Stellarium 0.10.4: planet orbits change over time? If I draw the orbit of Mars in stellarium 0.10.4 for 23 Sep 2015, it passes south of Chi Leonis:

If I advance the date to 17 Oct 2015, however, the orbit of Mars correctly goes right through Chi Leonis (Mars will occult Chi Leonis on 18 Oct 2015):

Is this a known bug in 0.10.4 that's been fixed? (I unfortunately can't upgrade)

Probably this is due to the fact that orbits of the Earth and Mars are inclined to the ecliptic differently so our viewing angle of the orbit of Mars changes slightly over the year.  The program is probably showing the orbit of Mars projected onto the Celestial Plane as seen by us on the one specific date. Note, this is different from the ephemeris orbital data which gives the apparent position as seen by us at each date/time.   Just a guess though.


When did people first measure that the Earth was closest to the Sun during January? When we talk about the reason for the seasons, we usually have to dispel the misconception that seasons are caused by being close and far away in the Earth's elliptical orbit.

And usually, we mention that the Earth is actually closest to the sun in January, in the dead of winter (for the Northern hemisphere).

But when did astronomers first have the Earth's orbit measured carefully enough that they knew the Earth was slightly closer during January?  How was that measurement made?  How accurate were the first measurements?

They didn't they measure the size of the disk of the sun very, very carefully, did they?  Perhaps with a pin-hole camera?  That seems like it would be very difficult to do.

Additional

I guess if we are talking about long enough ago, they would have thought it was the Sun's orbit that brought it closer because either because of an eccentric (the idea that the orbit of an ancient planet had a center that was offset) or because of epicycles bringing the Sun closer on the circle on a circle.

I just wonder what sort of observation they might have made.

One of the nicest comparisons I've seen can be found here.

If it was me with the tools available in ancient times, I'd probably use a rotatable camera obscura, and maybe a cone with markings to place at the center of the image of the sun, to exaggerate the effect of the size differences.

Second Additional

Based on JdeBP's answer I want to see if I have the correct concept.  (I would put this in comments, but comments can't be nicely formatted.)

Doing a search for the dates and times of the solstices and equinoxes, and finding the time between those dates and times, I found the lengths of the upcoming seasons.

Summer 2020 is 93 days, 15 hours, 47 minutes

Fall 2020 is 89 days, 23 hours, 0 minutes

Winter 2020 is 88 days, 21 hours, 7 minutes

Spring 2021 is 92 days, 17 hours, 54 minutes

Summer 2021 is 93 days, 15 hours, 49 minutes

If we subtract from 1/4 of an astronomical year, we get about:
$$
\begin{matrix}
Spring & +1.4 \: days & & Summer & +2.4 \: days \\
Fall & -1.4 \: days & & Winter & -2.4 \: days
\end{matrix}
$$
From there it seems like with a geocentric model with an eccentric, we could get a good approximation for the date of perihelion.

I'll have to think about the details of how to get there, though.

Hipparchus, not Kepler

Kepler got the conic sections right, and Newton gave us the mechanics.  But the question is about when people knew that the Earth was closer to Sol in one part of the year than others, and Hipparchus knew that, even though he wasn't too hot on the values of the orbital radii.  Hipparchus' version of the eccentric model had Sol's (purported) circular orbit around the Earth not centred upon the Earth, but 1/24th of an AU away.  Therefore Sol (purportedly) orbited at varying distances from Earth.  This was, after all, the whole point of the eccentric model, to explain non-uniform apparent motion through variation in distance.

Perigee and apogee were known in the times of Hipparchus and Ptolemy.  Hipparchus even worked out when the furthest point (apogee) was.  Ptolemy furthermore made an error based upon knowing that his placement of the apogee in Gemini was the same as that of Hipparchus 280 years beforehand, declaring that perigee and apogee were fixed.

They of course were not.  Hipparchus placed apogee at 5.30° Gemini.  Astronomers in the 9th century in Baghdad applied the same calculations to their measurements and placed it at 20.45° Gemini.

As for how this was observed, it was not done by measuring the Sun's appearance at all (although Hipparchus did do that).  Ptolemy and Hipparchus had a geometric model of a true geocentric circular orbit versus the (purported)  eccentric circular orbit of Sol.  It incorporated the equinoxes and the solstices.  By observing the times of the equinoxes and solstices, the lengths of the periods between them, they were able to determine trigonometrically all of the other orbital parameters, which included placement of perigee and apogee.

That points of closest and furthest approach existed was known in the 2nd century BC, as was their angular locations relative to the solstices; they've been in the models from then onwards.  That they moved around took about 11 centuries after that to discover.  The correct conic sections and the idea of both bodies orbiting around a barycentre came somewhat later, but that wasn't the question.

Further reading

Hugh Thurston. (1994) "Appendix 2: Calculation of the Eccentric Quotient for the Sun, and the Longitude of its Apogee". Early Astronomy. New York NY: Springer.  DOI 10.1007/978-1-4612-4322-9_1
James Evans (1998). The History and Practice of Ancient Astronomy.  Oxford University Press. ISBN 978019509539.
Viggo M. Petersen and Olaf Schmidt (June 1968). "The Determination of the Longitude of the Apogee of the Orbit of the Sun according to Hipparchus and Ptolemy".  Centaurus.  Volume 12.  Issue 2.  Pages 73–96.  DOI 10.1111/j.1600-0498.1968.tb00080.x


traveling past the speed of light According to the theory of relativity (If I understand correctly), nothing is supposed to be able to travel faster then the speed of light. I believe, to my limited knowledge and grasp of the concept, that quantum theorie speaks against, i.e. extend, the theory of relativity, hence my question:

Suppose we find (or imagine) a place in the universe where there is no obstruction for this thought experiment to take place. We take a spherical object and attach a spire at both sides so you get an object shaped like this: -O-

We start turning the object so the ends of the spire travel in an orbital mather according to the spherical object. What if the sphere starts to orbit near the spead of light, wouldn't the ends of the spires travel faster than the speed of light?

Sorry if this is the wrong place for this question.

This would be more appropiate at Physics.s.c but here is the answer nonetheless.

Your object may be perfectly rigid or not.

If the object is perfectly rigid, you would not be able to accelerate it to the point at which the extreme points of the spires reach the speed of light, because when they are approaching it, their mass will increment towards infinity and the energy you will need to continue accelerating will increment to infinity as well.

So you will simply not be able to accelerate the whole thing to a rotation speed where the extreme points will reach the speed of light.

If the object is not rigid, well, it will simply bend, plus the previous explanation.


How to calculate the mean molecular weight of the Sun I have a homework question in which I need to estimate a parameter known as $\beta_{P}$ and also the core temperature, both for the Sun.

However, prior to doing this I need to know the mean molecular weight of the Sun.

First question is, how would I go about doing this.

Second question is, can I assume that it is about 70% hydrogen and 30% Helium?

For your second question, it depends upon whether you are talking about composition by number, or composition by mass (the numbers you give are roughly the composition by mass).  If you want to get really specific, you can add in the other elements too.

To answer the first, you add them up proportionally.


How much of the Milky Way is visible to the naked eye from earth? When we see the Milky Way on a dark night are we seeing the bulk of the galaxy, or just our local arm? How much of the milky way is visible to the naked eye on a dark night?

At any one time, an average observer can see about 2,500 stars in a clear dark sky. Note that eyesight varies and sharp-eyed individuals may be able to see a half-magnitude dimmer stars than the average eye (apparent magnitude is a scale in which each integer is $2.51$ ($100^{0.2}$) times brighter or dimmer than the next consecutive integer.) A very dark sky may enable magnitude $+7.5$ or even $+8$ stars to be seen, but in a typical "dark" non-urban sky the limit is often $+5.5$ to $+6.5$.

Supernovae can potentially be seen as far as 13 billion light-years (ly) away, essentially from the 'edge' of our observable universe. So, it's not saying much to say that a supernova might be seen across a distance of 100,000 ly.

Some types of supernovae can be over $-22$ in absolute magnitude — where absolute magnitude is defined as their apparent magnitude if they were observed from a 32.6 ly distance (to be honest, I'm not sure if supernovae absolute brightness is defined in exactly this same way). By way of comparison, the Sun's abs. mag. is $+4.8$ (lower numbers indicate brighter stars).

Because of the dust and interstellar medium, the possible distance we can see is just a small fraction of the size of the Milky Way. In reality, few stars are bright enough to be seen over 400 ly away. Deneb which has an estimated distance of 2,600 ly (but may be as close as 1,550 ly, the large uncertainty is due to its variation in brightness). Only 6 visible stars are thought to be farther from us than 1,000 ly.

So, to sum up, looking inwards into the Milky Way, our visibility is very restricted to the nearest 1,000–2,000 ly (while the MW disk's radius is 50,000 to 90,000 ly and we're about 27,000 ly on this side of center, but only the brightest stars are visible from more than about 400 ly away.

While looking away, we can see the Andromeda Galaxy (but not its individual stars) which is 2,500,000 ly from us. In other words, most people blame dust for the poor visibility, but that's mostly only relevant for telescopes. For us, our limited eye-sight is the real barrier (not to mention the dearth of really dark skies).


What is the exact time Earth takes to revolve around its axis? I read that our day is not exactly 24 hours long. If it is not then why weather seasons come at the same time (Summer, Winter, Autumn Spring) ? how it is equalized ?

Update: Sorry I could not make it clear. My meaning was if 3.8 seconds are shorter of everyday the how is the calculation made that summer starts in May and winter start december everytime why it does not move further since. How time is neutralized ? I hope you guys understood this time.

Our day is 23 hours and 56 minutes long, and slowing by an infinitesimal (but measurable) amount each year due to tidal losses.

Our day has a connection with the weather, in that the sun drives all our weather systems, so heating over each part of the globe happens every day, but aside from that, your question doesn't make much sense.

Weather changes may come at the same time where you live (on a 24 hour cycle) but here in Scotland, we still have low reliability on even a 3 day weather forecast, because the weather systems that impact the UK are so complex as heating from the sun drives various air flows.

After your update, I still cannot understand what you mean. There is slippage in accuracy, but this is counteracted by leap seconds and leap years. It has nothing to do with weather. The shortest and longest days happen when they happen and are measurable. They help us define the year.


Could there be additional Neptunian satellites between Larissa and Hippocamp? Plotting the moons of Neptune in terms of semimajor axis and eccentricity using the values given on the Wikipedia page for the moons of Neptune, there appears to be a fairly large gap between the orbits of Larissa and the newly-named satellite Hippocamp.

Are there stable orbits within the gap, and are there likely to be additional moons there?

[My answer may be a bit off as astronomy is just my hobby]

First off, I would assume eccentricity has little function into the investigation because why would it be a moon when it can escape Neptune's gravity?[assuming you mean Neptunian eccentricity] I mean, unless you are thinking about a moon escaping?
Now, let's move onto the distances. You notice when you graph the moons that there is a considerable gap in distance between S/2004 N 1 and Larissa. I believe, after checking this on multiple sites, that there is indeed such a gap.
Neptune Moon Origin
First, let's take a look at one of the most accepted theories on Neptune and its moons. It is well known that Neptune is very close to the Kuiper Belt. In the Kuiper belt, there are objects like Pluto in orbit around the Sun. Well, Neptune having a very strong gravitational pull would actually yank objects out of orbit into its orbit. Most people believe that this is what happened with Triton. Making up more than 80% of the mass of all of Neptune's Moons,  if would have wreaked havoc on the then stable orbits of its original moons.
This consequently could have cause many moons that were large[relatively speaking] to gain/lose eccentricity which would have cause some to spiral inside of the Roche Limit of Neptune ultimately causing the formation of several rings around Neptune. However, in the case of Saturn, we see moons inside of the Roche limit, so we can also deduce that the Roche limit of a planet varies on how massive/large an object is.
We can now conclude that almost all of Neptune's current moons and their orbits are not its original ones. To answer your question, we look at the three body problem. I am not a physicist to I don't know much on the relative equations for the three body system to calculate the exact orbits of possible moons or calculate proof why there isn't a moon. However, I will give you a relative, not full equation blast, explanation.
Three body problem
The three body system states that in long term, the three body problem may lead to either and for the most part, stabilization of the orbits of moons around a planet or, for some period ratios, be the cause of the chaotic motions.
In Neptune's case, Triton introduction has cause for a short period, chaotic motions, which then stabilized into new stable orbits.

In the image here, it shows the relative orbits of stable moons and Neptune's rings. We can conclude that the respective Roche limit of Neptune would cause any possibly try of ring matter collapsing into a moon to be foiled if the mass was too high. This means that only small moons form between rings Adams and Galle, with Galle marking the "will tear anything apart no matter the size" Roche limit.
Moons between the gaps
There can be many moons there in the gaps between the rings, as long as the moon is small enough to fit, and its gravity is not influencing the rings so much that it is noticeable, most likely very small to be picked up by our telescopes. It brings up another problem: How small must a moon be to be no longer a moon?
Moon definition
There isn't a restriction. However, the most widely accepted one is that a moon is an object with a relatively clear orbit[little debris]. We can say that there may be many moons between the rings, but are too small to be detected.
Moons in the large gap you notice
Now moving on, we have a gap between Larissa and Proteus / S/2004 N 1. Clearly, no rings call this area a home. So this gap is somehow mysteriously "uninhabited".
I doubt that there isn't a "moon" there, even if it were really small. Hippocamp is a very small object. In fact, it is the smallest moon discovered around Neptune. The relative Roche limit is not here, so it probably is possible for ring material to recondense into a moon with a stable orbit, and many times, a moon might happen to be covered up when we search. As I said, there is no size limit to a moon, so, because this area is clear of debris, there might be quite a few that we can't find.
A possible scenario [saying that there are a lot of moons] might be that the moons will gravitationally interact with each other, such as kick each other out, part of the three body problem. However, this can be easily detected, as rings would be seemingly affected. This obviously isn't so, due to everything being in a stable orbit, because the "chaotic" period is already passed, and debris settles down and recondense whenever possible.
Note: There can be moons also at the lagrange point if the point is near the "gap" you are taking about.
Another Note: Similar to lagrange points, uhoh mentioned another very good concept of co-orbital configurations, which is the interaction between two celetrial objects. Places with extremely possible stable orbits are called Trojan Points. One example of this is the Trojan asteroid groups which share the orbit with Jupiter. There are two Trojan Points near a given object so those points might harbor many stable moon orbits there too.
Answer
So my guess is that yes, there are stable orbits within this gap, and there is a good chance there are additional moons out there.
We are discovering more and more moons today. Jupiter is one good example.

@uhoh I would think that a considerable gap must be necessary, because moons would kick each other out or collide if the space were too small, forming another chaotic period. Orbits must have settled for moons to reform and since there isn't another gravitational disturbance, there shouldn't be more chaos. Depending on the size of a given moon, the gap must be large enough so that the moon's gravity shouldn't disturb another moon's orbit. Such events would quickly settle if it would happen because objects tend to move to stable places.


Was this an astronomical phenomenon observed in 1689? In a Swedish church record from 1689 a phenomenon in the sky is described. With my very limited understanding it could be a meteorological or an astronomical phenomenon, so here I ask if it could have have an astronomical phenomenon, and what it was in that case.
The place
The observations were made from the village of Fagerhult in Kalmar län in Sweden, at 57°08′55″N 15°39′52″E, in the sky to the North-East.
The time
This was 18 December 1689 (according to the Gregorian calendar – actual date given in the document is Julian 8 December).
It was "around 4 in the afternoon, around sunset". (I think that sunset actually was at about 3:15 PM local time.)
Description
It is described as an elongated cross with the long arm being about 6 times as long as the short arm. I think it means to say that the long arm was vertical, but I'm not sure about that. The cross was shining clearly on the sky, a burning light in the north-east for about an hour. It was almost a clear sky and where the cross was there were no clouds at all.
The account is accompanied by this illustration. It doesn't have the same proportions as I understand the text to say. Maybe the image is better than the text.

(Undoubtely this led to religious ponderings, and that was presumably why this was recorded, but there is actually nothing of that in the note – just this description.)
Was there anything special happening in the sky then that could have to do with this?

This is a really interesting question!  One thing to note is that since it appeared in the northeast at sunset, it is nowhere near the sun - in fact, it is pretty much in the opposite direction as the setting sun.  In the winter in the north, the sun would set in the southwest.  And regarding your comment about the text describing the event as "around sunset" when it would have been 45 minutes later, I think that's not surprising - at northern latitudes in the winter, around the winter solstice, the path of the sun is at a very oblique angle to the horizon, so twilight lasts quite a long time.  So it could easily feel like "sunset" for an hour or more.  Though the sun set at 3:15 PM as you note, at 4:00 PM the sun would still only be 5.3 degrees below the horizon, so civil twilight had not yet ended. Plus, no one in those days was wearing a wristwatch, so a given observer's estimate of time of a particular event may have been less precise than we would be accustomed to now.
So, my best guess is that this might have been related to anticrepuscular rays.  Pros and cons for this hypothesis:
Pros:

Appears at the right time of day, in the right location (opposite the setting sun).
Would give a vertical ray.
Doesn't need clouds in the direction that the image is seen.  (But see my comment below about the crossbar.)

Cons:

Most of the cases I can find pictured have multiple rays, which isn't what is described here.
There is no obvious explanation for the horizontal part of the cross.
I'm not sure if this would persist without obvious changes for as long as described.

Another thought I had, which looks very similar in appearance, but is in completely the wrong direction, is a sun pillar, combined with a stratus cloud, like this (from here):

The appearance is striking, but (a) it would have to be in the sunset direction, and (b) there would have to be clouds.  So it doesn't seem to quite work.
I also thought of the moon, and whether that could give something similar, e.g. if it was just below the horizon.  But that doesn't quite work, either.  I did some calculations, and the moon was 38% full that day (waxing crescent, one day short of first quarter).  At 4:00 PM (assuming that the location is UTC+1 hour), the moon would have been 23 degrees above the horizon, at an azimuth of 161 degrees, i.e. low in the south-southeast.  So that doesn't seem like it would contribute much to something in the northeast.
So clearly I don't have a definitive answer, but maybe something here will be helpful or give others some ideas.


Can I slice off a piece of the Sun? Bear with me. I want to explain this to my 6 and half year old.

If, all things being equal, we could reach out to the Sun with a suitable implement, a gigantic space knife, could we cut a piece off the Sun? Take a slice from it? Do we end up with a body of Hydrogen and Helium, or is it impossible to extract anything from it?

I know that the Sun is a massive ball of plasma, the sheer mass compressing the gas through gravity until fusion is achieved. What I have been asked is, effectively, what happens if you try to take a piece of that plasma? Of our sun?

The idea of removing material from the sun for either fuel or to lengthen its lifespan is called "star lifting". Various hypothetical methods have been suggested, including heating parts of the atmosphere or squeezing it using magnetic fields. My own tentative suggestion is to combine these with chaos control to manipulate turbulence into making coronal mass ejections in desired directions.

The problem is not just that the sun is hot (5777 K at the surface), but that the stuff that is easy to scoop up with a huge magnetic field is a fairly sparse plasma - you don't get that much material despite the vast scales. Of course, you may want to compress the plasma into a smaller and more convenient ball, but now the ideal gas law tells you that it will heat up proportionally - to get a cool hydrogen ball you will need to bleed off all that heat slowly.

Which is why I think Dyson spheres are probably the easiest way to start.


Can a magnetic field of an object be stronger than its gravity? Can a planet, star or otherwise have a magnetic field that is stronger or have more range than its gravity?

It depends on what object it's acting on.  There are many objects, including stars, that have magnetic fields where Lorentz forces on charged particles like electrons and protons are stronger than the gravitational force on them.

Also remember that the strength of the Lorentz force depends on the speed of the particle moving through it, so a fast enough moving electron even here on Earth will receive a larger magnetic force than a force of gravity. This is how the Earth's magnetic field is able to contain charged particles in the Van Allen belts that its gravity could not contain.


Astronomical animations of deep space objects Is there any real animations from NASA or others (not simulation or Artist's impression) like this

https://svs.gsfc.nasa.gov/vis/a000000/a004400/a004442/frames/730x730_1x1_30p/

or this

https://svs.gsfc.nasa.gov/vis/a000000/a004000/a004038/frames/4096x4096_1x1_30p/304A-Frames/

for deep space objects like galaxies, nebula, Binary star, etc?

Or it is really not possible to get animation, because it is too far and everything is looks very slow from earth?

Because whatever I found so far is only solar system animations (planets, moon, Sun, etc), software simulation or Artist's impression.

Yes there are! Here are some examples:

A giant flare in the XZ Tauri system
The light echo from the outburst of V838 Monocerotis
The planetary system at HR 8799

The first two are obviously sequences of still images taken a while apart, the third one uses interpolation to smooth out the motion.


Looking for a Solar System Simulator including the smaller celestial body, for macOS I am researching for a sci-fi novel that will happen in 2136. For this I would like to know specific positions of "smaller" celestial bodies in our Solar System.
An example can be the dwarf planet Ceres and other big rocks in the Main Asteroid Belt.

There will be a lot of traveling in the Inner Solar System and I need to know the relative distances between two objects.

If possible, I would like the tool to be open-source.

Edit: I am looking for a tool like NASA's Eyes. That tool can give a detailed and global view on the Solar System. It has the orbits of different bodies. It also lets me play with time and distance. And most importantly, it can give me the distance between two bodies (https://eyes.nasa.gov/).

The main problem with NASA's app is that it only let me travel to until 2050 :/

Celestia can run on MacOS. With is, you can travel in the solar system and in time. It can give you a good idea of how far things are from each other.


Stabilizers/dampers for refractor telescope I have a basic refractor telescope. It's pretty light and so it catches vibrations and they never really settle down. Can I attach some dampers on it to cushion these?

It depend where vibration are. Often (if you have a good mount for the telescope) all the structure vibrate and there are some good anti-vibration feet that can be useful. If the tripod isn't the problem and the vibration are only near the telescope, probably the problem is the mount that is under-sized for you telescope. In this case the only good way is change the mount with a sturdier one.


What object could I have seen at late June on the Sky? In around late June, I saw two star-like bodies in the sky. At first they appeared to be twinkling, but when I looked closely they appeared to change color; blue, green, a slight red, then yellow.

It freaked me out.

But I'm wondering, did I see dying stars or was it some sort of rare phenomenon occurring because of the earth's atmosphere or was it something else?

Checked on Sky Map on my phone for late June, 2016 in Kolkata - and Saturn and Antares were close to each other around then in the Eastern sky, so these may be the two objects you saw.  The colour change sounds like atmospheric effects affecting the seeing that night.  I don't know if June in Kolkata is particularly worse than usual for atmospheric conditions, but even on a seemingly clear night there can be a layer of turbulent air above you that will diffract the light from the sources and produce unsteady images.  This is usually worse closer to the horizon as well (as these two objects were).
Planets don't usually twinkle, so if Saturn was one of the objects, then it must have been a strong effect.
Nothing to freak out about, and keep looking up! :)


Photodetector Question: Does converting an RGB image to grayscale produce the same result as using a grayscale image detector? FYI my application is not astronomy related, but I know astronomy folks have a lot of knowledge of photodetection and spectrometry instrumentation, so I am seeking this community's help.

I'm interested in doing both spectral and RGB imaging. I'm wondering if I can do both with just an RGB CMOS camera... The way I would approach the spectral imaging is to illuminate my sample with monochromatic light one wavelength at a time and then convert the images to grayscale. Each grayscale image would represent the intensity of light reflecting from my sample at a particular wavelength. I would love for this to work, but I am doubtful that the converted image will actually represent what a grayscale camera would record.

Bit of an odd scenario... Wasn't sure how to search the web for this. Any help is appreciated! Thanks!

Really, all cameras/detectors are greyscale detectors.  Every pixel in a CCD detector puts out a single number for the number of photons it detected. To create an RGB image you need three filters.  If you have 3 detectors, you can put a different filter in front of each and get 3 numbers at once.  If you have 1 detector, then you take 3 measurements with a different filter each time.  To create the color image, one combines these values but you have freedom to weight them as you like.  Typically, one tries to imitate the sensitivity of the eye, but that is a complicated issue since the sensitivity of the eye to different colors changes with total intensity and the filters can not exactly reproduce the eye's bandpasses.  So, there is no right way to do this.  Some people like red more than blue, so they weight the R-band more. Once, you get aR+bG+cB, you loose the information you need to derive R, G, and B separately.

So, to do your experiment properly, you need to first get the response function (response vs wavelength) of the filters + CCD of your camera at each wavelength that you are using.  You do this by imaging without the sample.  Then repeat with the sample and divide "with" by "without", where "with" is the output in the R, G, or B channel that has the most response in the "without" case.


Why does the number of extrasolar planets vary in different sources? There are at least three catalogues that are counting up-to-date discovered exoplanets. What criteria do they use to count the number of planets? Every catalogue has a different count value.

So at this moment various catalogues as:

exoplanet.eu
Open exoplanet catalogue
NASA exoplanet archive
etc.

show values of confirmed number of exoplanets between approx. 3500 and 4050.

Why do they disagree with each other?

As of April 6, 2020:

exoplanet.eu shows 4241 planets;
the Open Exoplanet Catalog shows 3852 planets of which 3504 are confirmed;
the Nasa Exoplanet Archive shows 4144 confirmed planets;
exoplanets.org indicates a total of 5747 planets of which 3262 are confirmed planets;
...

The number of confirmed planets are different from one website to another simply because what exactly is being counted is different. Each one of these databases has a different criteria for deciding whether an exoplanet is going to be counted or not. To understand the differences between these numbers, you have to ask

How is an exoplanet defined?
How solid must the evidence be for the exoplanet to be counted?
What detection methods are included?

The answers to these questions are usually in the methodology section of the website, or in a dedicated article.

Here are a few examples of differences:

exoplanets.org uses a cutoff of 24 Jupiter masses for what constitutes a planet or a brown dwarf, whereas the NASA Exoplanet archive uses 30 Jupiter masses. I haven't found a methodology section for exoplanet.eu, so I emailed the people maintaining the database, and they indicated they use a 60 Jupiter mass cutoff.
exoplanets.org only shows planets that appear in peer-reviewed publications with well-measured orbital parameters, rather than giving a comprehensive list of all planet announcements.
the Open Exoplanet Catalog is decentralized and community driven, so on different instances of the database, you might have different numbers (which the authors consider to be a feature, not a bug).

In addition to this, each of these websites might also be updated at different times. For instance, the NASA Expolanet Archive is updated once a week, but exoplanet.eu is updated daily. But other websites might be updated at different times and with different frequencies. Because of this, two websites with the same criteria might still show different numbers.


Is it possible to define a state of zero absolute angular momentum anywhere in the universe? Conservation of angular momentum doesn't seem correct to me. I think this because radiation is present everywhere, no space is completely void, and no object perfect. Thus all objects will have uneven radiation pressure. My question is there anything in space that is not increasing or decreasing in rotation or is fixed in space? Is the rate in which an object spins is determined by surrounding celestial bodies? Or in other words any object in space depending on its position in the galaxy will slow or increase to an ideal rotation?

Angular momentum does not change unless a torque acts upon a body. So your question is just whether the torque on a body could ever be exactly zero. The answer is of course that it can be arbitrarily small but we can never be sure it is exactly zero.

More interesting questions would have been "does the CMB exert a torque on an object?", or "is it possible to define a state of zero absolute angular momentum?".


Why was there a gap in the number of asteroid detections between 1807 and 1845? The first asteroid, Ceres, was discovered in 1801, although it wasn't called an asteroid yet.  Pallas, Juno and Vesta were discovered shortly after. Then no new asteroids were discovered for 38 years, until Astraea in 1845 when the rate of discovery picked up again, with the 10th asteroid (Hygiea) discovered in 1849.
Why was there such a decrease in new discoveries after the first four asteroids were discovered? And why did it pick up again a few decades later?
Was there a technological advance in the 1840s that made it possible to observe asteroids that were un-observable in the 1810s? Was the drop due to a decrease of interest in the astronomical community for objects between Mars and Jupiter? Did astronomers think they had found them all and stopped searching?

Ceres, Pallas, Juno and Vesta were all discovered between 1801 and 1807. After that, astronomers looked in vain for 38 years until the 5th, Astraea was spotted on December 8, 1845 by German amateur astronomer Karl L. Hencke by accident. He stumbled on Astraea while looking for Vesta one night.
So, what took so long to discover Astrea? There is a paper1 which discusses 4 possibilities:

Astronomers believed that there should be only four asteroids and thus didn't bother to look anymore. This conception came from Biblical theories and ideas.
Astronomers decided to postpone search for asteroids because of inadequacy of good charts and instruments.
Asteroids that were discovered later were not that bright as compared to the four asteroids that were discovered earlier. So, they had to wait for good telescopes to be able to see them.
Political unrest in Europe during that time. So, no one was that invested in astronomy.

The paper concluded that points (1) and (2) are more important and relevant and points (3) and (4) were met with speculation/contradictions. Refer to the paper for more details.
Reference:

Only the first four asteroids, Hughes, D. W.,1997, Journal of the British Astronomical Association, vol.107, no.4, p.211- 213. (link)


Boötes Void numbers, 2016 As a general science (hence Wikipedia!) reader, the latest information I have is that 60 galaxies "have been found and counted" inside the Boötes void, this of 1997.

(1) What is the latest count of galaxies "known" inside the Boötes void?

The Boötes void is a ball 250 million light-years across. (Say, .002 of the width of the visible universe?)  It seems - unbelievable - it has only 60 galaxies.

The "60" value (or some new number since 1997), maybe I am misunderstanding it, or it is misreported, or has a different meaning?  Galaxies are counted in the thousands or tens of thousands, it seems hard to believe in a cosmological space, the number "60" would come up.

So, what is (2) the thinking on how many galaxies there likely are in the Boötes Void? An order of magnitude.  Is it "about 100," or "about a trillion," or what?

What do you (3) normally quote galaxy density in?  Galaxies per Gpc3, or?

Indeed, I am trying to get a (4) comparative sense of galactic density in the Boötes void versus elsewhere. What is the galaxy density in the Boötes void, versus the universe in general, or if it's more meaningful versus perhaps superclusters, walls, or?

Is it "one tenth as dense"?  One zillionth?  One half?  Just how void is that sucker?

Further...

BruceS below has usefully pointed out that, indeed, the "60" galaxies are anyway only in a small tube which apparently cuts through the Boötes Void.

Does this mean that, incredibly, the actual main body of the Boötes Void - putting aside that tube - is literally empty, no galaxies at all??

That seems totally incredible.  I feel we really need a galaxy-counting astronomer to clarify the nature of the Boötes Void.

An ADS search on refereed astronomical papers including the words "Boötes Void" in the title returns only two papers in this millennium, in 2001 and 2002, and even these do not report any new data, but use data from the beginning of the 1990's. I can't find any newer references for the number density of galaxies in the Boötes Void in particular, but a typical value is roughly one-tenth of the average number density in the Universe.

Theoretical approach

I previously answered the similar question of the number of galaxies in an overdensity. This answer is based on an anlytical fit to (observationally calibrated) simulations of galaxy halo formation, giving the so-called halo mass function, i.e. the number of galaxy halos per halo mass. The total number $N_\mathrm{gal}$ of galaxies in a volume $V$ can be expressed as
$$
N_\mathrm{gal} = n_\mathrm{gal} \times V \times \delta,
$$
where $n_\mathrm{gal}$ is the average number density of galaxies in the Universe, and $\delta$ is the relative density in the volume. As I wrote in the other answer, $n_\mathrm{gal}$ is a number that impels you to define a lower threshold of galaxy size. The reason is that the lower you go, the more there are, and there is no formal threshold for how larger a clump of stars you need before you call it a galaxy. But as in the other answer, for the sake of this discussion we may use Small Magellanic Cloud-sized galaxies as our lower minimum. In that case, with $\delta\sim0.1$ and $V\sim236,\!000\,\mathrm{Mpc}^3$, the total number becomes
$$
N_\mathrm{gal,Böotes} = 0.17\,\mathrm{Mpc}^{-3} \, \times \, 236,\!000\,\mathrm{Mpc}^3 \, \times \, 0.1 \, \simeq \, 4000\,\mathrm{galaxies}.
$$

Units of galaxy number density

The way I write this result also answers your question 3: Galaxy number densities are almost always written in $\mathrm{Mpc}^{-3}$. In theoretical/numerical work, you'll often see the factor $h^3$ in front of the unit. This is simply the Hubble constant divided by 100 (i.e. $h=0.7$ for $H_0 = 70\,\mathrm{km}\,\mathrm{s}^{-1}\,\mathrm{Mpc}^{-1}$), allowing people to compare results more easily without knowing the exact value of $H_0$.

Observational approach

The observations of the Boötes Void are old, and seem to have been carried out on 1m-class telescopes. Hence, they won't be able to observe the smallest galaxies. In addition to telescope specifications, weather, etc., the exact detection limit (in terms of a limiting magnitude $m_\mathrm{lim}$) depends on how long they integrate (i.e. expose). Without reading the old papers in detail, I can't say what this is, but a typical value for such surveys would be, very approximately, $m_\mathrm{lim} \sim 20$ (if someone has a more realistic value than this, please edit). That is, objects fainter (i.e. larger values due to the backward astronomical system) than $m=20$ would be missed.

The distance to the Boötes Void implies a distance modulus of $\mu\sim37$, so the minimum absolute magnitude is
$$
M_\mathrm{lim} = m_\mathrm{lim} - \mu \simeq -17,
$$
which is somewhere between the Small and the Large Magellanic Cloud.

The figure below (from Wyder et al. 2005) shows the local-Universe luminosity function for UV-selected galaxies.
That is, it shows the number density of galaxies at a given magnitude. For instance, it shows (with the green dashed line) that the number density (here called $\Phi$) of galaxies with magnitudes around $M=-17$ is roughly $10^{-2.5}\sim0.003\,\mathrm{Mpc}^{-3}\,\mathrm{mag}^{-1}$.

Integrating over magnitudes from the $M=-17$ doesn't change the 0.003 much, since the density of brighter galaxies quckly declines. I get 0.004, i.e. the number density of galaxies at least as bright as $M=-17$ is $0.004\,\mathrm{Mpc}^{-3}$, smaller than the theoretical result above by 1.5 orders of magnitude. Multiplying this by the volume $V$ and the relative density $\delta$ yields $N_\mathrm{gal,Böotes} \simeq 100$ galaxies, not far from the 60 you quote.

To conclude, the number 60 seems in rough accordance with what is expected observationally, but theoretically, we'd expect there to be many more galaxies (although they're very small).

Location of the galaxies

They detected galaxies seem to be lying in a "tube" extending across the void. In general, galaxies and the underlying dark matter mass field tend not to be evenly distributed, but to form knots, sheets and filaments, separated by voids. My guess is that this "tube" is such a filament. Outside this filament, the void is voidier, but not completely void. There will still be galaxies, although few and small.


What is the orientation of the M87 black hole image relative to the jet? On the image of the M87 black hole released today by the Event Horizon Telescope team, which direction points towards the jet? Is it on the brighter side of the ring?

To resolve ambiguity, I'm talking about the main observed jet, not the fainter counterjet.

The jet is projected onto the image roughly in the E-W direction with the (main) jet coming towards us to the right of the black hole.

The brightening in the ring is perpendicular to this. It is probably caused by the spin of the black hole (aligned with the jet), which drags emitting material around and Doppler boosts the emission seen to the south.

This picture is from the fifth event horizon telescope paper published today. The jet is E-W (actually, I believe it is at PA=288 degrees, so rotated 18 degrees anticlockwise) and the black hole spin axis (indicated with the black arrows) is projected at the same PA and into the screen (the two left hand plots).


Is it be correct to say that we live in a young (only 14 billion years old) universe? From what I have read, it seems that our universe is expected to function more or less as it does now for some $10^{12}$ years, possibly more. If that is correct, our universe's current age of $14$ billion years seems like this place we live in is still a baby, or at most a child.

I realize we are a long way from being sure about making such a broad characterization. Nevertheless, does it generally accord with current astronomical and cosmological thinking to say that our universe is still young?

An alternative way of looking at it is that we live in a dying universe. It may have a long, drawn out end, but the peak of star forming activity happened about 10 billion years ago, both in our own Galaxy (see Is there a stellar database that indicates how long ago stars in our Galaxy formed?) and in the universe as a whole (see Will new stars stop forming at some point of time? ) and most quasar activity petered out about 5 billion years ago (e.g. see  Why are all quasars so far away? ).

On the other hand, it is true that the number of (low-mass, very low luminosity) stars in the universe is still increasing (e.g. When will the number of stars be a maximum?), thanks to their very long lifetimes. So although the fireworks are over, the bonfire is still smouldering (to use a UK metaphor that will have others baffled!).

I guess it all depends on whether you are a glass half-full or half-empty kind of person.

Another thing to consider from a philosophical point of view is the Carter catastrophe. The basic idea here is that it is unlikely for us to be alive at a special time just after the beginning of the human race, and that this indicates we are shortly to be doomed. This is normally framed in terms of humans and our survival on this planet, but I suppose could also be applied to the universe as a whole. The arguments that the universe could last trillions of years are based on concordance cosmology with the parameters for dark matter and dark energy fixed. The problem is we don't know what dark energy is or whether it varies in time or exactly what the relationship is between dark energy density and the (negative) pressure it exerts. There are big rip scenarios that may see the end of the universe within a mere 100 billion years!


Kepler's second law of motion Planets sweeps out equal areas in equal times.

Is the area being calculated here the area of a triangle? You draw a straight line from the center of the sun to the earth at point A. The Earth moves to Point B and you have another straight line from point A to point B and the finally another straight line from point B to the Sun? Even though the Earth makes an arc, I guess we make it a straight line.

I guess my question is whether I have understood the second law correctly? That the area of a triangle was being calculated in an elliptical orbit, no matter how elliptical the orbit is.

That is not correct.

The area is the total area between the two radius lines, so there is a curved side.

Imagine you have two points almost 180 degrees from each other. Using just a triangle, the area is close to zero. Now, two points placed closely together can have the same area in between them. Then you have an equal are, but not equal time, so your hypothesis can not be correct.


Relation between age and spectral type of star Does the spectral type of a star change as it ages? Since its temperature changes, I suppose the spectral type should also change, but I'm a bit confused about this.
Also, is the spectral type entirely based on the star's spectrum? If yes, then why do we say O types are hotter and M types are cooler. I mean the statement is true but why not say that O types have very few lines and M types have a large number of lines? Or is it just a phrase stuck through history just like O type being early and M type being late stars (although this one is wrong)?

Yes, the spectral type changes with age. The spectral type is a function of temperature, gravity and chemical composition at the photosphere. All of these can change during a star's life.
A star spends most of its life on the main sequence and changes in temperature and gravity are relatively slow. But thereafter there are comparatively rapid changes. For example, the Sun will become a K-type giant when it is older.
Second question. Spectral type is based on the appearance of the spectrum. However it turns out that when you put the spectral types in the correct order, they are also a sequence of temperatures and gravity.
We say O stars are hotter than M stars because that is a physically meaningful thing to say. The appearance of the spectrum and categorisation is just taxonomy.


Why do sun spots appear near the equator of the Sun? I noticed on many examples sun spots are near the equator of the Sun. Why are there no sun spots closer to the poles of the Sun?

Indeed the regions where sunspots  form tend to be equatorial, with sunspots concentrated near the regions at +/- 40° latitude.

Sunspots are regions at temperature lower than that of the surrounding as for they are closing point of the sun magnetic field. At sunspots, the magnetic field is much higher than average and that hinders heat transport to the photosphere due to a reduced convection.

While the magnetic field of the sun is loosely speaking and as seen by far described as a dipole field (similar to that of Earth or of a magnetic bar), its local distribution is complicated.

Picture from https://www.windows2universe.org/sun/sun_magnetic_field.html showing the magnetic field and spots forming at the loops closing areas.

Besides the fact that the magnetic field flips every roughly 11 years, and keeps changing in meantime, sun, which is not rock solid, has a latitude depending rotational speed. This, superimposed to the dynamo mechanism for stars magnetic field generation, is responsible for "carrying the local magnetic field  along with rotation" and forces the field closing before the geometrical poles.

In the words of Wikipedia under the entry stellar magnetic field https://en.m.wikipedia.org/wiki/Stellar_magnetic_field

"spots are regions of intense magnetic activity on the surface of a star. (On the Sun they are termed sunspots.) These form a visible component of magnetic flux tubes that are formed within a star's convection zone. Due to the differential rotation of the star, the tube becomes curled up and stretched, inhibiting convection and producing zones of lower than normal temperature".

This doesn't provide a clear picture of why the magnetic field behaves as such, but at least does tell that sunspots are a consequence of the magnetic field behaviour.

Understanding the magnetic field of stars and even that of Earth is indeed ongoing research. A little more articulated answer is given by the above linked Wikipedia page, particularly by sections Field generation and Surface activity.


Age of the universe and age of stars The age of the universe is 13.798±0.037 billion years, yet the age of HD 140283 is 14.46±0.8 billion years, how this can be the case?

Wikipedia reports the age of HD 140283 as the figure you cited, 14.46±0.8 billion years. Now look at the reference it refers to. This is the paper by Bond et al that presents measurements of some characteristics of the star and the subsequent calculations, as well as a lengthy section on where the error came from (800 million years is, after all, quite a lot - even in Astronomy!).

The paper describes an approach to determining ages of stars without all that tedious mucking around with globular clusters. It suggests determining "ages of extreme Population II subgiants in the solar neighborhood based on direct trigonometric parallaxes, combined with state-of-the-art theoretical isochrones appropriate to the  detailed composition of each star." In short, the idea is to test stars to see if there ages are consistent with the calculated age of the universe (13.77±0.06 billion years - note the much smaller uncertainty).

The paper cites some previous estimates of the star's age - ~14 billion years and 13.5±1.5 billion years. It's worth noting this because the authors appear to pride themselves on how they tried to reduce the error of their measurements. The team next measured the star's parallax, also attempting to reduce error. With that in hand, as well as measurements of temperature and visual magnitude, they used stellar evolution models to determine its age. They also compensated for extinction from their measurements. The authors list the error from parallax, photometry, and extinction as 310 million years. The measurements then (page 11) list the age "implied" by the models and measurements to be 14.46 billion years.

Where is the extra 490-million year-uncertainty? Take a look at Figure 1, near the end. It is due to a variety of factors, the largest of which is the uncertainty of the "initial oxygen abundance" - although more properly the uncertainty in the ratio of oxygen to hydrogen.

It does come down to what David H said - the error measurements due comply with a universal age of 13.77 billion years. I would note, though, that the uncertainty in the measurement of the age of HD 140283 is much greater (800 million years) than the uncertainty of the age of the universe (37 million years). If the uncertainty due to the oxygen/hydrogen ratio could be eliminated, a more accurate age could be calculated, but until this error is addressed (preferably by another team), I think it's safe to say that it's more likely that the measurements for HD 140283 are off than that the measurements for the universe, and so what could have been a paradox is somewhat resolved.

TL;DR - There's a heck of a lot of error in the age of HD 140283, which fits reasonably with current models of the age of the universe, and I would trust the measurements of the age of the universe much more than the measurements of the age of this star, especially because the method used here is not widely used.


Are there neutron stars whose magnetic axis and rotating axis are the same, and if so what will happen? I know that there's probably a higher chance of having a neutron star that has its magnetic axis inclined to the rotational axis rather than having it perfectly aligned.
If they are not aligned, the neutron star will create beams of radiation that sweep through space like the beams of light from a lighthouse. But what will happen if they were to be aligned?

It is believed that old pulsars may have their rotational axes closely aligned with their magnetic field. This would happen over a timescale of $\tau\sim10^7$ years (Lyne & Manchester (1988)). There are three sets of phenomena driving the dynamics of the alignment (Casini & Montemayor (1998)):

Short-term ($\sim50$ days) variations caused by glitches
Intermediate increases of the alignment angle $\theta$, moving the magnetic dipole moment towards the equator
A set of long-term ($\sim10^7$ years) dynamics leading to a decrease in $\theta$

Only in older pulsars does this last set of dynamics dominate. What this means is that the pulses from the neutron star would appear wider than pulses from neutron stars with significant misalignment. We also would not see interpulses, signals from the opposite magnetic pole, which we see in a number of young (or simply short-period) pulsars; we would just observe one pulse per rotation period.

I will add that Lyne & Manchester identified some young pulsars (including PSR 1800-21 and PSR 1823-13) that exhibited near alignment, a possible indication that the initial offset between the two axes may have a uniform distribution. Therefore, there should be examples of young pulsars with magnetic and rotation axes almost aligned. I would assume the intermediate-timescale mechanisms would then lead to misalignment before the long-term dynamics began to dominate.


Amount of EM radiation (and particles) Earth "receives" from other planets in our solar system? Can it be (or has it been already?) measured how much and what kind of EM radiation (apart from visible light)/particles Earth "receives" from a specific planet in our solar system?

Can other planets (giants like Jupiter/Saturn) also emit neutrinos or some other high energetic particles (like Sun)?

Of course all the planets have been observed by many telescopes at different wavelengths. Yes, they do emit energy at wavelengths other than in the visible part of the spectrum.

Broadly speaking, the peak of their output depends on their temperatures. Most of the light from the outer giant planets emerges in the infrared for example.

However, some planets also are strong radio emitters. For example Jupiter has a strong magnetic field that accelerates charged particles which then emit radio waves.

Neutrinos are hard to pinpoint - telescopes currently struggle to say exactly where they come from in the sky. The planets are not expected to be strong neutrino emitters.


Would studying massive stars that are experiencing time dilation due to proximity to a black hole be useful? I saw this question (Could a star closely orbit a black hole long enough for the star to have lost 0.5B+ years to time dilation?) and came up with an interesting thought. Massive stars live for less than 100 million years, but if it is traveling at relativistic speeds ($\sim 0.1c$) by orbiting an SMBH, we could observe it for slightly longer. Would this be useful, or would it have to be the other way around?
NB: By "other way around," I mean an observer moving at $\sim 0.5c$ or so watching stars age and die.

I don't think this is feasible, because of two competing effects.
To have significant time dilation, the velocity needs to be large.  The velocity in a Keplerian orbit of radius $r$ scales as $r^{-1/2}$, so the star would need to be close to the black hole.
Tidal forces from the black hole (the difference in gravitational force on the near side vs. far side of the star) scale as $r^{-3}$, so they get much stronger for orbits closer to the black hole.  So there would be strong tidal forces trying to pull the star apart.  Massive stars have outer layers that are only loosely bound in the first place (they are close to the Eddington luminosity) and thus have significant mass loss due to radiation pressure pushing on gas in their outer layers.  Tidal forces would exacerbate this, so the star would quickly start shedding mass.
The mass loss would have (at least) two effects.  First, it would generate a lot of luminosity as it heats up and spirals into the black hole; these accretion disks are in fact one of the ways that black holes are observed.  So the star itself would probably be hard to observe separately.  Second, the star would feel a drag from the gas in the accretion disk, and would spiral in toward the black hole, which would make the tidal forces stronger.  It might not seem like the star should feel drag from the gas if both are orbiting at the same distance, and thus should have the same orbital velocity.  But the gas disk is hot, so its internal pressure (or more fundamentally, its pressure gradient with $r$) partially supports it, allowing the gas to have a stable orbit with a slightly sub-Keplerian velocity.  The star, as a macroscopic object, has to orbit with the regular Keplerian velocity, so it feels a "headwind" from the gas.
I would guess that complete tidal disruption of the star would happen before it inspirals all the way to the event horizon, but either way, I don't think it would last long.
EDIT:  As @ProfRob points out in the comments, this analysis doesn’t work for supermassive black holes.  They have very low densities inside the event horizon, and thus relatively weak tidal forces there.


Can Space-Time Itself Have Energy Qualities Like Momentum? Hypothetically, if space could be moved or scaled, what would happen to everything inside?

I'd like to know if an element of momentum could be transferred to objects by contorting space-time.

Other than gravity, what effects does space-time contortion have on mass?

I'm going to build off of chris's original comment. I haven't had a response from him during the 24 hours since I asked the question, and there doesn't appear to have been any activity from him recently (Note: don't confuse him with another user who goes by Chris), so I might as well expand upon what he said.

Gravitational waves do appear to be what you're looking for. They are emitted by systems with varying quadrupole moments (see https://en.wikipedia.org/wiki/Gravitational_wave and https://en.wikipedia.org/wiki/Quadrupole for more information); commonly cited examples are binary neutron stars. In fact, one such binary, the Hulse-Taylor binary, was the first discovered system to emit gravitational waves.

Gravitational waves carry energy away from the system, at a rate of
$$\frac{dE}{dt}=-\frac{32}{5}\frac{G^4}{c^5}\frac{(m_1m_2)^2(m_1+m_2)}{r^5}$$
where $E$ is eneryg, $t$ is time, $m_1$ and $m_2$ are the masses of the objects in the system, $r$ is the distance between them, and $G$ and $c$ are the constants, the universal gravitational constant and the speed of light. I invite you to do the calculations for a given system, if you please. I can assure you that it's one of the easier calculations in general relativity! This release of energy causes the orbits of the two neutron stars to gradually decay, and it is thought that eventually the two will merge.

The answer boils down to this: Yes, gravitational waves can carry [angular] momentum, just like many other types of waves. They also have frequency, amplitude, wavelength, and speed, just like the "normal" waves we are familiar with.

I hope this helps.


What it would look like to observe people with a different time flows? As I learned, that the bigger gravity source you are influenced by the more slow time ticks for you, the farther away you are from a gravity source the faster times ticks.

So Imagine two different planets where on each people live, one planet is massively big (time ticks slow), the other one very small compared (time ticks faster). How would it look like for bigger planet people to observe smaller planet and for smaller planet people to observe them. Hope it makes some sense.

Normal case:

The effect would not be very noticeable unless the large planet was extremely large / high gravity (e.g. it would look "almost" normal) looking at one another.  If the planet was that large, it would probably not be able to support life, but to see what would happen, lets assume it can support life.

Relativistic case, from the large planet, observing the small planet:

If the large planet were extremely "heavy / massive" (think more like a neutron star), the people on the large planet would observe the people on the small planet moving much faster.  It would be like watching a movie with "fast forward" on.  However, light would also be blue shifted, which would produce other interesting visual effects (e.g. what was red might now be blue, and what was in invisible "infrared" might now be red depending on the amount of the shift.)  The amount of blue shift (light wave compression) and "fast forwarding" would increase if the planet were made bigger.  The rest of the universe would have similar effects when observed from the large planet.

Relativistic case, from the smaller planet, observing the large planet:

The people on the smaller planet would observe the people on the extremely large planet moving in "slow motion".  It would be like watching a movie in slow motion.  The people on the large planet would also be "red shifted".  For example, something that was blue might appear red, and something in the ultraviolet range might now appear blue.  The "slow motion" effect and red shift (light wave expansion) would increase if the extremely large planet were made bigger.


Does the edge of the Universe travel faster than the speed of light? The Universe is expanding faster than the speed of light then does that mean matter at the edge of the universe is also traveling at the speed of light or are ether opposite sides of the universe are expanding over half the speed of light equaling more than the speed of light ? Looking from outside of the universe to the nearest edge would light proceed slower or faster combined with the speed of matter traveling on the edge of the universe?

The universe has no edge, as far as we can tell.

Now, the observable universe does have an "edge" - it's the most distant places we can see. But that's just us and our limited instruments.

Due to the expansion of the universe, it is true that two objects, far enough from each other, could actually move faster than light relative to each other. That means light sent from one of them would never reach the other.

However, the speed of light, no matter how you measure it, is always the same. It does not matter how fast or slow its source was moving - when you measure the speed of that light, it's always, ALWAYS the same, for you, locally.

So how can that be?

Well, it's because the universe is expanding because space itself is growing. There's more space all the time in between distant objects. And the further apart two objects are, the faster the space between them is growing.

It's like a canvas with galaxies painted on it, but the canvas itself is growing. However, when light is traveling across the canvas, its speed is always the same relative to whatever galaxies it may find along the way.


Exactly how elliptical is Mercury's orbit, visually, without exaggeration? Reposting question from https://www.reddit.com/r/askscience/comments/5e1zhy/what_does_mercurys_orbit_really_look_like/ which reads:

  First the full question: What does the orbit of Mercury really look
  like visually, without artistic liberties taken to amplify the
  eccentricity of the orbit, or simplifying it to appear as a perfect
  circle? What eccentricities are needed for orbits that readily appear
  elliptical?

I originally did this as a self-answer, but my answer was of such poor quality I've now deleted it. I hope to do a better answer backed by some calculations, but, as always, anyone else is free to answer.

Some things we know about Mercury's orbit:

Semi-major axis: 0.387 AU, about 57.9 million km
Eccentricity: 0.205

We can calculate the semi-minor axis, $b$ from the equation
$$e=\sqrt{1-\frac{b^2}{a^2}}\to b=0.379\text{ AU}$$
We can also calculate the distance to the focus from the center of the ellipse, $f$, as
$$f=ae=0.078\text{ AU}$$
I used Mathematica's ParametricPlot to plot Mercury's orbit using these parameters, along with Earth's. Earth's semi-major axis is 1 AU, and with an eccentricity of 0.016, its semi-minor axis is about 1 to three significant figures; I've treated it as a circle. Mathematica gives

ParametricPlot[{{0.387*Sin[x] - 0.0793, 0.379*Cos[x]}, {Sin[x],
  Cos[x]}}, {x, 0, 2*Pi}, AxesLabel -> {AU, AU}, PlotLegends ->
  Placed[LineLegend[{Red, Blue}, {"Mercury", "Earth"}], {1, 0.25}],
  PlotStyle -> {Red, Blue}]

I'd consider the deviation from a perfect circle to be noticeable insofar as the focus is shifted from the point $(0, 0)$, but it's not incredible.


Why when we look through a telescope in space, do the billions of stars not block our view from seeing further? I know that we are able to "see back into time" by looking very far away through telescopes such as the Hubble telescope, but my question is, wouldn't you run into at least a couple of stars that were blocking your vision when you were trying to look even farther?

Imagine a big Boulder in front of another smaller Boulder  and if we are looking from the side of the larger Boulder, we wouldn't be able to see the smaller one because the bigger one was in the way. Hopefully that simplifies my question. If it is as simple as that, wouldn't we miss stars and not be able to view them?

OK, Imagine the stars at distance x block an area of the sky. At a distance of 2x there should be four times as many stars, but they would seem four times smaller in terms of area covered. Thus, the blocking grows linearly until a significant part of the sky is blocked. The stars within 20 light years (excluding the Sun), blocks approximately $4.3 \cdot 10^{-16}$ of the sky. Even if we scale that up to the radius of the universe, 45 billion light years, only about $2 \cdot 10^{-6}$ of the sky is covered.

Even that tiny number is an overestimate, as we live in an extremely star-dense part of the universe, our galaxy.

Space is simply too big.


Why does the moon sometimes appear giant and a orange red color near the horizon? I've read various ideas about why the moon looks larger on the horizon. The most reasonable one in my opinion is that it is due to how our brain calculates (perceives) distance, with objects high above the horizon being generally further away than objects closer to the horizon.

But every once in a while, the moon looks absolutely huge and has a orange red color to it. Booth the size and color diminish as it moves further above the horizon. This does not seem to fit in with the regular perceived size changes that I already mentioned.

So what is the name of this giant orange red effect and what causes it?

(Source, Wikipedia Commons)

The moon is generally called a "Harvest Moon" when it appears that way (i.e. large and red) in autumn, amongst a few other names.  There are other names that are associated with specific timeframes as well.  The colour is due to atmospheric scattering (Also known as Rayleigh scattering):

  may have noticed that they always occur when the Sun or Moon is close to the horizon. If you think about it, sunlight or moonlight must travel through the maximum amount of atmosphere to get to your eyes when the Sun or Moon is on the horizon (remember that that atmosphere is a sphere around the Earth). So, you expect more blue light to be scattered from Sunlight or Moonlight when the Sun or Moon is on the horizon than when it is, say, overhead; this makes the object look redder.

As to the size, that is commonly referred to as the "Moon Illusion", which may be a combination of many factors.  The most common explanation is that the frame of reference just tricks our brains.  Also, if you look straight up, the perceived distance is much smaller to our brains than the distance to the horizon.  We don't perceive the sky to be a hemispherical bowl over us, but rather a much more shallow bowl.  Just ask anyone to point to the halfway point between the horizon and zenith, and you will see that the angle tends to be closer to 30 degrees as opposed to the 45 it should be.

University of Wisconsin discussion on the Moon Illusion.

NASA discussion on moon illusion.

A graphical representation of this:

Dr. Phil Plait discusses the illusion in detail.


Why do Auroras only occur in polar regions? Why is it that Auroras only occur in the North and South poles. Why don't they occur in the equator or anywhere for that matter?

The reason is because of Earth's global magnetic field. Take a look at the image below which shows a representation of what the magnetic field lines around Earth look like.

What you see above is what's known as a Magnetic Dipole field. The reason being that there are two (Di means two) magnetic poles, namely the north pole and the south pole. This is completely analogous to a giant bar magnet you may have played with in science class. You can see here that the field lines leave the magnetic south pole and travel around, going up into the magnetic north pole. The field lines don't really leave from or enter the Earth's equator.

But why does this mean aurora's happen at the poles? Because, according to the theory of electromagnetism, charged particles are forced to travel only along magnetic field lines. They cannot travel across them or break free from them. Once they get stuck, they're stuck (this isn't 100% true but for the purposes of this answer, it is). Note that the particles can travel both forward and backward on the field lines. They're not constrained to going only in the direction of the field line. This means that the primary location in which these charged particles can enter the Earth's atmosphere is where the field lines tend to hit the atmosphere and we already noted that that location was the magnetic north and south pole (which coincidentally is near the physical north and south pole).

Auroras themselves are just emissions from our atmosphere in response to being bombarded by high energy particles. To put this all together, here's what happens: The Sun spews out huge amounts of energy and tons of charged particles. Some of those charged particles hit the Earth's magnetic field and get trapped in it, being forced to travel along the magnetic field lines. They are then constrained to primarily hit the Earth's atmosphere at the magnetic poles where their extreme energy excites the atmosphere, causing it to emit light in shimmering red and green sheets. The auroras can't occur at the equator simply because not enough particles can hit the atmosphere there to cause auroras.

One day though, we may see auroras at the equator. The Earth's magnetic field is not fixed and moves with time. About every 450,000 years the field flips around completely. Its been about 750,000 years since the last reversal so we're due for one potentially soon (and by soon I mean in the next few thousand or tens of thousands of years). During the flip, while the magnetic poles are over the equator, you'll see auroras there.


Does the Zwicky Transient Facility only run when triggered by a cell phone? A really enjoyable interview with Mansi Kasliwal & Samaya Nissanke ("From discovery to making precision measurements") is available as a short audio podcast in the BBC News article Gravitational waves hunt now in overdrive describes the very recent gravitational wave events and the way optical searches for the source were initiated by pressing a "go button" on a cell phone!

It's a excerpt from the BBC's Science in Action episode What is behind the Indian Ocean Cyclones?

The article is quite a good read as well, and talks about the Zwicky Transient Facility (ZTF), based on the 48-inch Samuel Oschin Schmidt telescope with several upgrades including a state of the art silicon CCD array replacing the 14 inch square photographic plates it originally used, faster slewing and AI-based recognition of candidate objects.

The huge aperture allows for almost 50 square degree field to be checked down to +20 magnitude every five minutes. That's about one steradian in 5 hours. According to the podcast, it can run as fast as 4000 square degrees per hour, about 1.2 sr/h presumably with a brighter limiting magnitude.

Question: Does the ZTF only start surveying when triggered by a cellphone or other alert, or does it continue to search for new objects and events on some nominal routine whenever there is good visibility?

The ZTF is operating on most clear nights on an automated queue of pointings. When they discover transients, they inform TNS which may trigger other observatories to point at the same object, just as ZTF may respond to transient events from other observatories.


What lies at the center of a neutron star if any? What does the current theory tell us about what is going on at the core of a neutron star, I'm expecting a black hole hopefully I won't be disappointed!

Certainly not a black hole! That would not be stable situation at all.

The content of the core of a neutron star is the subject of much speculation. The possibilities fall into a number of categories. (i) An increasingly hard neutron equation of state, such that neutrons retain their identities as they are squeezed closer, but an increasingly repulsive many-body strong nuclear force provides support. (ii) Additional hadronic degrees of freedom, such that the neutrons (and protons) transform into other heavy hadrons such as lambda or sigma particles. (iii) Some sort of quark plasma. (iv) Boson condensations involving the neutrons decaying into pions or kaons with zero momentum.

There are a number of diagnostics of these possibilities: primarily, the maximum possible mass of a neutron star should decrease from about 3 solar masses for (i) down to about 1.5 solar masses for (iv). Secure measurements of a 2 solar mass neutron star would seem to rule out (iv), but even that seems not completely agreed. Another diagnostic is how quickly neutron stars can cool. The presence of quark matter or boson condensations should lead to much more rapid cooling by neutrino emission. Again, nothing conclusive has emerged yet.


Have we seen a black hole? Have we actually seen a black hole out there? By "seen" I mean "recorded a em-spectrum image of gravitational lensing caused by object believed to be black hole".

The answer was no prior to 10 April 2019. There were no resolved images at any wavelength of black holes or black hole candidates that demonstrated their lensing effect.

There were of course lensing images due to massive objects that probably have black holes at their centres (e.g. Courbin et al. 2010 and see below), but that is not the same thing.

A quasar acting as a gravitational lens - Courbin et al. (2010)

All this has changed with the release of a reconstructed millimetre-wave image of the supermassive black hole at the centre of M87.

The image below, obtained by the "Event Horizon Telescope" (a global interferometer) is formed by intense light from accreting matter being lensed by a 6.5 billion solar mass black hole. The dark circle is known as the "photon shadow" and is about two times the size of the black hole event horizon. The bright ring is caused by the lensing of light from around and behind the black hole. It is thought to be asymmetric and off-centre because of the black hole rotation.


Are there large underground caverns on Mars? Scientists talk about the possibility of life underground on Mars, and I assume they are talking about microscopic life that can live in small gaps in the soil or rock.  But is there any evidence that there might be large caverns under the surface of Mars relative to the human scale or even larger?

Then I wonder how could we even detect or map such subsurface features on Mars or other rocky bodies in the solar system such as Titan, Venus, or even the Moon.  Could we use radar or maybe sonar for bodies with substantial atmospheres from an orbiting spacecraft?  Are there other methods for detecting sub-surface cavities?

As evidence of caverns detected on Mars, consider the following recent image taken from HiRISE instrument on the Martian Reconnaissance Orbiter :

Image source: NASA

Scientists believe that

  The hole appears to be an opening to an underground cavern, partly illuminated on the image right. Analysis of this and follow-up images revealed the opening to be about 35 meters across, while the interior shadow angle indicates that the underlying cavern is roughly 20 meters deep.

Which accompany larger openings, reported in the National Geographic article Mars Has Cave Networks, New Photos Suggest (Norris, 2007) of several larger pits detected by Odyssey, and they offer a possible origin:

  The surface of Mars is strewn with craters from meteor impacts and depressions formed by the collapse of underground chambers formed by flowing lava, the experts said.

There are many examples of lave tubes (and indeed, in other places in the Solar System), caverns etc reported in the article Mars Cave-Exploration Mission Entices Scientists (Wall, 2012), who also state, in regards to detection and exploration of these caverns that robotic means are being explored.

However, according to this NASA article, a method to find and characterise caves on Mars (or indeed Titan, and maybe other places, including the Moon) is to detect them via thermal differences caves cause in comparison to the surrounding surface.  This method is used on Earth, and

  Techniques developed
  through this research will ultimately be applied to
  locating subterranean cavities on the Martian surface.
  We anticipate one of the best techniques for systematically
  finding caves on Mars will be via remotely
  sensed thermal imagery.


What's the smallest scale at which dark matter has been measured? What is the smallest scale at which we have detected "dark matter" (more kinetic energy than would be predicted)?

If you don't mind indulging a follow-up: Would its detection on smaller scales be beneficial at all to advancing our understanding?

(aside: I base this question purely on the notion that our instruments have surely improved a bit since the 1930s.)

It looks like Segue 2, found by the Keck Observatory, is a candidate for the lightest galaxy, but I don't know if its the smallest one.


Are the darker blue/black areas in this picture what the atmosphere of Jupiter looks like without clouds? This is a picture (mosaic?) of Jupiter's south pole taken by Juno (also shown below). It has gotten a lot of press, but I haven't seen anything, even in the scientific paper that accompanied its release, that answers the first question that came to mind when I saw it : is this a view into the inside of Jupiter? I seem to recall seeing IR images of Jupiter where the poles appear quite hot, which would be consistent with a "clear sky" providing a view of deeper, hotter layers. But how deep can see in this picture?

Edit: I am aware that this is a visible light image, and that visible light does not penetrate the cloud layer of Jupiter.

My question is whether part of the south pole of Jupiter is cloud-free, at least to a depth where atmospheric extinction limits our ability to see the clouds. This interpretation of the picture would assume that blue light penetrates deeper than red light into the atmosphere (clouds not included) of Jupiter, and that this is the reason that some of the clouds, presumed to be deeper, look blue, while some areas between them are essentially black.

Here is an example of a picture of Jupiter in IR, which shows the poles as being relatively hot. The troposphere of Jupiter (or any planet) is generally hotter the deeper you go, so the poles appearing hotter is consistent with less cloud cover at the poles. [

I'm aware that perhaps no-one, including the Juno team, knows the answer to my question. Some useful partial answers would include an estimate of the depth to which visible light could penetrate Jupiter's atmosphere in the absence of clouds, and whether blue light would penetrate deeper than red.

userLTK's comment is correct, you cannot see very deep at all.

Information about the image

First off, there are three imagers on the Juno spacecraft: JunoCam, UVS, and JIRCAM. You can see a full list of instruments here. JunoCam is described as a:

  visible-light camera/telescope of the Juno Jupiter orbiter, a NASA space probe launched to the planet Jupiter on 5 August 2011. It was built by Malin Space Science Systems. The telescope/camera has a field of view of 58 degrees with four filters (3 for visible light).

The image you're seeing was taken by JunoCam and is a visible image. In fact, this image is combined by images taken over multiple days and from multiple orbits to get different vantage points and show all areas lit up. It isn't however a mosaic $-$ the 58 degree field of view allows JunoCam to image the entirety of Jupiter in one image. Instead, this image is simply the carefully crafted combination of many different images of the south pole.

FWIW, UVS is an imager in the ultraviolet and JIRAM is an imager in the infrared.

How deep are you seeing into Jupiter?

Not very deep at all. Because the image is in the visible spectrum, the depth into Jupiter before it becomes too opaque is going to be very, very short. You may not even be seeing down to Jupiter's "surface" where the atmospheric pressure equals 1 bar.

Incidentally, JIRCAM can see down to about 5-7 bars if its lucky, enough to penetrate the very upper layers of the atmosphere.

Infrared images of the poles

Obviously I can't be certain what you saw before concerning infrared images of Jupiter's poles, but my guess is they may have been something like the image below. This image was taken by JIRAM


Are we looking for brown dwarfs where the dark matter has to be? We have been repeatedly assured that their ain't enough brown dwarfs to account for all the dark matter in our galaxy.

My question is have we been looking in the right places?

Or in more detail. Where exactly does computer modelling put the missing mass?

I would expect the bulk of it to be on the galaxy's plain and on the edge. And if I am not mistaken there is too much junk in the way to look.

Further I can't see any reason why the dark matter won't be unevenly distributed in clumps just like everything else.

So, given the above an extrapolation from what we have detected will be way low if it assumes an even distribution in all directions, and horribly low if the LMG and GMG happen to be between clumps.

The dark matter has to have a roughly spherical distribution and a smooth radial distribution if it is to account for the dynamics of gas and stars in our and other Galaxies.

The exact shape of any required dark matter halo is still the subject of debate and research. Whilst computer simulations of galaxy formation predict mildly non-spherical or triaxial profiles (e.g. Kunting et al. 2018) and some stellar kinematic data appear to confirm sphericity (e.g. Smith et al. 2009 ); others disagree and suggest there is some significant prolateness ( Bowden et al. 2016) or oblateness suggested by large scale surveys (e.g. Loebman et al. 2014).

Any concentration of dark matter into a disk is totally ruled out by observations of the dynamics of stars above and below the plane.
e.g. The latest word, using Gaia DR2 data, is that there can only be about 10% of the local mass density in the form of dark matter of any kind (Buch et al. 2019, agreeing with lots of previous work. For instance this was established fact at least 30 years ago (Kuijken & Gilmore 1989).

The amount of dark matter required at different radii (assuming Newtonian gravity is the correct formulation)  is deduced from the rotation curve. The image below, courtesy of Nick Strobel at www.astronomynotes.com, shows a sketch of the current thinking on how much dark matter needs to be at what radius (labelled as "Corona") and takes the dark matter to be distributed spherically symmetrically.

Dark matter already "outweighs" luminous matter by 2:1 at the solar radius, yet there is room for only 10% of the local density to be in a "dark" form of any kind.

Brown dwarfs are not actually "dark objects" that cannot be found. They emit infrared light and surveys near the Sun reveal that stars outnumber them by 4:1 and that they contribute less than a few percent the local mass density.
They can also be found via gravitational microlensing, in surveys that have been performed towards the Magellanic clouds (sampling the halo) and towards the Galactic bulge (sampling the inner disk). In neither case has evidence been found for huge numbers of brown dwarfs (or cold white dwarfs or black holes) and these have been ruled out as significant contributors to the Galactic dark matter problem (e.g. Pietrzynski 2018). In particular, the numbers of microlensing events towards the Galactic bulge is perfectly consistent with the brown dwarf/star fraction in the local neighborhood (Niikura et al. (2019).

As a final remark, dark matter (whether in the form of brown dwarfs or non-baryonic particles) doesn't clump very easily because it cannot readily dissipate it's kinetic energy. There is little evidence for any clumping of dark matter on scales smaller than dwarf galaxies. In particular no evidence for dark matter is found in globular or open clusters of stars.

The stellar disk forms because the stars formed more-or-less in a disk that had already formed from dissipative baryonic gas that had already collapsed into a disk.  As brown dwarfs must also form from baryonic gas then there are really only two alternatives for their distribution. Either they follow the disk and should be associated with disk stars or they should be very old and associated with halo stars that formed in a spherical gas cloud before it collapsed to form a disk and therefore still have a spherical distribution.

The dark matter needs to be much more extended than the luminous matter of the Galaxy. In neither scenario described above do we end up with tens of trillions of brown dwarfs (for that is what would be required) distributed a long way outside the solar circle and unassociated with any luminous matter.


Will the E-ELT use Adaptive Optics at visible wavelengths? In this recent BBC article I read that the European Extremely Large Telescope or E-ELT is in final design and is planned to be on line by 2024, and that (of course) it will rely heavily on adaptive optics (AO) technology.

In a great answer and discussion related to my previous question, it seems that while most AO is actually implemented for infrared imaging, work is being done to push the technology to visible wavelengths.

Will the E-ELT have AO available for visible light imaging in the beginning, will it be phased in later, or is there currently no plan for visible light AO?

note: Adaptive Optics (AO) techniques allow ground based observatories to dramatically improve resolution by actively compensating for the effects of Astronomical Seeing through dynamically deformable optics. AO is also implemented in radio astronomy computationally applied when data from arrays is combined, as described in this answer.

A drawing of the E-ELT from from Wikipedia (note scale of humans):

From looking at the E-ELT website, it appears that at first light, the AO will only work for near-IR. Specifically, the instrument that can use AO is the MICADO instrument. The description page for this instrument states

MICADO, or the Multi-Adaptive Optics Imaging Camera for Deep
Observations, is one of the first-light instruments for the European
Extremely Large Telescope (E-ELT) instrument and takes the Adaptive
Optics technique to the next level. It will be the first dedicated
imaging camera for the E-ELT and works with the multi-conjugate
adaptive optics module, MAORY.
MICADO will equip the E-ELT with a first light capability for
diffraction limited imaging at near-infrared wavelengths.

The MAORY instrument will be the specific AO instrument which, at first light will operate at

wavelengths from 0.8–2.4µm

So in short, the answer appears to be that the E-ELT will only have AO capabilities for near-IR at first light.
As to whether or not there are plans for visible AO capabilities, I couldn't find anything specific to that question. Given that first light for this project is 8 years away at best, I think that plans for future upgrades are tentative at best, and likely not easily accessible to those not on the project. I will say that while the MAORY instrument appears to be the intended AO system at first light, there is listed a second AO instrument called ATLAS that will eventually make its way onto the telescope. While I can find no specific details of this particular instrument, its possible that this will provide for visible AO.
On a tangentially related note, this telescope will also have active optics that will help to improve their imaging capabilities.


Are there ways to determine a star's spectral type without the spectrum itself? Because of the lack of available data online, I am trying to find other ways to determine an exoplanet's host star's spectral type.

The color index of the star can give you that information. I can look it up for you if I know which exoplanet you're interested in, or you can find it in the NASA Exoplanet Database here.


Apparent magnitude of Milky Way? We have apparent magnitude of stars, planets, DSOs. What about the apparent magnitude of Milky Way?
Can it be estimated?

We have apparent magnitude of stars, planets, DSOs.

That's true, and these are calculated from the total integrated brightness across the apparent size of the object for an observer on Earth.

What about the apparent magnitude of Milky Way?
Can it be estimated?

There are two options if one wants an apparent value:

Try to get the total integrated brightness of the milky way as seen from Earth, either integrated over the whole celestial sphere or by choosing a place on earth, season and time of night and using that part, and converting that to a magnitude
Switch over to a surface brightness approach, i.e. convert a brightness per unit solid angle to a magnitude per unit solid angle.

While option 1 can be executed in a straightforward way, and might be useful if one is trying to photograph a scene on Earth lit by the Milky Way in a similar way to how one might light a scene by blue sky brightness by blocking out only the Sun, it's not very helpful if one asks how bright the Milky Way appears, because it is huge compared to a tiny fuzzy nebula or planet, stretching across the whole sky rather than being a barely-resolved spot.
Option 2 is what's normally done for a large extended object. This large table of apparent magnitudes only includes objects of small angular size, the largest being that of the Sun or Moon (each roughly 30 arc minutes in diameter as seen from Earth).
Wikipedia's Sky_brightness; Relative contributions explains some of the concepts required for this kind of calculation.
                      Surface brightness [S10]
Airglow                     145
Zodiacal light               60
Scattered starlight         ~15

and S10 is the brightness of a 10th magnitude star smeared over the solid angle of one square degree.
So to "Can it be estimated" my answer is yes!
From Wikipedia's Milky Way; Appearance:

The Milky Way is visible from Earth as a hazy band of white light, some 30° wide, arching across the night sky.[38] In night sky observing, although all the individual naked-eye stars in the entire sky are part of the Milky Way Galaxy, the term "Milky Way" is limited to this band of light.

and

The Milky Way has a relatively low surface brightness. Its visibility can be greatly reduced by background light, such as light pollution or moonlight. The sky needs to be darker than about 20.2 magnitude per square arcsecond in order for the Milky Way to be visible.41
Crumey 2014 open access in MNRAS: 41Human contrast threshold and astronomical visibility also readable in arXiv

Further reading here in Astronomy SE related to apparent magnitude and smaller objects in contrast to something as large as the Milky Way:

What is the difference between albedo, absolute magnitude or aparent magnitude?
Venus' magnitude during inferior conjunction
How (the heck) was this photo of Venus at inferior conjunction (between us and the Sun) taken?
Can you see the difference with the naked eye whether Venus is at the other side of the Sun or at the Earths' side?


Converting an RGB image to fits (astropy) I am trying to save a NumPy RGB array (a 3d array with dimension 512x512x3), to a .fits file using astropy.io.
Saving this array to a .png file works fine, but when I'm trying to save it to a .fits file using HDUList.writeto, it produces just garbage colors.
Also, I have to reshape the image like so:

img = img.reshape(img.shape[2], img.shape[0], img.shape[1])

or it will try to store just 3 column of the image instead of 512.

Each element in the 3d array contains a value between 0 and 255.

I haven't done much astronomical image processing before, but as this question is unanswered I'll give it a shot - hopefully to some avail. If the problem is more specific, a code sample/image sample would probably be useful for further diagnosis, but otherwise this example may help. It discusses the process of writing a 3-channel image to separate FITS images. I would try this first, and check if this output is also garbage - it might just be an issue with how you're using the HDUList.writeto() function. The relevant code is pasted below (with a couple of edits, as the example uses the Pillow Image class at first):

Split the three channels (RGB):

r, g, b = img[:, :, 0], img[:, :, 1], img[:, :, 2]

Write out the channels as separate FITS images

red = fits.PrimaryHDU(data=r)
red.writeto('red.fits')

green = fits.PrimaryHDU(data=g)
green.writeto('green.fits')

blue = fits.PrimaryHDU(data=b)
blue.writeto('blue.fits')

Also out of curiosity - why is your "numpy RGB array" storing values between 0 and 512? The standard for images is usually integers between 0-255 or floats between 0 and 1; but my guess is that astronomy requires a larger bit depth? In any case, you should also make sure the dtype of your numpy array is large enough for the desired bit depth - often images are stored with dtype="uint8" which ranges from 0 to 255.


How did I flip some mirrors around in the dark at 3 AM and change the focal length of a 24 inch Boller and Chivens? Current answer(s) to How do telescopes "zoom" and change angle of view? are "they don't", but traditional large genera-purpose observatory telescopes do sometimes (often?) have multiple focuses with different f/ numbers.
I vaguely recall about 1 mya a 24 inch (0.61 meter) Boller and Chivens with a primary focus at the back for photographic plates, and a little door somewhere along the side of the telescope where you would reach in with one arm, feel your way around and somehow rotate something into place that would send the light path down though the mount into a big room on the floor below where a pair of grating spectrographs sat on concrete pedestals, a "normal" one and an Echelle, both with photographic plate holders.
During the procedure fiddling around inside the telescope, one had to be extremely cautious not to touch any optical surfaces; which at 3 AM in the dark it was quite a challenge.
Presumably this presented a far longer f/no. focus for the slits. We;d place the slit across Jupiter, develop the Echelele plate, and measure it's rotation rate on a Mann measuring machine if I remember correctly.
Question: How did I flip some mirrors around in the dark at 3 AM and change the focal length of a 24 inch Boller and Chivens?

Example of a similar instrument found on the internet: Eastern Iowa Observatory offers a chance to see the stars and so much more. The "mirror-swapping door" of which I speak is probably on the other side of the telescope.

What you did was to switch in a Coudé mirror, which sends the light down the equatorial mount as you described, to other instruments (besides the original focal point of the secondary mirror)
There are several designs to a Coudé path. The simplest uses a flat mirror and then places a second camera at the re-positioned focal plane, or re-images that focal plane using relay lenses.

Other designs can use a convex mirror as the first Coudé mirror to extend the focal length and send the light down into a "Coudé room" where other instruments are located.


Is it true that the sun goes down to the west? I know the sun doesn't go down but we are moving and is it true that the sun goes down to the west? I've read on many webpages that this is not entirely true.

Earth rotates. That gives us the feeling of Sunrise and Sunset. Actually there are no such things. See Below animated image. Earth completes one rotation in a day (24hrs). (Actually 1 rotation of earth is called a "Day").

This image also depicts the cause of Solstice and Equinox, earth's tilted axis.