy7NzjCmUf0?t=435

you will see that the narrator says:

  "We simply don't know what else is out there because the light from these incredibly distant places has not yet had enough time in the universes history to reach us yet back on Earth. And the light from some places might never reach us at all. Because some parts of space, very far from Earth, are expanding away from us, faster then the speed of light, that means that the light from these places will never, in an infinite amount of time reach Earth"

Am I understanding this correctly? Some parts in the universe are expanding faster then the speed of light? Is it yet proven that there are definitely things faster then the speed of light?

I feel like I am misunderstanding the above statement.

P.S, try watching the entire video, it will be 10 minutes well spent.

You've been confused by a subtle error that seems always to be made when trying to express the speed of light limitation of Relativity in words.

What's fundamental is something called the Lorentz Symmetry AKA Lorentz Invariance which is the basis of Relativity. This symmetry is a local symmetry and it's a strong symmetry of of the physical universe. Basically, what it says is that the laws of physics are independent of velocity. A consequence of this symmetry is that two objects can't pass each other at a speed greater than that of light. (The reason why is not especially difficult, but best explained using mathematics rather than words.)

General Relativity is not a local theory -- it's a theory of the structure of spacetime -- but its local structure is that of Special Relativity, and hence it has Lorentz symmetry.  One of the surprising (utterly surprising -- Einstein himself didn't believe it for many years) consequences of General Relativity is that spacetime isn't stable, but it must either expand or contract.  This expansion is not the galaxies flying further and further into the void like the bits of an exploded hand grenade, but is spacetime itself expanding everywhere (there is no center of expansion) and the galaxies everywhere more-or-less sorta sitting still in the expanding spacetime and being carried along with it.

If you pick two points which are far enough apart, they can be receding from each other at a speed greater than that of light -- or, alternatively and just as correctly -- more than 300,000 km of new space can be created between them each second.  Because nothing is moving past each other at FTL speeds, local Lorentz symmetry is preserved and Einstein is happy and his equations are satisfied and all's well with the universe.

Note that everything I've said is wrong, but I hope it's not misleadingly wrong.
You really need to do the math to see what's actually happening.

But the upshot is that we can't fly to the Moon (or Mars or Alpha Centauri or wherever) and come back at a speed faster than light, but many distant galaxies -- which we will never see or hear from again -- have just passed c moving away from us, and more do so every year.


Time-frame regarding cosmic expansion and the bound state of the Virgo Cluster Due to accelerating cosmic inflation, other galaxies and galaxy clusters will eventually race toward the Hubble horizon apparently away from us, placing them effectively out of our sphere of influence. Due to Virgocentric flow, nearby galaxies, including our own, may become gravitationally bound to the Virgo Supercluster, resisting cosmic expansion (for at least some time).

My question is, Is there any hypothetical time-frame for when this event happens? That is, in what supposed number of billions or trillions of years will the Virgo Supercluster become the only region within the Hubble sphere with which we may interact with?

Edit: Is such an estimate currently impossible? I know I've read one in the past, I just don't remember it well. Somewhen in the ballpark of a trillion years. All I'm looking for is an educated guess.

I don't know how to calculate but Wikipedia has a page about it:
Timeline of the Universe

The galaxies in the Local Group, the cluster of galaxies which includes the Milky Way and the Andromeda Galaxy, are gravitationally bound to each other. It is expected that between 10^11 (100 billion) and 10^12 (1 trillion) years from now, their orbits will decay and the entire Local Group will merge into one large galaxy.[4]
Assuming that dark energy continues to make the universe expand at an accelerating rate, in about 150 billion years all galaxies outside the Local Supercluster will pass behind the cosmological horizon. It will then be impossible for events in the Local Group to affect other galaxies. Similarly it will be impossible for events after 150 billion years, as seen by observers in distant galaxies, to affect events in the Local Group.[3] However, an observer in the Local Supercluster will continue to see distant galaxies, but events they observe will become exponentially more red shifted as the galaxy approaches the horizon until time in the distant galaxy seems to stop. The observer in the Local Supercluster never observes events after 150 billion years in their local time, and eventually all light and background radiation lying outside the local supercluster will appear to blink out as light becomes so redshifted that its wavelength has become longer than the physical diameter of the horizon.
Technically, it will take an infinitely long time for all casual interaction between our local supercluster and this light; however, due to a the redshifting explained above, the light will not necessarily be observed fit an infinite amount of time, and after 150 billion years, no new causal interaction will be observed.
Therefore, after 150 billion years intergalactic transportation and communication beyond the Local Supercluster becomes causally impossible.

This wikipedia data, comes from here, which is used in the text as reference (4)


Is this a Black Hole? While researching about galaxies in Google sky, I came upon a strange area.
Co-ordinates: 11h 6m 15.0 seconds | -77° 22' 9.8"
A lot of space around this body is empty, is it possible that the body is a black hole?
The reason I'm asking is because of this image -
This is a photograph and readings are of a real black hole..
It looks like the one on the co-ordinates which I observed with all the dust around it.
The co-ordinates of the real black hole are - 03h 19m 47.60s | +41° 30' 37.00"

Going by those coordinates, the dark area you're seeing in Google Sky is the Chamaeleon I cloud. It's a nearby star-forming region that is part of the Chamaeleon complex (see also the "Notable features" section of this article on the Chamaeleon constellation). The star nearest to the coordinates you provided is the T Tauri star TYC 9414-787-1.


what would a captured planet do if it was in a retrograde orbit? If our solar system some how captured a rogue planet, and it orbited the sun in the opposite direction to all the other planets,

what major affect would it have on the stability, would its drag cause all the other planets to slow down ?

could it be possible for it to be sustained at all ?

Totally dependent on the details.  A rogue planet could be any mass from Pluto to 10 Jupiter masses - quite different gravitationally.

And exactly what it's trajectory was makes a huge difference.  If it came barreling into the inner system it could cause chaos, particularly a large body.

It would certainly have to pass thought the Oort cloud and possibly the Kuiper Belt.  Any appreciable mass doing would disturb many, many objects which have the potential to fall into the inner solar system (or not).  So that alone could be dangerous for us (or not).  It's a dice throw - quite random from our point of view.

Stable is relative.  If it disturbed orbits in such a way that in a hundred thousand years Earth would become uninhabitable that might be not too bad - a lot can happen to a species in that time scale, especially one armed with nukes. :-)  So this isn't a question with a single answer.


What is a UBV source for stars or stellar objects? I was using the V19(VizieR) catalog and the main identifier of the stars was in terms of an 'ID' number which they mentioned as being 'Star number in UBV source' I have no idea what that means. How would I find a more common identifier say  HD/HIP/HR number?

According to the associated paper (to get to this, click on the Bibcode link near the top right in VizieR), this refers to the source of the UBV photometry.
I believe the relevant references are the ones in the V/19/clusters table, e.g. for the second cluster (NGC 188) the reference is to Upgren, Mesrobian & Kerridge (1972). This does provide cross-references, but only to another catalogue of the cluster by Sandage (1962). It isn't particularly surprising that there aren't cross-references to more general catalogues, as identifying which star is which in such crowded fields can be challenging depending on how accurately positions and stellar properties are given.
Tracking some of the other references down is challenging, this is not helped by the fact that the ADS seems to have Romanized several of the names in various different ways, and searching for one Romanization does not return results for the others (at least, nothing came back for "Lavdovskij", although there are papers attributed to a "V. V. Lavdovsky" and "V. V. Lavdovskii"). I'm not sure how complete ADS coverage of Soviet publications (e.g. Izv. Glav. Astron. Obs. Pulkovo) is.


Does the sun blow a bubble into the interstellar medium? I once read that the solar wind is more or less blowing a "bubble" into the interstellar medium. However, the numbers I found while trying to verify this don't give me a clear picture yet:

The interplanetary medium seems to have an average density of 5 particles per cm³ (at least near earth).
The interstellar medium density varies widely, from 10⁻⁴ to 10⁶ particles per cm³.

What I did not find was the density of the interstellar medium directly around the solar system. I know that the solar wind is interacting with the interstellar medium, which is responsible for the termination shock, but I'm wondering if the density difference is large enough that one can say that the sun is actually blowing a "bubble" into the interstellar medium.

So, is the density inside the heliosphere much less than directly outside of it?

At the moment, the Solar System is inside the Local Interstellar Cloud (which is itself within the Local Bubble). The LIC has a neutral hydrogen number density of $\sim0.1\text{ cm}^{-3}$, which is a bit lower than the mean galactic neutral hydrogen number density. The Local Bubble itself is less dense than the LIC, by about one or two orders of magnitude. On the whole, this region of space is less dense than the ISM, on average.

So, if we take atomic hydrogen density to be a good benchmark, then the LIC and surrounding regions are in fact less dense than the Solar System on average.


Is it practical to hand grind a convex parabolic or hyperbolic mirror? I know it's practical to hand grind a convex spherical mirror and that it's practical to make a concave parabolic mirror from a spherical one. But as I understand it, the procedure for doing so depend on tracking progress by using interference patters generated by focusing light off the mirror, and that procedure clearly can't be used (unmodified) with a convex mirror.

Short of floating the blank in a bath of index of refraction matched optical fluid and apply the concave procedure to the back side, does anyone know of a way to do that sort of grinding?

Also, I've never actually run across a procedure for finishing a hyperbolic mirror for either convex or concave, though I haven't spent much time searching on that one.

If you're asking in regards to testing methods, as indicated in the comments, the simplest setup for interferometrically testing convex conic mirrors is with a Hindle Test, shown below in a figure from the University of Arizona College of Optical Sciences.  This setup can achieve a perfect null after adjusting the reference sphere to be focused at the focus of the test optic - the catch is that the sphere needs to be larger than your test optic, with a hole through it as shown.

In industry, it is much more common to use an aperture-stitching interferometer for small quantities of non-research-level optics.  Larger, more precise, and higher quantity aspheres may use a set of nulling optics, or a diffractive/holographic element to create a null wavefront, as covered in better detail in the link below, which is a slideset from U of Arizona's optical fabrication and testing course.

http://fp.optics.arizona.edu/jcwyant/Short_Courses/SIRA/7-TestingAsphericSurfaces.pdf

If you're feeling especially ambitious, there is a concept for measuring surface form of a mirror by displaying points on a monitor and using an HD camera to see where the reflections come from, thus telling you the angle of the optical surface at that location.  The data is then integrated to form a full surface map.  In theory, this system could be developed at low cost, with relatively high performance.

https://www.osapublishing.org/ao/abstract.cfm?uri=ao-13-11-2693

Hope this (or at least some of it) helps!

-J


Site to see direction to the Voyager by entering time and coordinates of my position on the Earth? Where is Voyager relative to me right now in realtime?

TheSkyLive has charts showing the positions of a few interplanetary spacecraft in the sky for a given time and place on Earth.
Their positions for Voyagers 1 and 2 agree with the authoritative JPL HORIZONS, which can generate a table of equatorial coordinates for a range of dates.

Voyager 1 is in northern Ophiuchus.
This is in the August evening sky for most of the Earth.
As of August 2018, it is 143 AU away from the Sun and receding 3.6 AU/year.
Voyager 2
is in northern Pavo, in the September evening sky for the tropics and southern hemisphere.
Its current distance from the Sun is 118 AU, receding 3.2 AU/year.

Since both probes are so far away (compare Neptune at 30 AU) and moving away from the Sun, their apparent positions among the stars will not change much in the next few years.
Of course they are not visible in any telescope, and a very large dish is needed to receive their radio transmissions.

If you like using desktop planetarium software, you can try importing hyperbolic orbital elements for the Voyagers.


Is the Sun prograde or retrograde with respect to the rotation of the Milky Way? I recently learned about the terms prograde and retrograde. I've seen these terms used to describe the rotations and orbits of moons with respect to their planet, and planets with respect to their star. But I haven't been able to find what the relationship is between the rotation and orbit of the Sun with respect to the rotation of the Milky Way.

So, basically what I'm trying to do here is ask two questions:

Is the orbit of the Sun around the center of the Milky Way prograde or retrograde with respect to the rotation of the Milky Way?
Is the axial rotation of the Sun prograde or retrograde with respect to the rotation of the Milky Way?

Regarding question #2, I know it might be considered a slightly weird question to ask, simply because the plane of the Solar System is significantly tilted relative to the plane of the Milky Way (by about 60°), and furthermore the Sun itself has its equatorial plane tilted by a few more degrees relative to the plane of the Solar System (about 7°). But if we mentally "flatten out" the Solar System such that the Sun's equatorial plane aligns with the plane of the Milky Way, then we can talk about whether its axial rotation is in the same direction as the rotation of the Milky Way, so I think this is a valid application of the concept.

Prograde. This is usually the case for stars in spiral galaxies outside the most central regions.

A galaxy can be either rotation-dominated or dispersion-dominated, depending on whether its kinematics is dominated by ordered or random motion. Spiral galaxies such as the Milky Way belong to the first class, while ellipticals belong to the second. Irregular galaxies can be somewhere in between, with random motions but with an overall rotation.

In spirals, stars and gas generally rotate in the same direction (although they can have more random motion in their bulges, i.e. in the central parts). The Sun follows the overall rotation of the Milky Way, except for roughly 20 km/s in the direction wrt. the overall rotation; this direction is known as the Solar apex.

However, following a major merger event, the product can be two kinematically decoupled components. This is described well in Corsini (2014). Such a merger is required to perturb the system sufficiently.

On smaller scales, however, the turbulence of the gas clouds giving birth to stars are more random. Hence the rotation of a star may very well be in another direction than the overall disk rotation. As you say, the plane of the Solar System is tilted by 63º wrt. the plane of the Milky Way, in the direction that the Sun is traveling. This can be seen e.g. in this infrared IRAS image, where the angle between the bright Milky Way disk and the blue stripes — which is emission from dust in the plane of the Solar System — crosses at roughly 60º:

credit: Caltech.

The rotation of the Sun is also prograde wrt. the Milky Way (or else we would say that the tilt was 180º – 63º = 117º).


How to orient ESO's Milky Way Panorama in a 3D model I have been trying hard to find the star "Polaris" in the following image from ESO and Serge Brunier:

From peering at other maps and simulation apps, I think it should be in the top-left quarter, relatively close to the middle (vertically), however taking the brightest starts one by one, I cannot piece together the other stars belonging to Ursa Minor (small dipper).

I want to use this image as a background in a 3D model of Earth. The projection is perfect for that purpose, I only need to rotate it in such a way that Polaris lines up with the north pole. Once that is accomplished I can rotate it around the Earth's polar axis to set the right time of day at a certain part of the year (it does not have to be 100% accurate for my purpose, but it should at least look a bit realistic to an astronomer).

One thing that makes this task quite hard with this high resolution image is that stars seem to pop in and out of existence while zooming in and out, probably due to lossy fast re-sampling algorithms used in my image viewers.

If someone with a trained eye could point out the Ursa Minor constellation for me it would save me a ton of time!

Here is a star map (from http://www.datasync.com/~rsf1/fun/sm-newd.gif ) given in Galactic coordinates. It appears to be presented in a similar (but not the same, since it is rectangular) projection to the one you show above.

The Galactic centre is in the middle and the Large and Small Magellanic clouds are to the lower right and oriented in the same way.

As you can see from this map, your assumption is completely correct. Ursa Major and Polaris should be situated towards the upper left hand side of the picture. Conveniently it should be on the opposite side and roughly equi-distant from the Galactic plane as M31 (which is clearly visible in the image) and at a similar (slightly lower) Galactic longitude.

I have added a copy of your image at the bottom and marked where I think Polaris is. I suggest you get hold of the higher resolution image available from Serge Brunier if you want to pick out individual stars. The way you transfrom this into equatorial coordinates for different times of the year is not as simple as you suggest.


Device used to measure stellar radii I remember seeing in a video an instrument that was used to measure a star's radius with nothing more than the light it emitted.
The device worked by opening or closing its mechanism in increments, while watching a display. The display started out showing some shape, maybe some lines or a circle, with blurriness. The closest you could get to making the shape blurry-less would determine the instrument's approximation of the star's radius.
My question is, what is the name of this instrument?

I am not sure what you are referring to; possibly it is intensity interferometry using the Hanbury Brown & Twiss method, but even this requires two telescopes. This kind of interferometry was used to estimate angular diameters for some tens of stars but is not generally used these days for this purpose. Instead, one constructs interferometers from separated telescopes using delay lines to compensate for path-length differences and maintain coherence between the different paths that the light takes. This then uses amplitude interferometry to reconstruct the angular diameter of a star from the interference pattern seen in different telescope baseline configurations.

The radii (or to be more exact, the angular size) of many nearby stars and also of many giant stars at rather larger distances, have been measured using interferometric techniques. An accurate distance is then required to estimate an absolute radius.

Relevant papers are by Boyajian 2010 and Boyajian et al. (2013). There are a number of such interferometric experiments, mostly working in the infrared.

There are also direct radius measurements for stars that are occulted by the Moon and other solar system bodies - the time-dependent diffraction pattern can yield an angular size for the object.

Absolute radii are also provided by eclipsing binary systems (independent of whether they have a known distance).

See also How do astronomer measures the size of any celestial objects?


Source of the odor in space If space is a vacuum, then how can it contain an odor? I read that after spacewalks, astronauts detected an unusual odor after removing their spacesuits.  It was compared to the smell of a candle burning. Perhaps this is the smell from the "Big Bang"?

Here's a former astronaut describing the smell: Astronaut Chris Hadfield Debunks Space Myths | WIRED "This one is true! (gunpowder, burnt steak, brimstone)" and What does space smell like? Astronaut Chris Hadfield answers your questions "a little bit like a shooting range, gunpower/cordite, brimstone"

If outer space was a perfect vacuum, then there would be no planets or other objects occupying it. How much space are we talking about, and where is this space you refer to? for practical purposes, outer space is defined to as starting at a location past the Kármán line heading outwards from planet Earth. That does not however mean that it is a perfect vacuum. There are still particles, atoms, molecules and other stuff there.
I have also read that Astronauts detect the smell of slight traces of something burnt or burning. This can be caused by a physical effect to do with the way the brain reacts to certain conditions. It may not actually be due to the human olfactory system.
https://www.nbcnews.com/health/health-news/phantom-smells-may-be-sign-trouble-n890271
Even though such things may affect Astronauts, NASA or other space agency medical staff may not take it as a long term effect that cannot be corrected or stabilized. And as such still allow Astronauts to space walk with periods of recovery in between. Thus allowing the astronauts to effectively work in outer space, even though they experience such olfactory sensations.
Yeh, I also have seen that video with Astronaut Chris Hadfield. One of the biggest issues with Astronaut habitats in outer space is that they accumulate large amounts of dead skin cells. I mean, huge amounts! So if some of those dead skin cells deposited themselves on the space suits, and radiation outside the craft frazzled the cells, it would smell like burnt steak, when you come back inside the habitat/craft.


How do large solar flares compare to flares on other stars? Solar are violent releases of solar magnetic energy. Other stars are also known to have magnetic fields, in some cases much stronger than the Sun. How do the largest stellar flares compare to the largest solar flares in terms of energy released? It would also be interesting to normalize the answer by the strength of the magnetic field to be able to compare the efficiency of magnetic energy release on other stars compared to the Sun.

This is a part answer, a comparison of sun-like star superflare behaviour with our sun.

According to the article Superflares on Solar Type Stars Observed with Kepler
I. Statistical Properties of Superflares (Shibayama et al. 2013), observations were made on solar-like stars (type G), over 500 days.

One key observation was that they

  found 1547 superflares on 279 G-type dwarfs

Despite this seemingly massive amount, they deduced that

  the occurrence frequency
  of superflares with energy of 10^34 − 10^35 erg is once in 800-5000 years.

and

  in some G-type dwarfs the occurrence frequency of superflares was
  extremely high, ∼ 57 superflares in 500 days (i.e., once in 10 days). In the case of
  Sun-like stars, the most active stars show the frequency of one superflares (with
  10^34 erg) in 100 days.

These are associated with very large star-spots, far larger than those on our sun.

There was an earlier theory that the presence of Hot Jupiters were a major contributor to super flares, hence the reason that our Sun did not exhibit this phenomenon often. However, there is some evidence of a possible past superflare occurred from our Sun:

  an occurrence of a energetic cosmic ray event in 8th century
  recorded in a tree ring of Japanese cedar trees. There is a possibility that this event was
  produced by a superflare (with energy of ∼ 10^35 erg) on our Sun.

and no hot Jupiters were detected around many of the stars observed either, so this theory is largely ruled out by the authors. Rather, they postulate that G-type stars 'store' magnetic energy.


Trying to understand the way Saturn's ring look in this famous Cassini image You all probably know the famous The day the Earth Smiled image. For the life of me I can't understand what's going on with the rings there. Seems like the "bottom" half of the rings is the half that goes "behind" Saturn (relative to the camera), but then what's going on with the distortion of the rings on the upper half when they (presumably) cross in front of Saturn? I can't make sense of of this image.

Higher resolution and annotated versions available

What's going on with the distortion of the rings on the upper half when they (presumably) cross in front of Saturn?

The brownish areas you see on Saturn are ring light, analogous to seeing the Earth by moonlight. Saturn's rings light up Saturn's night sky, particularly just after sunrise and just before sunset. The two dark bands across the face of Saturn are the A ring (upper dark band) and B ring (lower dark band). Of all of Saturn's rings, these two are the most opaque. The parts of Saturn underneath those two dark bands are brilliantly lit by ring light, but Cassini can't see those parts of Saturn because the opaque A and B rings block that light.

For a much better and more thorough explanation of all of the features in this incredible image, I suggestion you spend eleven minutes watching this youTube video by Emily Lakdawalla.

This is one of the very few times I will post a link to a youTube video. It is worth every eleven minutes.


Impact of Atmospheric Water vapour on Optical Observations I would like to ask; how does Water Vapour in the atmosphere interfere with astronomical observations? Is it in the same manner as Carbon dioxide?

What brought this to mind is the following: A member of Monash University staff claimed recently in a newspaper in Australia that both CO2 and H2O absorbed heat and hence their presence needed to be reduced for astronimical observations. Hence the siting of optical observatories on mountain tops where the absolute amount of water vapour in the atmosphere is reduced by the relatively low temperatures there.

Yes, water vapour interferes even stronger with astronomical observations than $\rm CO_2$.

In the optical, water easily forms droplets which scatter light. But also in the gaseous phase water has strong absorption bands, some of those reach into the optical, while most of those however lie in the infrared, making water a greenhouse gas at the same time.
Let's have a look at the absorption spectrum of various gaseous species in the atmosphere:

Here we see with data what would otherwise take too many words to explain. Take note that the observable spectrum for humans is at the wavelengths $\rm \lambda = 0.4-0.8 \mu m$. The many absorbtion bands of water make clear why this interferes with astronomical observations:
In the same way that outgoing thermal radiation from Earth is stopped at going into space, also infrared coming from space is absorbed in the atmosphere and thus not reaching telescopes.

To deal with this there is a number of ways to cope:

Go high up above all the water vapour, to a dry place like Chile. Or a dry place like space. Then the absorption spectrum looks much more benevolent, and that's part of the reason we have all the major observatories in Chile, Mauna Kea, etc.
Try to observe in one of the absorption bands, but fly above all the water vapour. This is what the two airborne-based observatories SOFIA and its predecessor do/did.
Observe through the infrared-'windows' that let light pass through, from the ground. This is a bit exotic science wise nowadays - AFAIK this is a method mostly used in history, as today it is hard to justify building a massive telescope when most of your prospective signal gets absorbed. This was employed at Mt. Wilson Observatory, but also many others.


Our universe the surface of a 4-dimensional sphere? The cosmic microwave background that we observe uniformly around us is usually explained by assuming that our universe is the surface of a four dimensional sphere. That way the uniformity makes sense since there is no center. My question is if this is true then what is the explanation that describes the fact that the farther we look into space, the further we look back in time. I can't perfectly picture this and see how it would coexist. Help me out.

The surface of the 4-dimensional ball (called 3-sphere) is a slice through the universe as a whole for a fixed cosmic time. This slice describes just three spatial dimensions. The observable universe is a tiny part of this 3-sphere; hence it looks flat (3-dimensional Euclidean space) up to measurement precision (about 0.4% at the moment).

Adding time makes the universe 4-dimensional. The observable part is similar to a 3+1-dimensional Minkowski space-time. The universe as a whole may be a de Sitter space-time. A de Sitter space-time is the analogon of a sphere ( = surface of a ball), embedded in a Minkowski space, instead of an Euclidean space, but it's not literally a sphere.

If time would be taken as an additional spatial dimension, the de Sitter space would ressemble a hyperboloid of revolution, if embedded in a 5-dimensional hyperspace.
The difference to an Euclidean space is due to the different definition of the distance: In a 4-dimensional Euclidean space the distance between two points is defined by
$l = \sqrt{\Delta x^2+\Delta y^2+\Delta z^2+\Delta t^2};$ for a 3+1-dimensional Minkowski space it's $l = \sqrt{\Delta x^2+\Delta y^2+\Delta z^2-\Delta t^2}.$ For simplicity the speed of light has been set to $1$.

This model of the universe as a whole can hold, if it actually originated (almost) as a (0-dimensional) singularity (a point); but our horizon is restricted to the observable part, so everything beyond is a theoretical model; other theoretical models could be defined in a way to be similar in the observable part of the universe, but different far beyond, see e.g. this Planck paper.


How do scientists know that distant parts of the universe obey the physical laws exactly as we observe around us? How do scientists know that distant parts of the universe obey the physical laws exactly as we observe around us?
The question might look a bit odd but I am really stuck on my head. We know, scientists (with tools) explored physically only our solar system and some parts of our galaxy which is really a tiny part of the observable universe. And now they are constantly using these knowledge along with 'tested physical laws' to measure the properties of distant parts of our universe.
For example, we tested and found the speed of the light is constant within our local periphery many times (within our Earth and Space around Earth). But yet we presume that the speed of the light is constant even at the farthest part of our universe. Certainly we did not test it in the other distant part of the universe because we have no way until now. Not only light but also the other physical properties like, luminosity, gravity, and etc related physical laws are agreed upon based on tests within our solar system. And based on these laws we deduced the properties of other parts of the universe (i.e. age, distance, mass, luminosity of stars in millions/billions light years away).
My question is, how do we know that these physical laws which we tested within a tiny area of the universe are consistently working in the distant parts of it? Is there any probability that the distant part of our universe obeys physical laws differently and our prediction based on applied physical laws gave us an unreal illusion of the actual reality, yet consistently?

We don't know in general but to the extent we can measure, the laws seem to be the same, even if conditions are not.
For example radioactive decay: We know how fast various elements decay, and we can observe the results of radioactive decay in distant supernovae.  The conclusion is that, for at least some elements, the rate of radioactive decay is the same on Earth as it is in distant supernovae.
After accounting for redshift, spectral emission lines remain unchanged by distance. This implies that the fine-structure constant is indeed constant.
Distant galaxies have gravitational fields, and interactions between galaxies proceeds in the same way in distant galaxies as it does in local ones. Eventually, the justification is philosophical:  There is no observational reason to believe gravity behaves differently in distant parts of the universe, and so we believe that it does not,
In the extreme conditions of the early universe, some physical laws were different.  For example, instead of distinct electromagnetic and weak fields, there was a single Electroweak field.  But this can be described as single "law" with the electromagnetic and weak interactions being just the low energy approximation of the electroweak interaction.
So if it were discovered that Gravity (for example) was working differently in distant parts of the universe, but that there was a consistent pattern or rule for how it varied, then that would simply become the new theory of gravity (with general relativity becoming only the local approximation to this new law).
There is a more fundamental assumption: that the behaviour of matter and energy in the universe can be modelled by "laws".  There are no angels dancing on pinheads. The justification for this is strictly in the realm of philosophy.


How much clearer are stars in earths orbit? As a child I remember my parents taking camping in California, and upon stepping out of the car I was awestruck at how thick the Milky Way galaxy was and at the number of stars everywhere. I've always wondered how much clearer it could get. What percentage of visible light from stars are absorbed by all of earths atmosphere at the clearest locations on earth? Are we taking about a small percentage or perhaps a significant amount?

It varies.

The best astronomical sites have a visual band extinction of 0.1 mag, which means that only $\sim 10$ per cent of light is absorbed/scattered in the atmosphere.

In dusty, smoggy or polluted sites, this can easily reach one magnitude of extinction, which means that 60 per cent of the light is scattered.

These numbers are per airmass - which means looking straight up at zenith. At lower altitudes, the baseline extinction figure would be multiplied by $\sec z$ where $z$ is the angle from the zenith.

This amount of absorption doesn't sound like a huge difference, but what it means is that a large number of extra stars become visible, because roughly-speaking, you get a factor of two increase in the number of stars for every 0.5 magnitudes deeper you can see, and I think that is what is most striking about going to a clear, dark astronomical site.

Thus there wouldn't be much of an improvement going into space over the best astronomical sites (in the visible band), at least not due to the decrease in extinction. You do also escape light pollution, which is a different effect. With light pollution it is the fact that the background sky becomes too bright for your eye to pick up the fainter stars. [As Carl Witthoft notes, it is a different ball game at other wavelengths because the atmosphere is significantly more opaque to UV, X-ray and IR wavelengths.]

Another thing to consider is what you are breathing! Lack of oxygen can hinder visual acuity. http://navyaviation.tpub.com/14020/css/Effects-Of-Hypoxia-141.htm


Why one cannot escape from a black hole The usual explanation is that escape velocity of a black hole is above speed of light, therefore nothing can escape because nothing can move faster than light.

This is however a fallacy, because escape velocity only pertains to bodies "without propulsion", or in other words without any force other than gravity acting on them. As an example I could theoretically escape Earth's gravity at a walking speed, just give me a staircase and I could walk all the way to the Moon. No need to accelerate to 11 km/s at any point.

How does this affect black holes? Why cannot I have a staircase and climb out?

I know the staircase idea must be wrong, but I would like to know why. My guess is that I am applying Newtonian mechanics to a problem which requires general relativity approach, because Newtonian mechanics approximation fails badly around such massive gravity fields?

Anyway, according to Newtonian mechanics one can easily escape a black hole, even if we assume that speed of light cannot be exceeded.

A great explanation; all credit goes to: https://groups.google.com/forum/#!original/sci.physics.research/PZh0yawL2nA/YKbv-EpWJPsJ

  The explaination of a black hole in terms of escape velocity is not a
  good way to think of a black hole. The escape velocity is merely the
  intial speed at which a projectile must be fired from a large body in
  order so that, given that only gravitational forces act on it (which
  is a scenario very far from reality) it will never return to that
  body.

  However, anything that passes through the event horizon of a black
  hole will never be on the other side. Even if it were to constantly
  accelerate away from the black hole, it would still be going inward.
  This is not true of Newtonian gravity. In fact, in Newtonian gravity,
  there are no black holes, because there is no universal speed limit.

  The best way to think of an event horizon is where spacetime is so
  bent that all paths 'curve inwards', IE, the lightcones all point
  inward, so there is no chance of escape.

  BTW, in GR, it's not good to explain the effects of gravity as a
  FORCE. It is as much a force as the Centrifugal Force or the Coriolis
  Force. There is an amount of force needed to stay the same distance
  from an object, IE, to overcome gravity, however. And as you approach
  the black hole, the amount of force required approaches infinity. But
  to say that there is an infinite amount of force PULLING on someone is
  not very accurate.


Do other planets affect the Earth? Let say Mercury, Venus, the Moon were all in alignment between the Earth and Sun. They are all closest to each other in respective elliptical orbit.  On the other side of the Sun you have the rest of the planets.

Would their gravity combine to have any gravitational effects on the Earth?
Or are they all too far away?

TL;DR: No. Well, yes, but they are negligible.

Full answer: a conjunction like this 'nearly' happened on May 5th, 2000. NASA published an article about this event, stating:

For example, the combined gravitational effect of all the planets together is much less than the effect of the Sun or the Moon on the Earth.

and

While unusual, such alignments have happened in the past without any consequences. The planets are simply too far away to have an effect on anything here on Earth - except our imaginations.

For hard numbers, see this article by Truman Collins which is linked there:

Conclusion
What I discovered was that the tidal forces exerted on Earth from the five planets on the other side of the Sun are vastly smaller than those exerted by the Moon or the Sun. In fact, with the planets in the position they will be in on the big day, the largest tidal force from one of them will be from Jupiter, and it is about one five hundred thousandth the size of the tidal force exerted by the Moon on an average day!


How were the distances to stars measure before parallax? A comment under When was the distance to a star measured for the first time without using parallax? mentions that the distance to stars was measured before parallax was possible. How was this done?

As far as I know direct parallax measurements are the only way to directly measure the distances to stars.
Once parallaxs of hundreds of stars were known and diagrams of the relationship between stellar luminosity and spectral types, such as the Hertzsrpung-Russel diagram, were made, it became possible to estimate a star's absolute magnitude more or less accurately and thus calculate its distance from its apparent magnitude.
It may be noted that stars in some spectral types can have any one of up to nine luminosity classes.  A star's luminosity class is included as Roman numerals in its spectral classification if it is known.  Calculating the distance to a star without knowing its luminosity class can be very inaccurate.
https://en.wikipedia.org/wiki/Stellar_classification#Yerkes_spectral_classification[1]
The heliocentric theory, that the Earth revolves around the Sun, was first suggested in classical Greece.  Aristotle (384-322 BC) rejected that theory, because of the lack of detectable stellar parallax caused by Earth's motion around the Sun.
Exploration of the Universe Brief Edition, George Abell, 1964, 1969, page 18.
That same argument was used against the heliocentric theory in the early modern age.

Stellar parallax is so small that it was unobservable until the 19th century, and its apparent absence was used as a scientific argument against heliocentrism during the early modern age. It is clear from Euclid's geometry that the effect would be undetectable if the stars were far enough away, but for various reasons, such gigantic distances involved seemed entirely implausible: it was one of Tycho Brahe's principal objections to Copernican heliocentrism that for it to be compatible with the lack of observable stellar parallax, there would have to be an enormous and unlikely void between the orbit of Saturn and the eighth sphere (the fixed stars).2

https://en.wikipedia.org/wiki/Stellar_parallax[2]
As astronomers gradually accepted the heiocentric theory after about 1600, there were attempts to meaure stellar parallaxes which ended in failure after failure.  And each astronomer who failed to measure stellar parallax could have calculated the smallest stellar parallax he could have measured, and thus the distance that the star(s) he studied must have exceeded.  So astronomers should have gradually increased their estimates of the minimum possible distances to various stars whose parallaxes had failed to be measured, while perhaps hoping that some stars might possibly be a lot closer.
And as the distance between the Sun and the Earth was repeatedly measured more and more accurately, astronomers could have estimated the Sun's luminosity more and more accurately.  By assuming that any particular star was as luminous as the Sun, and by measuring its apparant magnitude, astronomers could have estimated how far away it was.
But of course the actual luminosities of stars vary greatly, and in the 17th and 18th centuries measurements of the apparent brightness of the Sun and of distant stars would probably have been very inaccurate.
I doubt if any estimates of the distances to stars were very accurate before the first stellar parallaxes were measured in the 1830s, because the first measured parallaxes seemed too small, and thus the distances too great, for astronomers to believe.
For example, the first actual measurement of a stellar parallax was made by Thomas Henderson, but his is usually listed as the second measurement.

The large proper motion of Alpha Centauri AB was discovered by Manuel John Johnson, observing from Saint Helena, who informed Thomas Henderson at the Royal Observatory, Cape of Good Hope of it. The parallax of Alpha Centauri was subsequently determined by Henderson from many exacting positional observations of the AB system between April 1832 and May 1833. He withheld his results, however, because he suspected they were too large to be true, but eventually published them in 1839 after Friedrich Wilhelm Bessel released his own accurately determined parallax for 61 Cygni in 1838.[62] For this reason, Alpha Centauri is sometimes considered as the second star to have its distance measured because Henderson's work was not fully acknowledged at first.[62)

https://en.wikipedia.org/wiki/Alpha_Centauri#Observational_history[3]

The distance to Vega can be determined by measuring its parallax shift against the background stars as the Earth orbits the Sun. The first person to publish a star's parallax was Friedrich G. W. von Struve, when he announced a value of 0.125 arcseconds (0.125″) for Vega.[37] Friedrich Bessel was skeptical about Struve's data, and, when Bessel published a parallax of 0.314″ for the star system 61 Cygni, Struve revised his value for Vega's parallax to nearly double the original estimate. This change cast further doubt on Struve's data. Thus most astronomers at the time, including Struve, credited Bessel with the first published parallax result. However, Struve's initial result was actually close to the currently accepted value of 0.129″,[38][39] as determined by the Hipparcos astrometry satellite.[4][40][41]

https://en.wikipedia.org/wiki/Vega#Observational_history[4]
So Henderson and Struve suffered failures of nerve, being unable to accept how distanct Alpha Centauri and Vega were according to their measurements, and so lost the honor of making the first measurement of stellar parallaxes to Bessel.
And in my opinion that would not have happened if astronomers of the time commonly accepted that even the nearest stars must be several hundred thosuand Astronomical Units, and multiples of light years and parsecs (units which had not yet been invented) from the Sun.
So any estimates of the distances to stars made before the first successful parallax measurements probably greatly underestimated the distances.


How could the polarizer in early coronagraphs make the Sun's corona more visible against the daytime sky? Wikipedia's Coronagraph; Invention says:

  High Altitude Observatory's Mark IV Coronagraph on top of Mauna Loa, uses polarization to distinguish sky brightness from the image of the corona: both coronal light and sky brightness are scattered sunlight and have similar spectral properties, but the coronal light is Thomson-scattered at nearly a right angle and therefore undergoes scattering polarization, while the superimposed light from the sky near the Sun is scattered at only a glancing angle and hence remains nearly unpolarized.

How does this work?

For example, I looked at the daytime sky through a polarizing filter and I didn't notice substantial sky darkening? How could a polarizer have darkened the sky sufficiently to make the solar corona more visible?

When you look at the Sun in white (visible) light, what you see is actually sunlight that has scattered from free electrons in the solar corona. This is known as "Thomson scattering".

If we view the corona around the Sun (from our perspective), then the scattered light that reaches us has scattered through a large angle (roughly 90 degrees). The sunlight incident upon the electrons in the corona is unpolarised (that is, the electric field vector of the light is randomly oriented but at right angles to the direction of travel of the light). Such light accelerates the free electrons which re-emit light (this is what scattering is). But the scattered light is polarised when it is scattered through a large angle, because we effectively can only "see" the electrons accelerating along a line that forms a tangent to the limb of the Sun. The component of the acceleration along the line of sight does not cause any radiation to be emitted towards us.

The light we see from the daylight sky is also produced by scattering. In this case it is "Rayleigh scattering" from electrons that are bound into molecules and atoms in the Earth's atmosphere. The polarisation effects of this scattering are almost identical in nature to that caused by Thomson scattering. However, when we look at the sky right next to the Sun, then this will have been scattered through a very small angle. In these circumstances the scattered light will be unpolarised.

Thus we have a means of seeing a contrast between (polarised) light coming from the corona and (unpolarised) light coming from the sky in the same line of sight. By viewing the coronal area using a linear polariser, and then presumably rotating the plane of polarisation and taking a series of pictures, and comparing this with images taken without a polarising filter, we can isolate the small fraction of light from around the Sun that is due to Thomson scattering in its corona.

You made the comment that you don't see the sky darken when looking through a polarising filter. Well, either your filter isn't working or you were looking at the sky roughly 90 degrees away from the Sun and had the polarising filter in just the right orientation, or your sky is very polluted (the scattering caused by particulates does not cause the same degree of polarisation). Daytime sky is most definitely polarised depending on angular separation from the Sun. This map shows how the degree of linear polarisation changes with position as the Sun moves across the sky. The Sun is where the black circle is (black indicating unpolarised light). See also this answer.


How much dimmer is a partial solar eclipse that goes from -26.71 to -25.89? Based on what I found in Stellarium, for Vlissingen, Netherlands, on July 2, 1666 there was a partial eclipse with a change of apparent brightness -26.71 to -25.89.
Is it valid to say based on the chart below that the sun would've appeared a little more than 50% dimmer at maximum compared to before the eclipse?
If not, approximately how much dimmer was the sun during this eclipse, based on the values given by Stellarium?

Source

above: Just before the start of the eclipse. below: At maximum

There are two ways to think about this change: firstly in terms of the amount of light energy.  The rule for magnitudes is that "five magnitudes is a factor of 100" So to find the change in luminosity between two magnitudes you can calculate
$$\Delta L=100^{(M_1-M_2)/5}$$
With your values I get 0.47, or about half as bright at full eclipse.
The second way is to note that your eyes (and brain) actually do a good job of compensating for huge variation in brightness.  Unless you are looking towards the sun (with your safety glasses of course) you probably won't notice the difference in illumination. In the 1999 eclipse, it was only at 90%+ coverage that there was any apparent change in illumination. The burghers of Vlissingen may have been blissfully unaware.