What is the best database for identification of spectral lines? What is the best database for the identification of spectral lines? For instance, I have a red spectrum of A0 star and I would like to recognize the most prominent lines, where to find them? Many thanks.
EDIT:
Why is O2 line not in NIST? It would be about 2676.99 A.

source: http://www.spektros.de/sirius/sirius.html
After advice:
How to install the database for molecules https://bitbucket.org/OES_muni/massiveoes/src/master/? I obtained the following error:
python setup.py develop
/home/linux/.local/lib/python2.7/site-packages/setuptools/dist.py:474: UserWarning: Normalizing '1.001' to '1.1'
  normalized_version,
running develop
error: can't create or remove files in install directory

The following error occurred while trying to add or remove files in the
installation directory:

    [Errno 13] Permission denied: '/usr/local/lib/python2.7/dist-packages/test-easy-install-547.write-test'

The installation directory you specified (via --install-dir, --prefix, or
the distutils default setting) was:

    /usr/local/lib/python2.7/dist-packages/

Perhaps your account does not have write access to this directory?  If the
installation directory is a system-owned directory, you may need to sign in
as the administrator or "root" account.  If you do not have administrative
access to this machine, you may wish to choose a different installation
directory, preferably one that is listed in your PYTHONPATH environment
variable.

For information on other options, you may wish to consult the
documentation at:

  https://setuptools.readthedocs.io/en/latest/easy_install.html

Please make the appropriate changes for your system and try again.

There is the NIST Atomic Spectra Database where you could browse by elements. This the reverse approach, meaning that you have to first query element by element and then see which of the lines you find in the spectrum - that's how we used to do it in our labs during studies:

Then, there is another NIST site where you can enter up to 4 spectral lines in units of electron Volt to be identified:

I am aware that there is also at least one old commercial software (some name starting with Win) to do that, running under Windows 3.1.
There is also custom written C-code used by research groups working on the topic, but I have to dig to figure out whether these software is still available. All I found is a software called Spectrum Analyzer which I have not tested.


Can Pluto be seen with the naked eye from Neptune when Pluto and Neptune are closest? When Neptune and Pluto are closest, about 100 million mi (160 million km) from each other, would an observer on Neptune (or rather on one of its moons, since Neptune is gaseous) be able to see Pluto, and maybe even Charon, with the naked eye? If not, could Pluto be seen in average binoculars? I think Pluto would appear a bit smaller than Mercury from Earth (but at much lower apparent brightness because of the distance from the Sun).

No, it cannot. Far from it.
The closest approach between both planets is roughly 16 AU due to the 3:2 orbit resonance. Pluto will even then be a tiny dot among many with a brightness around 14 mag.
You can try that with Stellarium yourself, placing the observer on Neptune and looking for Pluto. You just have to find the right time. One such time is approx. in the year 2877.


Has a gravitational microlensing event ever been predicted? If so, has it been observed? This answer to Are astronomers waiting to see something in an image from a gravitational lens that they've already seen in an adjacent image? describes "Deja vu all over again" (SN "Refsdal"); after multiple gravitationally lensed images of a supernova were observed with different path lengths an shifted light curves, one more "echo" was predicted then indeed was later observed.
While that's a rarity, occultations of objects by other objects are regularly predicted, and some of the nearer objects are pretty far, at least in the Kuiper belt. But alas they are not usually very massive.
Gravitational microlensing events are defined by a peak in a light curve rather than a measurable deflection. From How are microlensing events used to constrain the size of innermost stable circular orbits around spinning black holes?:

Example light curve of Gravitational Microlensing event - OGLE-2005-BLG-006 Source

Question: Has a gravitational microlensing event ever been predicted? If so, has it been observed?

Related to stellar occultations by distant objects:

Where and how to look up the coordinates and magnitude of the star in this occultation prediction?
What is the farthest reported distance from observer from which a solar system body has occulted a star?
What discoveries have come from the DASCH project (Digital Access to a Sky Century @ Harvard)
Could stellar occultations be used to survey Kuiper belt objects?
this answer to Is it possible to mount a telescope on a plane? Is it beneficial?
What caused this mysterious stellar occultation on July 10, 2017 from something ~100 km away from 486958 Arrokoth?
Is 486958 Arrokoth (2014 MU69 aka Ultima Thule) the only solar-system object determined to be binary by occultation?

In the pre-Gaia era, this was effectively impossible to do, since positions and proper motions weren't precise enough for a large enough sample of stars.  With the release of Gaia data, though, it has become an active area of research.
Bramich 2018 predicts 76 events, while Nielsen & Bramich 2018 extend this to predict an additional 27 events using Pan-STaRRS data for very low mass stars as additional lensing objects against Gaia background stars.
Klüter et al. 2018 also predict 3914 events, and there may be other papers as well.
Note that many of the predicted events are for astrometric microlensing, where the position of the background star would be measurably shifted, rather than the usual photometric microlensing, where it gets brighter.  The latter requires closer alignment for a measurable event than the former does. But some of the events would yield measurable brightening as well.
McGill et al. 2020 suggest that many of the predictions are flawed, being due to either binary companions of the lens star (which would be co-moving and thus would not be lensed, since both stars are moving together) or to cases where there is a duplicate detection in the Gaia catalog (i.e. the candidate lens star and background star are both the same object).  They revise the overall list of predictions, but still retain many events.
As far as I can tell none of these events has been observed yet, but the predictions are there.


Effect of Hot/Warm Jupiter atmosphere loss on other planets Let us suppose a solar system with a Hot/Warm Jupiter HJ (with a semi-major axis somewhere from 0.5 AU to 0.0001 AU) and an Earth-Like planet EL in it (with a semi-major axis of about 1 AU). Let their orbital inclinations be a couple of degrees apart. Let the star be a G2V or so, like our Sun.

According to this and other sources, we can tell that Hot Jupiters are "bleeding" atmosphere due to their proximity to their sun. Some of it, I presume, gets pushed out with the solar wind farther out into the solar system, at least as far as the EL is.

What effect will this bled atmosphere from HJ have on EL, if any?
If there is an effect, when does it happen? On all of EL's orbit, despite the different inclinations? Only when EL crosses a transit node? Only when there's a transit (or rather, when EL passes through the transit node a certain amount of time after HJ does, to account for the velocity of the atmospheric particles through space)?
Will this bled atmosphere be (naked-eye) visible from EL? What would it look like?
Would all of this have any effect on life/civilization on the EL?

The primary effects wouldn't be the things you ask about.   First, an object the size of Jupiter would change the day and night sky drastically.    The sky now is basically, Sun, Moon and a bunch of yellow dots (and one reddish dot) mostly fixed, 4 move around visibly to the naked eye (Venus, Mars, Jupiter, Saturn).

Venus stands out a little as larger and brighter, but it looks pretty much like an abnormally bright star.   All that changes if you put a Jupiter sized planet in the orbits around where Mercury or Venus are.   Jupiter is about 1/10th the diameter of the Sun.   If you put it about midway between Earth and the Sun it would be about 1/5th the diameter and transits over the sun would be obvious, unlike Venus transits which are observable (not a good idea without protective lenses), but easily missed.

Jupiter in an orbit between the Earth and the Sun would have apparent phases like the Moon and visible size variation.  It's likely that in such a scenario, the geocentric model would never have been adapted, cause Jupiter would raise too many questions in this hypothetical.   Aristotle and Ptolomy were smart.   This situation would have been enough to perhaps look at things differently.

There's also the 3 body gravitation problem.   Jupiter inside the Earth's orbit might wreck havoc with the Earth's orbit.  Earth's eccentricity might vary a lot more perhaps leading to more extreme climate change periods.   There's a good chance an Earth like planet's relatively stable orbit would be subject too a lot of undesirable changes with a hot Jupiter between it and the sun.  Comet impacts could be a problem too.   A distant Jupiter changing the orbit of passing comets helps the Earth avoid comet impacts.  A Jupiter that close to the sun could do the opposite.    There's also the migration problem.   If Jupiter forms out far from the sun and migrates inwards, that would wreck havoc with any planet orbits it passes.    So there's a huge gravity problem with your scenario.   If you're looking for Earth like planets, solar-systems with hot Jupiters are probably not the best place to look.

that said, onto your questions.

  What effect will this bled atmosphere from HJ have on EL, if any?

  If there is an effect, when does it happen? On all of EL's orbit,
  despite the different inclinations? Only when EL crosses a transit
  node? Only when there's a transit (or rather, when EL passes through
  the transit node a certain amount of time after HJ does, to account
  for the velocity of the atmospheric particles through space)?

  Will this bled atmosphere be (naked-eye) visible from EL? What would
  it look like?

It's difficult to say with precision.   Certainly if the bleeding was significant enough it might be visible, but on average I think probably not.   As a bad comparison, we can't see Coronal Mass Ejections.   Most of the "bleeding" from a hot Jupiter would be surface gas molecules, primarily hydrogen and helium, most of it uncharged.  Those gas molecules, spread out in space would be essentially transparent, unless it was an absolutely gonzo insane amount.    Very sensitive cameras might pick them up, but they should be largely invisible from Earth.

Comet tails are visible, even occasionally visible during the day, but comet tails are different.  They're full of dust that breaks off the comet and the escape velocity off a comet is low, so the dust that breaks off from the comet tends to follow in a similar orbit, hence the relative density and apparent cohesion of a comet tail.   With a hot Jupiter and gas escaping in a variety of directions presumably due to heat escape or Jeans escape primarily, there would be very little cohesion.   Escaped gas molecules would quickly spread out in space.   The charged particles that escape from this theoretical Jupiter might be enough to create a faint Aurora, but should be significantly smaller than the charged particles escaping the sun.   The uncharged particles, a small percentage would collide with Earth, most would miss.   There's no scenario I can imagine that would be visibly impressive from such bleeding unless the hot Jupiter was super-close to the sun, say closer than Mercury and the bleeding was extreme.   Even then, the primary visual would be the stream of gas falling into the sun, not the spread of gas moving towards and past the Earth's orbit.

Something not too different happened in the Earth's distant past.  Venus used to have oceans and it bled probably at least an Atlantic ocean of water, perhaps a Pacific ocean of water into space that drifted mostly past the Earth.  Likely there was very little effect to Earth drifting through a very disburse amount of water-vapor in it's orbit.

That's a ballpark answer anyway.  Corrections are welcome.


What is the distance of the Earth's orbit around the Moon? I realize the Earth and the Moon both orbit around their shared center of mass, and that in the case of the Earth and Moon this center is "inside" the Earth. However, I'm looking for an approach to determine the distance the Earth travels in one "orbit around the moon" (as small as it may be).

The mass of the Earth is about 81 times that of the Moon and the distance between their centres of mass is typically 384 400 km, so the centre of mass will be about 1/82 of this distance from the centre of mass of the Earth, which is about 4700km.

The orbits are not too far from circular, so the Earth travels about $2 \pi$ times that each month, so a little under 30000km.


How can we be certain that Ryugu is a remnant of the material that did not form planets? As mentioned here, that Ryugu is a part of the Solar System material that did not get swept into forming a planet.

But how can we be sure that it was not created later for example by an asteroid striking Mars or some other planet? Why do we think its "primordial"?

Most of my information is from this link. So here's a watered-down version.

When the Solar System first formed, numerous concentrations of mass, called 'planetesimals', formed over time from slowly accreting material. Some of these would go on to combine with other planetesimals and form planets in their own right, but some would simply float in space. Over time, these planetesimals would shatter apart into primordial asteroids from impacts with smaller bodies. However, these remnant asteroids would originate from a single body, so they would have about the same physical properties. In addition with our modern orbital calculation techniques*, we would able to identify groups of these primordial asteroids, and flag them as such. However, an asteroid created from a planetary impact would likely be ruled out as non-primordial, since there would be no "brother asteroids" to compare it with.

*To clarify, we can trace orbits of asteroids back over billions of years to see where the bodies might have been in the distant past. Asteroids that originated from a single parent body would be observed to converge to a single point.


How was Trappist-1 discovered? I was going through all the questions in this community related to TRAPPIST-1 in order to know how planets TRAPPIST-1b to TRAPPIST-1h were discovered, but there aren't any.

How were they discovered?

The star at the center of TRAPPIST-1 is called 2MASS J23062928-0502285.  It was discovered by the Two Micron All-Sky Survey (2MASS), which imaged the whole sky in the infrared between 1997 and 2001.  This resulted in a catalog of over 300 million objects.  TRAPPIST-1 itself was cataloged in 1999.  The name is actually its coordinates in right ascension and declination.

The planets of the TRAPPIST-1 were discovered by the method of transit photometry.  The way this works is a telescope watches a star for a period of time and records the amount of light coming from the star.  They chart how much light is coming from the star as a function of time, creating a light curve.  If they see periodic dips in the intensity from the star, there is a high likelihood that that star has a planet in orbit around it.  The planet blocks the light from the star every time it passes between us and the star.  This causes the dips in the light curve.  One advantage to this method is that you can scan multiple stars in the same field of view, analyzing them all for planets.

By measuring how long it takes planets to pass in front of the star, how much light it blocks, and how often they orbit, scientists can calculate the masses of these planets and how far away from the star they are by using Kepler's laws of motion.

TRAPPIST-1 was initially determined to have planets orbiting it by the Transiting Planets and Planetesimals Small Telescope - South team.  From their data they determined that it had at least 3 planets.  One of these planets was in the habitable zone of the star.  They published their results in the journal Nature in May 2016.

Once TRAPPIST had determined that the system had planets around it, NASA trained the Spitzer Space Telescope on it.  Ground-based observations of Trappist-1 are difficult because it is so dim.  Spitzer, an infrared telescope, made more precise measurements of the light curves and determined that there were at least 7 planets in orbit around it, 3 of which were in the habitable zone.  Additional observations were made by numerous other telescopes including the Very Large Telescope, UKIRT, the Liverpool Telescope, and the William Herschel Telescope.  The results were also published in Nature.

Here's an image showing the light curve of the TRAPPIST-1 system as measured by Spitzer.


When was the martian dichotomy first observed? The North and South hemispheres of Mars are very different one from another. They have different elevations, different crust thickness, different surface ages. This is known as the martian dichotomy.
When was this striking difference between the two hemispheres first observed?

It looks like the Mariner 9 era.
Based on a review paper by Watters et al. ("Hemispheres Apart: The Crustal Dichotomy on Mars"):  "The north-south asymmetry ... was clear from the first global image mosaics of Mars returned in the 1970s by Mariner 9 (Mutch et al., 1976) and the Viking Orbiters."  Other papers cite a 1973 paper by Hartmann ("Martian cratering 4, Mariner 9 initial analysis of cratering chronology") as one of the first papers to describe it, but it’s still based on Mariner 9 images.
I suppose in hindsight one could look at earlier flyby imagery and see that there is a dichotomy, but with 23 images from the first flyby, that's hardly representative of the surface and not enough to draw such conclusions.  Similarly, one could probably look at Earth-based telescopic observations from before the Space Age and retrofit observations into a north-south dichotomy paradigm, but I don't think one could legitimately claim earlier discovery of it.
So, 1971-1972 (Mariner 9 entered orbit in late 1971).


How many (or percentage) satellites of Solar system planets have their orbits synchronized like the Moon (always facing earth)? Mercury and Venus have no satellites. Then earth has moon (synchronized orbit). For Mars - both its satellites (Fobos and Deimos) also synchronized. Analyzing other planets is hard - too many satellites. Maybe someone knows percentage - all of them synchronized or which ones are not synchronized (if just a few ones are not synchronized). All 4 largest Jupiter satellites are also synchronized.

This is not really known; while the rotation period of most of the larger satellites is known, and they're mostly synchronized, for the smaller ones it's harder to determine and they're less likely to be synchronized:

  One conclusion is that, other things being equal ... a large moon will lock faster than a smaller moon at the same orbital distance from the planet. ... A possible example of this is in the Saturn system, where Hyperion is not tidally locked, whereas the larger Iapetus, which orbits at a greater distance, is.

Wikipedia has a list of synchronized satellites and 'suspects', if you want to investigate further.


Timing based on upper or lower part of sun/moon When you time an astronomical event, such as the moon/sun rising above the horizon, do you note the time when the the first sliver breeches the horizon or when the bottom of the moon/sun just becomes visible?

It is the first one, i.e. as soon as the heavenly body can be first seen. Here a definition from http://www.ga.gov.au/scientific-topics/astronomical/astronomical-definitions

Sunrise is defined as the instant in the morning under ideal meteorological conditions, with standard refraction of the Sun's rays, when the upper edge of the sun's disk is coincident with an ideal horizon.


What would an asteroid inside of Mercury's orbit be called? The first Vatira asteroid has been discovered. This makes me wonder if there is already a name for the hypothetical class of asteroids that orbit entirely inside of Mercury's orbit. It wouldn't be the first time that an astronomical class is named before being actually observed.

Is there a name to designate the hypothetical subclass of Atira asteroids whose aphelion is closer to the Sun than Mercury's perihelion ?

The name used for such objects is vulcanoids. This term refers to the hypothetical intra-Mercurian planet Vulcan, which was proposed by Urbain Le Verrier (best known for his successful prediction of the existence of Neptune) to account for the anomalous precession of Mercury's orbit that is now explained by general relativity.
Note that the expected number of vulcanoids is low: the Yarkovsky effect should  clear out objects smaller than 1 km on ~gigayear timescales (Vokrouhlický et al., 2000), while the YORP effect would be effective at destroying larger objects by spinning them up until they break apart (Collins, 2020). Observational constraints from NASA's STEREO mission suggest the number of objects larger than 1 km may be no more than 76 (Steffl et al., 2013).


Do black holes pull in and keep spacetime itself or merely warp it but ultimately let it pass through? Forgive my 'amazing' MS-Paint skills, but this is along the lines of what i'm wondering:

Is there any situation where Scenario A occurs, or is space always dragged back out as in Scenario B?

Black holes orbit things and galaxies move within space, so it would be safe to assume that black holes move through space-time and do not hold it in any particular state or position. They simply drag their "depression" along with them as they go.


How do we not know what's in the Solar System? It seems every once in a while there's a claim of a new planet in the Solar System.

I don't understand how local planets can stay undiscovered for this long when astronomers are accomplishing what seem to be far more difficult feats.

For example, it's been a whole century since they had accurate enough measurements to tell that Mercury was precessing in a way that didn't jive with Newtonian mechanics.
Now, if there are Earth-sized planets out there, shouldn't their presence have a pretty sizable effect on the orbits of bodies such as Neptune and Pluto? How can there be such large planets with no measurable effects until 2017 technology came along?

There could be another planet in the solar system that has not been discovered if that planet had a highly elliptical orbit with a single orbit taking many hundreds of earth years to complete. That planet could be in the part of its orbit further from sun and could stay in that area for a few hundred years, longer than we've had large telescopes.

At such a distance it would receive very little sunlight and the chances of visually detecting it would be very small. And we wouldn't know where to look either, which just compounds the problem.

Other more distant plants have been discovered but their discovery was due to the effects on the star or stars it is orbiting. We observe a planet passing in front of a star which briefly lowers the apparent magnitude of its parent star. This is a good clue of a planet in orbit. This is impossible in our solar system because an unknown planet would have to come between us and the sun. We would easily see it if that were the case.

We have observed some discrepancies in the orbits of some of the outer planets. This has led to the speculation of another planet. It is very difficult to determine the cause of these small discrepancies though. All bodies orbiting our sun are not only effected by the suns gravity but by the gravitational attraction of all other bodies (including all objects in the asteroid and Kuiper belts). This is a very intense calculation. This attraction constantly changes due to the motion of all the bodies.

Simulations have been developed to study this but must take a lot of computing time even with today's high speed computers.

I hope this helps explain the situation.


How long does it typically take to make a telescope yourself for the first time? How difficult is it to make a telescope oneself? How much time would it take for a first-timer? What are the typical pitfalls?

Making your own telescope is very do-able given basic working skills. Usually a DOB or Newtonian reflector is a good choice. You can buy a mirror and components all ready to assemble or you can choose to grind your own (which is more difficult). If you have an astronomy club in your area check to see if they can help putting your project together. If not, check online ( Youtube, etc) for guidance. Websites like Edmonds Optics may still be a good choice for components. Once you get all the components together, they can be assembled in a pretty short time. Good Luck!


Discovery of Near Earth Objects (NEO's) Near Earth Objects (NEO's) have been mapped by NASA and JPL since 1990's. The graph below shows how many of these potential planet-killers have been found and classified over the years. I am curious as to why there have been so many more 140 meter to 1 km NEO's discovered, as opposed to NEO's with a diameter of 1 km or more. I think it is as simple as there are just more "smaller" objects left over from the formation of the solar system? Am I correct in saying this, or are there deeper astronomical processes at play here?

This is because there are many more small NEOs than there are large ones and the larger ones are easier to find and so were found first. The numbers of NEOs follow what's know as a power law distribution with an exponent of ~1.75. This means that given the roughly factor 7 difference between 140m and 1000m NEOs, there will be 7^1.75 = ~31 times as many 140m NEOs as 1000m NEOs. This is shown in this plot below.  We estimate there are about 950 NEOs 1km or larger and about 25,000 NEOs larger than 140m. We have found over 93% of the 1+km NEOs and so there aren't many left to find. In comparison, over 2/3rds of the 140-1000m NEOs still remain to be found, most in the 140-300m size range. However this improved considerably since even 2010 when about 82% of the population was still to be found.


How come, in our lifetime, we will see the first stars which ever formed? Isn't it an unlikely coincidence that we live in such special times, that we will have observatories (JWST) that see all the way back to the first starlight in the universe? Are we dreaming, and is it possible that we will be surprised and keep finding evolved galaxies more than 13.8 billion years old?

We will not find Galaxies that have existed before the big bang.
There's a principle in Astronomy that states: The further you look, the earlier you look. That means that the more distance the light had to travel to reach our telescopes, the more time it needed to reach us, because the speed of light is not infinite. Therefore seeing the first galaxies of the universe isn't some lucky coincidence, we are just looking the right distance away (13 billion light years). If we had lived earlier we could still see these galaxies, but they would be closer to us. And the same goes vice versa; If we had lived at an earlier time, we would still see said galaxies, but much farther away. I hope this helps.


Are the most distant known objects in the universe more than 14 billion light years away? When I hear about the most distant objects in the universe, such as the recently discovered galaxy GN-z11, their distances are usually stated to be a little under 14 billion light years away....
But, the observable universe is 46.5 billion light years in radius... So..
Does the 13.4 billion light years or so distance for GN-z11 factor out the expansion of the universe itself?  So-called 'co-moving distance'?  Or is it actually, currently about 13.4 billion light years away?

According to Wikipedia, GN-z11 has a redshift of 11.09, which, according to Ned Wright's cosmology calculator, corresponds to a light travel time of about 13 billion years and a comoving radial distance of about 32 billion light years.
The comoving radial distance is the distance measure with respect to which the diameter of the observable universe is about 46 billion light years, so that's the one you want in this context.
A lot of people don't understand the implications of cosmological expansion and think that if the light travel time is 13 billion years, then the distance to the galaxy must be (or must have been) 13 billion light years. They're sure enough of it that they'll report that distance as fact, without disclosing that they calculated it themselves. As a result, distances in popular articles generally can't be trusted. You can trust the redshift if it's reported, and you can probably trust times (either light travel time or time since the big bang).


Why would all places appear to be at the center of the universe's expansion? I just watched an iMax planetarium video that claimed that no matter where you are in the universe, it will appear that you are at the center of the universe and all things are expanding away from you.

What the heck?? Can someone please explain this?

This is a famous analogy, which was prbably given to you in the video. Imagne that you are on a ball of dough full of raisins. You obviously perceive yourself as the center. Then, the dough starts to grow. The raisins are still the same size, but there is constantly more and more dough between them. From your position it would seem like there is always more and more space between you and everything else. You don't see yourself moving, but you see everything else moving away from you. This effect would happen anywhere in the universe, not only at the center. In fact, you would think - correctly, relatively speaking - that the center (if you can see it) is being pushed away from you. So basically, it all comes down to your point of view.

This expansion is thoroughly studied presently. The most common theory is that the universe contains a giant amount of energy (called dark energy) that is so far undetectable and that is causing the the space to expand. Think of it as yeast. There are other theories that are much more complicated, which a non-professional such as I wouldn't be able to explain. (For instance, there is one that suggests that our universe adheres to other universes, like soap bubbles, which adds space). But dark energy is the most likely candidate, so far.


Moon changes position So most nights the Moon usually rises on the right of the sky when I look through the window just like the previous night.

But this night the Moon was on the left of the sky rising. Does anyone know what this is?

In the absence of any detailed information, such as @Uhoh asks for, this is just a guess, but here goes. I'm assuming you live at mid northern latitude.

The moon rises between south east and north east.  If we consider full moons, then during  the summer, the full moon will rise in the south east.  As we move into the autumn and winter, the full moon progressively rises further north.  If you are looking out of an east facing window, then through mid May to mid July the moon will appear to rise to the right side of your window, but will gradually move northwards, ie to the left of your window.

The reason behind this is that at full moon, the moon is diametrically opposite the sun.  At mid summer, the sun rises in the north east, and in mid winter it rises in the south east.


Does the earth's atmosphere act as a spherical lens and refract light from space? If so by how much does it "spoil" the view of stars and galaxies etc.

Is this error noticable, when compared to pictures taken from outer space.

If so by how much does it "spoil" the view of stars and galaxies etc.

There are several very different issues related to your question. Let's tackle them one by one.

Atmospheric refraction

Yes, the Earth's atmosphere refracts light. One notable effect is that objects near horizon appear higher than they should be.

Therefore, the Sun (or any other object) rises earlier than it should, and sets later.

http://en.wikipedia.org/wiki/Atmospheric_refraction

It affects astrometry, because it messes up the coordinates of objects near the horizon. Other than that, it does not cause great harm to astronomy.

And then there are several effects harmful to astronomy:

1. Seeing

What we call in astronomy "seeing" is just air turbulence. Because air moves around due to heat, it distorts the image continuously. This is how the Moon looks like in a telescope - notice the shifting around due to seeing (animated GIF, reload if it's not moving, or open in new tab):

If we lived in a vacuum, that image would be static, and perfectly clear. But turbulence causes the shifting. Also, it causes a blur - the image is less sharp than it could be. It's also what causes the stars to "twinkle".

It's basically the same phenomenon that you see in summer along the hot walls of a house under sunlight - the same shimmering effect of the image seen through the hot rising air along the walls.

For amateur astronomers, seeing affects high-resolution targets such as planets, the Moon, double stars. It does not affect targets that are perceived to be "low-resolution" in an amateur telescope: galaxies, nebulae, star clusters.

More reading:

http://en.wikipedia.org/wiki/Astronomical_seeing

Since seeing depends on weather, it can be predicted to some extent:

http://cleardarksky.com/csk/

2. Light pollution

Atmosphere also scatters and reflects light back to Earth. All the city lights around you are scattered by haze and dust in the air, and some of that light goes back down into your eye, creating sort of a "light fog".

As a result, objects that are not too bright are masked by light pollution and become less visible, or invisible altogether.

A bigger telescope will help you pierce through the haze of light pollution, but only to some extent. The best way to see faint objects is to put the scope in the car and drive far away from the city.

Here's a light pollution map:

http://www.jshine.net/astronomy/dark_sky/

For amateur astronomers, light pollution affects the "faint fuzzies": galaxies, nebulae, star clusters. It has no effect whatsoever on bright objects such as: planets, the Moon, many double stars.

Other ways the atmosphere can negatively impact observations:

clouds - this is an obvious one
transparency - different from clouds, it includes haze, dust, particulate matter, water vapor. It affects low contrast objects such as galaxies.


Measure an angle in the sky from a photo? Let's say I have an picture of the sky or of an landscape and I want to measure an angle in the celestial sphere. Is there any software for that? In real life I would use the empirical hand gestures or a theodolite.

A plate solver matches the stars in an image to a catalogue of star positions.
Astrometry.net provides a web service and a software API for this.
Given only another user's cell phone photo, Astrometry.net determined that the image scale was 112 arcseconds per pixel.
It also estimated an image size of 72 by 54 degrees, apparently using an approximation which is only valid for small angles.

Assuming low distortion (not a fisheye lens), the image is a gnomonic projection of part of the celestial sphere.
If $r$ is the distance from the image center in pixels, $p$ is the pixel scale in radians per pixel, and $\theta$ is the angular separation from the center in radians, then

$$rp = \tan \theta$$

For example, I used the measurement tools in GIMP and Stellarium on two bright stars near the center to verify the central image scale of 112 arcsec/pixel or 0.54 mrad/pixel.
The edges of the 2320x1740 image are 1160 and 870 pixels away from the center.
Solving for $\theta$, the edges are 0.56 and 0.44 radian away from the center, so the image's angular dimensions are 64 by 50 degrees from edge to edge.

Daytime images would of course have the same pixel scale and angular dimensions as astrophotos taken with the same equipment.


How thick was Earth's primordial atmosphere? Earth's first atmosphere was made of hydrogen and helium accumulated before fusion started in the Sun. As soon as this happened it was stripped away by the Sun. How many atm of hydrogen and helium had Earth accumulated at the peak pressure of the first atmosphere?

We have no way of knowing how much hydrogen and helium was in the Earth's primordial atmosphere. What is more important is what the atmosphere consisted of when the primeval soup from which life arose was formed, just over 4 billion years ago. We have a fairly good idea what this atmosphere consisted of: mainly carbon dioxide and nitrogen, rather like Mars at the present time, but with substantial amounts of ammonia and methane. Also very small amounts of hydrogen and helium, most of which had by then been blown away by solar radiation. There was hardly any oxygen. As for pressure, all we can say for sure is that on Earth it was a good deal higher than it is today. Conditions at that time were similar, though not identical, on Venus, Earth and Mars, perhaps on Titan as well, so it is possible that all three terrestrial planets developed a version of the Earth's primeval soup, which was essential for the creation of life. Whether life emerged on any planet other than Earth, we don't yet know. Because all three planets had oceans, water vapour would also have been an important atmospheric constituent.


Is it true that black holes emit only photons, electrons, and positrons in the form of Hawking Radiation? According to this comment, if the peak photon energy is 511 KeV, electrons and antielectrons begin to be produced along with photons. So when the peak energy is ~940 MeV, the rest mass of a proton, will they be produced as well, and what types of subatomic particles can be generated via Hawking Radiation?

When you look at Hawking's 1974 Nature paper (https://www.nature.com/articles/248030a0), it is very clear that like thermodynamic blackbodies, black holes can emit anything they want. Or to be more precise, the thermal radiation that is produced by gravitational vacuum polarization in the vicinity of the black hole event horizon is not restricted to photons.
But what else could it be?
Photons are there, obviously. And their peak wavelength is determined by the temperature of the black hole or blackbody. Unless that temperature is several billion degrees kelvin, there just isn't enough energy to form electron-positron pairs in meaningful quantities. (Astrophysical black holes have temperatures in the nanokelvin range.) Neutrino-antineutrino pairs perhaps, but the extreme short range of the weak interaction effectively limits their radiation. Gravitational thermal radiation (assuming gravity is quantized and gravitons exist - see Dyson's Poincaré prize lecture, section 5 in particular: https://publications.ias.edu/sites/default/files/poincare2012.pdf ) might also be present but again, close to twenty orders of magnitude less intense than photons.
If the temperature is high enough for electron-positron pairs to form, sure, those can also come out of the black hole. As well as proton-antiproton or neutron-antineutron pairs. But these hot black holes would be so small (just a few billion metric tons, with Schwarzschild radii in the subatomic range), they would have to be either primordial black holes or astrophysical black holes near the end of their evaporation, in the extreme distant future.
What else? There are no other stable particles for the black hole to emit. If a sufficiently hot black hole produces, say, a W+/W- pair, those would very rapidly decay the usual way into positrons (electrons) and electron (antielectron) neutrinos, or their muon/tau flavor counterparts (but those, too, would decay.)
So in the end, just like in the case of a thermodynamic blackbody, we find that although in principle, a black hole can emit other things, at non-extreme temperatures, the only thing it can emit with any efficiency is photons.


How were the orbits of planets first mapped out? I'd like to know how the orbits of planets were first 'mapped out' and to understand the maths behind it. e.g. How do we know what position in space a planet will be at certain time so that spacecraft trips to planets could be planned etc.?

I'd also like to know the most primitive way that this could be done and if there is anyway this could be done using only maths and simple measuring tools assuming no previous knowledge about the planets positions.

I think your question covers a lot of ground cause you're asking it from 2 different perspectives.

  How were the orbits of planets first mapped out?

and

  How do we know what position in space a planet will be at certain time
  so that spacecraft trips to planets could be planned etc.

These should probably be separate questions, as they're quite different.

First one first, here's a bit of history:  Using parallax Hipparchus was able to get a pretty good estimate of the distance to the Moon about 200 BC.  The distance to the sun was much more difficult but Aristarchus was the first to come up with a method and others were able to improve upon it to get more accurate measurements.

The relative orbits of the planets wasn't hard to calculate once you had one of them, more on that here.

We can measure the length of a year on Mercury or Venus quite easily, as well as their relative angle to the sun (though Mercury's orbit is quite eccentric, you'd want to take several measurements).  Even the wacky Copernican model with just circles on top of other circles made pretty good distance calculations with the sun in the center and the planets orbiting the sun.

Kepler was the first to recognize that the planets orbited at slightly different orbital planes, and from what I've read, Kepler was prouder of that discovery than the discovery of the elliptical orbits that he's most often remembered for.  The combination of slightly different orbital planes and elliptical orbits and Brache's improved accuracy to his measurements was a huge step forward in planet mapping.  Kepler still didn't have the distance to the sun well plotted out, but he figured out the model for planetary orbits.   After that, it was just a matter of improving the distance measurements.

As to how NASA does it now.   I'm not an expert, so somebody else might be able to answer that one better.   It gets pretty mathematical, 3 position vectors, 3 direction vectors and velocity changes based on position, but we have nearly pinpoint laser measurements now.

  I'd also like to know the most primitive way that this could be done
  and if there is anyway this could be done using only maths and simple
  measuring tools assuming no previous knowledge about the planets
  positions.

It depends what you mean by primitive.    Galileo's telescope enabled him to see phases of Venus, and that isn't possible without a telescope.  The telescope also made it possible for Galileo to see 4 of Jupiter's moons, which orbited Jupiter and not Earth.

Aristarchus tried to eyeball the difference in shadow from the sun between the earth and the moon and he estimated the Sun was some 18-20 times more distant than the moon.  Details here.  To his credit, he successfully deduced that the Sun is much further than the moon, and he estimated the angle ratio to be 87 degrees when it was 89.83 degrees.  Despite his efforts, his estimate was off by a factor of about 20, in distance though he was only off by 2.83 degrees.

Brahe used the best equipment available to him at the time, much of it made specifically for his observations.  More on that here.  Brahe also had the advantage of having accurate time pieces.

Brahe believed in the Ptolemaic model but he was unable to get it to match his data and he urged Kepler to figure it out when he was old and knew he wasn't going to figure it out himself.  (There are rumors that Kepler poisoned Brahe to get more credit for himself, but I don't think that actually happened, Brahe's body was even exhumed to test that theory some time later but no traces of poison were found).

Kepler, unlike Brahe, liked the Copernican model but he couldn't get that to work because he believed in Copernican circles.   He tried ellipses, more out of desperation because he couldn't get circles to work and the ellipse model matched the data.   Kepler had not only Brahe's but years of Copernicus' records to work with as well as observations preceding them, some going all the way back to ancient Greece.

I'm not sure it would be possible to figure out that planets have elliptical orbits and how far they were from Earth without good equipment.  Kepler had a lot to work with and a determination to find a simple explanation.

Hope that's not too wordy.  I can try to clean it up a bit later if you like.


Which type of telescope should I make? I am not very much interested in observational physics but I just like to look at the night sky. It gives me a lot of inspiration. So, i was thinking about making a telescope for basic observations. I googled about the design and found two models. One consisted of two convex lenses of different focal lengths and the other one consisted of a concave lens as the objective lens and a convex lens as the eye-piece(that's called a Galilean one I guess).

I am confused about which one should I make. Can anyone give me some insight into both these types?

One consisted of two convex lenses of different focal lengths and the
  other one consisted of a concave lens as the objective lens and a
  convex lens as the eye-piece(that's called a Galilean one I guess).

That's the ABC of refractor design - the most basic types of refractor. They are only interesting from a historic perspective.

http://en.wikipedia.org/wiki/Refracting_telescope

You can certainly purchase a large convergent lens (bi-convex, or plan-convex) and use that as an objective. Then, for the eyepiece (ocular), you can use either a small convergent lens (bi-convex, or plan-convex), or small divergent (bi-concave, or plan-concave). Probably only the first one will work well: the image will be inverted, but the field of view will be nice and wide. The second design (divergent lens for eyepiece, Galilean refractor) provides a non-inverted image, but the field of view is extremely narrow and hard to use; avoid it, it's frustrating.

However, this sort of bare-bones design is only interesting if you want to reconstruct the history of astronomy. At least for the eyepiece you could just "steal" any cheap 1-1/4" eyepiece from a telescope or microscope. Even the cheapest one will work much better than your single-lens "eyepiece".

http://www.telescope.com/Accessories/Telescope-Eyepieces/pc/3/47.uts?&refinementValueIds=4469

For the objective, again, I suggest you don't use a single convergent lens. It will work, but it will be full of aberrations (image distortions, color fringes, etc). Instead, go on Surplus Shed, click on the Lens Finder. For lens type, choose Achromat. For diameter, anything over 45 mm. For focal length, anything over 190 mm.

http://www.surplusshed.com/lens.cfm

An achromat is a combination of two lenses, glued together, that are designed to reduce aberrations. The image will be much better.

Their stock varies greatly. Right now, I've found these two:

http://www.surplusshed.com/pages/item/pa1038.html

http://www.surplusshed.com/pages/item/pa1039.html

The 50 mm diameter, 300 mm focal length achromat, combined with a 15 mm focal length eyepiece, will provide 300 / 15 = 20x magnification. With a 5 mm eyepiece instead, it will give 300 / 5 = 60x magnification. Don't go much higher than this (more magnification is not always better).

The focal length of the objective dictates the length of the telescope tube. E.g. with the 300 mm achromat, the telescope length will be 300 mm plus some extra at the back for the eyepiece.

EDIT: Be mindful of the f-ratio. Basically, divide the focal length by the diameter, and that gives you the focal ratio.

A lens with a diameter of 50 mm and focal length of 500 mm has an f-ratio of f/10. A lens with a diameter of 50 mm and focal length of 300 mm has an f-ratio of f/6.

It's best to use an achromat with an f-ratio between f/8 and f/20. If you go below f/8 it will be very hard to collimate (fine tune) the scope. If you go above f/20, it will be too long.

Sometimes you find super-cheap eyepieces there, too, like this one:

http://www.surplusshed.com/pages/item/l2019.html

Also keep digging around that site, maybe you'll find various other parts necessary for building the refractor.

Enormous care must be given to keeping the objective and the eyepiece collimated (aligned and centered). Even a tiny collimation error will greatly affect performance. This is not a trivial engineering problem.

There's also an easier way. You could purchase a complete kit, like the Galileoscope:

http://galileoscope.org/

It's a 50 mm (2") f/10 refractor that has about the same performance like good binoculars. The price is very affordable (and very cheap for what it does). As a telescope and mirror maker, I was impressed by the quality of the achromat doublet objective that they provide (50 mm diameter, 500 mm focal length, f/10). For the price and the size, it is well made and well corrected, and punches well above its price category.

The kit comes with everything you need: all the lenses, the tube, various small parts, etc., including components for the ocular (eyepiece). Another nice thing is that you can swap its eyepiece for a standard 1-1/4" eyepiece used in a regular telescope (like a cheapie on Surplus Shed). Whoever designed this thing put some good thought in it. There are several different instruction documents on the site, that you can choose from.

My 9 and 12 year old sons put it together in an afternoon. It's that easy.

EDIT: What can you do with the Galileoscope?

Obviously, you can watch some of the planets. You'll see the two big equatorial belts on Jupiter, and the 4 galilean moons of Jupiter. Saturn's rings will be quite obvious. Venus will show a crescent - it will look just like the Moon, only much smaller.

You can watch the Moon, too. The big craters are quite visible in an instrument this size.

You can watch some of the deep space objects too. The Orion Nebula is quite visible actually. The Andromeda galaxy is somewhat visible from a place outside the city.

Double stars such as Albireo, which appear as single star to the naked eye, are easily split by a 50 mm aperture. Albireo is very pretty, and easy to see in a Galileoscope - an orange/blue double, easy and wide, very beautiful.

Mizar, one of the main stars in the Big Dipper, is also a double. This one is a lot more tight. I think you should be able to split it in a G-scope anyway.

http://www.synapses.co.uk/astro/bearings.html

For the planets and the Moon, you'll probably want high magnification. 50x or more, up to 100x (the maximum supported by this instrument).

Make sure the instrument is sitting on a stable, firm support. A flimsy, shaky tripod is the enemy of high magnification.

Look on Surplus Shed and other online stores for 1-1/4" eyepieces. A 5 mm eyepiece will give you 100x magnification - the maximum. A 10 mm eyepiece will give 50x. A 20 mm eyepiece will give 25x - nice and wide, good for big objects like the Orion Nebula.

Look for eyepieces with a wide field of view (at least 50 degrees, up to 80 degrees, or more, if you can find them). A wide field makes everything easier.

It is also possible to build everything from scratch, including making the lenses and/or mirrors yourself from a piece of glass. But that is a MUCH bigger commitment, and a much more long-term project (a few months to a year).


Data reduction and photometry without IRAF? The IRAF package is old.

I've been looking around for a more modern software to replace it in the processes of CCD data reduction and photometry, but haven't been able to find any.

The closest I've found is the PyRAF tool, but this seems more like a Python wrapper around IRAF rather than a replacement for it.

Is there some new software I might've miss or is IRAF really the only option even today?

Add:

I forgot to mention this, but I'm looking for tools that work under Linux and are free (open source + no charge), if possible. I will not pay (neither for a Windows license nor for a software package) to get rid of IRAF.

I suspect that everything you want and more is available and written in python or has python wrappers.

Astropy
ccdproc
photutils


Why is the discovery of merging neutron stars important? I'm fairly certain people here will have heard about it, already, but apparently, two supernova leftovers clashed some 130 million years ago and some billion billion kilometres away ...

What I haven't heard yet, however, is why we should care.

I mean sure, it's an interesting phenomenon and measuring it can't have been easy.

But now that we've heard it ... what changes?

I'll admit it, I don't know particularly much about astronomy, but I'm curious:

What's the significance of having achieved this? Why does it matter whether or not we know?

Reasons why this is important:

It is the first simultaneous detection of a gravitational wave and electromagnetic signal, and the strongest GW signal yet in terms of signal to noise (Abbott et al. 2017a). It spectacularly corroborates the reality of the GW detection technology and analysis. The progenitor has been unambiguously located in a (relatively) nearby galaxy (Soares-Santos et al. 2017), allowing a host of other telescopes to obtain detailed measurements.

It shows that GWs travel at the speed of light, a further verification of Einstein's General Relativity (Abbott et al. 2017b).

It shows that most of the very heavy elements such as gold, platinum, osmium etc. are plausibly produced by merging neutron stars and constrains the rate of such mergers in the local universe (e.g. Chornock et al. 2017; Tanvir et al. 2017).

It shows that short gamma ray bursts — some of the most energetic explosions in the universe — can be caused by neutron star mergers (e.g. Savchenko et al. 2017; Goldstein et al. 2017).

It is the closest detected short gamma ray burst (with a known distance). That the progenitor has also been characterised allows a closer investigation of the interesting physics underlying the ejection and jet mechanisms thought to be responsible for the gamma rays and later X-ray and radio emission (e.g. Margutti et al. 2017; Alexander et al. 2017).

It provides observational constraints on how matter behaves at extremely high densities, testing our understanding of fundamental physics to its limits — for example, the details of the gravitational wave signal moments before merger are diagnostic of the interior conditions of neutron stars at densities of $\sim 10^{18}$ kg/m$^3$ (Hinderer et al. 2010; Postnikov et al. 2010).

It provides an independent way of measuring the expansion of the universe. Merging binary gravitational wave sources are known as "standard sirens", because the distance to the GW source pops straight out of the analysis and can be compared with the redshift of the identified host galaxy (Abbott et al. 2017c). The result agrees with measurements made using the cosmic microwave background and the distance-redshift relation calibrated by other means, verifying our estimation of distances, at least in the local universe.

Finally, this event will turn out to be important because it was lucky; in the sense that the source was detected well-inside the sensitivity horizon of LIGO (Abbott et al. 2017a). The detection itself, was not unexpected given the rates predicted based on studying the neutron star binary systems in our own Galaxy (e.g. Kim et al. 2015), but the fact that it was so close —
 within the closest 5% of the sensitive survey volume where it could have been detected — is fortunate.

In the end, if someone thinks none of the above is interesting or important, then nothing I can write will convince them otherwise. The vast majority of people I speak to are curious and fascinated to find out about our cosmic origins and how the universe works.


lunar calendar accuracy for predicting new moons I wonder, how accurate the lunar calendar is?
Can we predict the new moons precisely for tens of years in future?
If we know all factors that affects the movement of the moon, then why it is not possible to predict it?

The answer depends on what you mean with "precisely".  The ELP theory seems to have an accuracy of ~10-6° in longitude over a century, which comes down to about a millisecond for phases.


Where do local users of ALMA live and work? These days with high speed internet, observational astronomy can be done from remote locations. Still I believe from time to time an experimenter might travel to work on-site for particularly challenging or unusual/non-standard observations, where working closely with the instrument support team is necessary.

When astronomers travel to the Atacama Large Millimeter Array or ALMA:

Where do they live?
Where do they work during observing sessions?

Representative photos would be greatly appreciated!

I'm guessing it looks a little different than this:

Source: Cropped from Apertif - the focal-plane array system for the WSRT

  Figure 2. The WSRT telescope RT5 equipped with the APERTIF prototype. Shown in the inserts is the Vivaldi array in the lab (top right) and in the focal plane of the telescope (bottom right).

No one travels to ALMA to observe; the observations are all carried out by staff working at the Operations Support Facility. (This is at an altitude of 2900 meters; the actual telescopes are 28 km away at the Array Operations Site, which is at an altitude of 5000 meters and is not permanently staffed.) Any "unusual/non-standard observations" would have to be specified in advance and encoded into the Scheduling Blocks (prepared files specifying the observations you want).

https://almascience.nrao.edu/about-alma/alma-site

I believe the staff at the Operations Support Facility stays in the Residencia. (More pictures here and here; pictures of the OSF, including the telescope control room, can be found here.)

(I wonder if some future movie will film scenes there, as the James Bond movie Quantum of Solace did at the Paranal Observatory's Residencia in 2008...)

Visitors (who cannot stay overnight) are permitted on Saturdays and Sundays, though you have reserve your visit in advance.


How can orbital resonance sometimes have a stabilising effect, whilst other times, it has a destabilising effect? I have just started learning about orbital resonance. I understand how bodies in orbital resonance will line up according to the orbital ratio number, and there will be increased gravitational effects upon alignment.
However, I do not understand how resonance can often have a stabilising effect, while other times, it is destabilising? For example, in the Io-Europa-Ganymede system, the 4:2:1 resonance has a stabilising effect. But also, I read that the gravitational perturbances when the moons align cause them to have elliptical orbits, so is it really a stabilising effect? There are other examples where orbital resonance completely prevents stability, like in the Kirkwood Gaps of the asteroid belt.
So my question is, how do orbital resonances form in the first place, and what determines if they are stable/unstable? Thank you.

According to my understanding, for an orbital resonance to be stable, there must be at least three circumstances present:

The forces on the bodies in resonance have to cancel out over time,
The momentary forces shouldn't be that high that the orbits are changed significantly,
after a perturbation, the bodies should pull each other back into resonance.

For example, the Trojans are in a 1:1 resonance around the Lagrange points of Jupiter, and the Galilean moons of Jupiter are in a 4:2:1 resonance. A 2:1 resonance wouldn't be stable at it's own because the gravitational forces would always apply at the same point, but the perturbances are cancelled out by the third moon, and the outer and inner moon are in a stable 4:1 resonance.
In general, a resonance can be stable, if the encounters of the objects are equally distributed, so imaginary lines between these points have point symmetry / a regular shape (for example a straight line across the orbits (3:1; 3:2), an equilateral triangle (4:1; 4:3), a square (5:1; 5:2), a pentagon or pentagram, etc.) But, the closer the orbits get to each other, the more unstable they get, because the deflection gets bigger, so those resonances only work if the bodies aren't too massive. 2:1 resonances can be relatively stable too, but without any other resonance preventing it, the eccentricity will rise over time and one of the bodies may be ejected. However, orbital resonances are only stable in certain circumstances, because if the eccentricity and/or the inclination are too high, the forces don't cancel each other out enough. But there still are some configurations in which highly inclined and eccentric orbits are stable.


The formation of absorption lines in solar spectrum It is said that absorption lines originate from regions higher in the photosphere where the gas is cooler.

The gas should be absorbing photons, then re-emitting them... absorb, re-emit, absorb, re-emit...

My question is where are the photons produced by re-emission? Why are there absorption lines?

Although photons could be scattered, photons at other places could be scattered to our line our sight. Surely the total number of photons does not drop the further out you go? So while photons originally headed in the direction of an observer are possibly absorbed and re-emitted in a direction away from the observer, surely this will be made up for by a photon being emitted from another location in the direction of the observer? Otherwise wouldn't there be a location somewhere else where a bunch of extra scattered photons can be observed?

If we integrate all the photons at the lower surface of the photosphere A and integrated all the photons at the upper surface of the chromosphere C, surely there are the same number of photons? Otherwise, where have they gone? Where is the lost energy?

Are there less photons emitted from the surface C because some have been emitted back down into the Sun? But that has to reach an equilibrium, or there would be a cascading increase in the number of photons inside the sun.

In the upper photosphere, the electrons join with the ions to neutralize the atom. When a photon strikes this atom (the atom absorbs the photon), it promotes a electron from a low energy shell to an higher energy shell. The electron then falls back to the lower energy shell emitting a photon in a different direction. When you break the sunlight into wavelengths (by a prism), you will see dark bands where the light was absorbed by the atoms and emitted in a different direction. These bands correspond to the energy absorbed and released when an electron changes energy shells. Here is the sun's light spread out to show the absorption bands.

Added: Here is a high resolution of the wavelengths from the sun.

Apr 20: You have the wrong interpretation of Figure 16.9. At level A, there is about the same number of photons at all wavelengths coming at the observer on Earth. As the photons pass through the Chromosphere, the photons with the wavelengths of the atom's electron transition are absorbed and redirected away from the observer. The photons with different wavelengths of the atom's electron transition eventually arrive at the observer. This is why there are dark lines in the spectrum. It is because most of the photons are absorbed and redirected in a different direction.

The absorption lines are formed at a higher region since the photons are removed from the spectrum.


Variable and Multiple Stars in Hipparcos Is there any way I can tell from Hipparcos catalog which stars are Binary/Multiple Stars and which are Variable stars?

Yes. If you have the Hipparcos data from ftp (http://vizier.u-strasbg.fr/viz-bin/VizieR?-source=I/239&-to=3), you will have several data files.

In hip_main.dat you have a field MultFlag at position 347 that indicates whether the star is a double or multiple star:

Note on MultFlag: indicates that further details are given in the Double
     and Multiple Systems Annex:
     C : solutions for the components
     G : acceleration or higher order terms
     O : orbital solutions
     V : variability-induced movers (apparent motion arises from variability)
     X : stochastic solution (probably astrometric binaries with short period)

To determine whether the star is a double or multiple star system, look in one of the files for double star solutions. For instance in h_dm_com.dat there is a field at bytes 24-25 that gives the number of components (2 for double and >2 for multiple stars).

24- 25  I2     ---     Ncomp    Number of components in this solution   (DCM2)

The Hipparcos number identifying the star can be found in bytes 43-48.

Of course this only works for multiple stars with orbital solutions.

For variability, three fields are provided in the main Hipparcos file:

314-320  F7.2  d       Period    ? Variability period (days)              (H51)
     322  A1    ---     HvarType *[CDMPRU]? variability type               (H52)
     324  A1    ---     moreVar  *[12] Additional data about variability   (H53)

with two notes (NB. if HvarType is C then the star is constant, i.e. not variable!):

Note on HvarType: Hipparcos-defined type of variability (a blank entry
     signifies that the entry could not be classified as variable or constant):
     C : no variability detected ("constant")
     D : duplicity-induced variability
     M : possibly micro-variable (amplitude < 0.03mag)
     P : periodic variable
     R : V-I colour index was revised due to variability analysis
     U : unsolved variable which does not fall in the other categories
Note on moreVar: more data about periodic variability are provided

There are two additional files on variability: hip_va_1.dat and hip_va_2.dat, which provide information such as variable type (GCVS) and max and min magnitude and also the variable star name.
You can find a description of the variable type in the General Catalogue of Variable Stars (GCVS) in the file vartype.txt if you download the GCVS at http://cdsarc.u-strasbg.fr/viz-bin/Cat?B/gcvs


Historical Distance of Planets to Earth I'm curious if there is a readily accessible tool available, or simply a reliable way to find out what the distance of the planets in the solar system were on a given date.

For example, if I am trying to determine how far:

The Sun
Mercury
Venus
Mars

were from Earth on May 3, 1954 ... how would I do that?

Python

I see that you are quite active in Stack Overflow but have no posts tagged with python yet, so maybe it's time to give it a try!

Run pip install skyfield
read the simple documentation: https://rhodesmill.org/skyfield/
calculate x, y, z positions in the solar system's coordinates with a single line of Python; e.g. `x, y, z = venus.at(time).position.km, or
use Topos to define a viewing location on Earth and the .altaz() method to get the position of each in the sky.

NASA JPL Horizions

This is the same basic source of information that Skyfield uses.

Go to the Horizons website at: https://ssd.jpl.nasa.gov/horizons.cgi
Follow the instructions I've written up in this answer.
You can also read the documentation there to specify topographic coordinates and get positions in the sky, and you can do that viewing from other planets and bodies as well!

Have fun!


Would drinking melted ice water from Mars taste like sparkling water? Does water on Mars form from the atmosphere as ice free from salt?
Does water desalinize through the evaporation and transpiration?

When the fresh top ice is placed in a container then pressurized to melt, would the water be carbonated?

Particulates of salt and dust is less in the atmosphere during a Martian winter so would the ice that forms be also drinkably cleaner?

https://space.stackexchange.com/questions/33884/how-to-drink-water-on-mars
https://earthscience.stackexchange.com/questions/16076/how-salty-can-ice-be

https://kottke.org/18/12/a-massive-ice-filled-crater-on-mars

Any ice that forms from water will be free of salt -- the process of crystallization does an excellent job of removing impurities.

But that's only when freezing out of liquid water, which is not a process that happens today at the Martian surface. When ice forms out of the air, it forms as tiny crystals -- "hoar frost" we call it on Earth -- and forms on substrates, such as grains of sand.  Since the soil of Mars is laden with salts of various sorts (including things a lot nastier than NaCl, such as perchlorate salts), it's hard to believe that the Martian hoar frost wouldn't be contaminated with salty sand and dust.

And, of course, the Martian atmosphere has dust suspended in it which is continually precipitating out and dust which is blown up by the not-infrequent dust storms and dust devils and all of this falls on any newly-formed frost.

It's very hard to see how there could be any macroscopic ice deposits on Mars that don't have dust and salt as impurities.


Free source of printable star charts in the format of the Millennium Star Atlas? I've been looking for sources of printable electronic star charts - there seem to be many different ones available, either as part of planetarium software or just plain charting packages. But there's such a richness of offerings that I'm having trouble figuring out which of those, if any, can produce printable charts in the format that I'm looking for.

I like the general format of the Millennium Star Atlas - black stars on a white sky background, and with horizontal lines representing 1 degree of declination and vertical lines representing 4 minutes of RA (see sample page at http://www.skyandtelescope.com/wp-content/uploads/GD-Sept2011-Chart.pdf ).

I'd like to be able to specify a range of RA and declination and get that chart.

Can anyone suggest any free star chart printing software that can print similar output for any desired sector of the sky?

I'm looking at Cartes du Ciel (http://sourceforge.net/projects/skychart/) which might have that capability, but if so, I haven't yet figured out how to enable it.

OK, I think that I've found the answer - it looks like Cartes du Ciel will do what I'm looking for (although I haven't yet figured out the simplest workflow).

Click on the icon for Equatorial Coordinates

Then click on the icon for 'configure the program'

Under the Colour tab, click on 'black on white'

Under the Lines tab, make sure that 'show grid label' and 'show coordinates grid - line' are checked.

Zoom in to the desired magnification

Click on printer icon


Entropy of black hole A line from one of the answers on a different question got me thinking:

  The simplest way to see this is probably that a black hole has a much
  higher entropy than a star or even another type of stellar remnant of
  even vaguely similar mass and so there simply could not exist a
  spontaneous process by which a black hole develops back into a star.

Now, I agree a black hole turning into a star seems far-fetched since it's a one-way trip (like you can't recover a block of sugar from a glass of water to that exact form). But as far as I know entropy is the amount of disorder. A black hole is denser than a star. For a density that high, I assume a certain amount of order (inverse entropy?) is required. It's an enormous amount of mass in a small amount of space, keeping itself together. Sounds to me like a system, not like a random collection of mass.

How can the amount of order necessary for such dense objects as black holes be lower than that of the star they originate from?

But as far as I know entropy is the amount of disorder.

Entropy is a measure of the number of possible microscopic states consistent with an observed macroscopic state1, $S = k_\text{B}\ln N$. Fundamentally it has nothing to do with disorder, although as an analogy it sometimes works. For example, in simple situations like an $n$ point-particle gas in a box: there many more ways to put point-particles in a box in disorderly manner than an orderly one. However, the exact the opposite may be true if they have a positive size and the box is crowded enough. Overall, disorder is just a bad analogy.

1 Even that's not quite true, but it's better than disorder. Specifically, it's a simplification under the assumption that all microstates are equally likely.

  A black hole is denser than a star. For a density that high, I assume a certain amount of order (inverse entropy?) is required.

If an object is crushed inside ideal box that isolates it and prevents any leaks to the outside, the crushed object still has information about what it was before. And an event horizon is about as an ideal box as there can be.

Classically, black holes have no hair, meaning that the spacetime of an isolated black hole is characterized by mass, angular momentum, and electric charge. So there are two possible responses to this: either the black hole really has no structure other than those few parameters, in which case the information is destroyed, or it does have structure that's just not externally observable classically.

Thus, if information is not destroyed, we should expect the number of microstates of a black hole to be huge simply because there's a huge number of ways to produce a black hole. Roughly, at least the number of microstates of possible collapsing star remnants of the same mass, angular momentum, and charge (though this is idealized because a realistic collapsing process sheds a lot).

  For a density that high, I assume a certain amount of order (inverse entropy?) is required.

Quite the opposite; black hole are the most entropic objects for their size.

In the early 1970s, physicists have noticed an interesting analogies between how black holes behave and the laws of thermodynamics. Most relevantly here, the surface gravity $\kappa$ of a black hole is constant (paralleling zeroth law of thermodynamics) and the area $A$ of a black hole is classically nondecreasing (paralleling second law). This is extended further with analogies of the first and third laws of thermodynamics with $\kappa$ acting like temperature and $A$ as the entropy.

The problem is that for this to be more than an analogy, black holes should radiate with temperature that's (some multiple of) their surface gravity. But they do; this is called Hawking radiation. So the area can shrink as long as there is a compensating entropy emitted to the outside:
$$\delta\left(S_\text{outside} + A\frac{k_\text{B}c^3}{4\hbar G}\right)\geq 0\text{.}$$
Thus, semi-classically, the entropy of a black hole is proportional to its surface area. In natural units, it is simply $S_\text{BH} = A/4$, which is huge because Planck areas are very small.

Thus, we know that in a semi-classical approximation, a black hole must radiate with temperature proportional to its surface gravity and entropy proportional to its area. It's natural to wonder the next step: if a black hole has all this entropy, where is the structure? How can it have so many possible microstates if it's classically just a vacuum? But going there takes us to the land of quantum gravity, which is not yet firmly established, and is outside the scope for astronomy.


How did the reionization epoch make the universe "transparent"? I've been reading about the reionization epoch of the universe, and it isn't entirely clear to me.

As I understand it, very shortly after the big bang the universe was ionized, with the electrons stripped from the protons of hydrogen. The way that it's described is that the universe was "opaque" (since it was filled with ionized gas). Then the universe cooled enough for neutral hydrogen to form again, and the universe became "transparent". When the first stars and galaxies began to form they reionized the universe.

What I don't understand is that in illustrations of this reionization, during the reionization epoch, it seems that the universe isn't transparent, but filled with unionized IGM (neutral hydrogen). Then the ionizing radiation from the newly formed stars ionizes the medium and ... these bubbles of ionized regions grow and merge until the entire universe is ionized again (and is therefore transparent).

Have I misunderstood when the universe is transparent? Or the definition of transparent? Would it be accurate to say that the present day universe is completely ionized? (Does this just mean that most matter is in the form of stars which can be considered ionized? Since it doesn't really seem immediately evident in everyday life that "everything is ionized")

Nowadays, the universe is mostly ionized, and transparent.

Transparent means that photons are able to travel long distances.

So there comes the confusion: How could it be that recombination made the Universe transparent, and reionization keep it transparent?

After the Big Bang we had a very small Universe, filled with ionized matter. That amount of ionized matter caused the photons to be short-lived, and thus the Universe were opaque.

Then it came the recombination, and most baryonic matter became neutral, the photons were able to travel long distances, and the Universe thus became transparent.

And it was expanding. And at the same time clusters of matter were collapsing.

The combination of these two factors made the intergalactic medium density to fall a lot. Not only were there much more space for the same matter, but also the matter was concentrated in small bunchs making almost all the space almost empty.

  The second phase change occurred once objects started to condense in the early universe that were energetic enough to ionize neutral hydrogen. As these objects formed and radiated energy, the universe reverted from being neutral, to once again being an ionized plasma. This occurred between 150 million and one billion years after the Big Bang (at a redshift 6 < z < 20). At that time, however, matter had been diffused by the expansion of the universe, and the scattering interactions of photons and electrons were much less frequent than before electron-proton recombination. Thus a universe full of low density ionized hydrogen will remain transparent, as is the case today. [1]

So when the clusters of matter collapsed enough to become galaxies with stars and these stars reionized the intergallactic medium, there were almost no matter there, so photons continued being free to travel without obstacles, and so the Universe remained transparent.

So, it has been transparent for two different reasons: neutrality of matter, after the recombination, and scarcity of matter, after the reionization.

1: http://en.wikipedia.org/wiki/Reionization#Background


Voyager 1 and 2 detected a 20-fold increase in plasma density, significantly different refractive index? As reported by Nature.com, Voyager 1 and 2 detected a 20-fold increase in plasma density going from the heliosphere out to interstellar space. Does that 20-fold difference in density imply a difference in refractive index, and if so, how much?

Here's the abstract of the (paywalled) paper:

  The heliopause is the boundary between the hot heliospheric (solar wind) plasma and the relatively cold interstellar plasma. Pressure balance considerations show that there should be a large (factor of 20 to 50) density increase across the heliopause. Here we report electron density measurements from the Voyager 1 and 2 plasma wave instruments near and beyond the heliopause. The plasma density in the outer heliosphere is typically about 0.002 cm⁻³. The first electron density measured by the Voyager 2 plasma wave instrument in the interstellar medium, 0.039 cm⁻³ ± 15%, was on 30 January 2019 at a heliocentric radial distance of 119.7 au. The density jump, about a factor of 20, confirms that Voyager 2 crossed the heliopause. The new density is very similar to the first density measured in the interstellar medium by the Voyager 1 plasma wave instrument, 0.055 cm⁻³, on 23 October 2013 at a radial distance of 122.6 au. These small differences in the densities and radial distances are probably due to the relative locations of the spacecraft in the boundary layer that forms in the interstellar plasma just beyond the heliopause.

The index of refraction of a plasma is the square root of the permittivity:
$$n=\sqrt{\epsilon}=\sqrt{1-\frac{\omega_p^2}{\omega^2}},\quad \omega_p\equiv\left(\frac{n_ee^2}{\epsilon_0 m_e}\right)^{\frac{1}{2}}$$
where $n_e$ is the electron number density and $e$ is the elementary charge. Note that $n$ is frequency-dependent, and so the difference in refractive index is wavelength-dependent. A plot at small $\omega$ indicates that at $\omega\gtrsim 10^5\text{ rad s}^{-1}$, this difference is fairly small, and presumably negligible for optical light, where we have $\omega\sim10^{16}\text{ rad s}^{-1}$:

The area of the spectrum where there is a significant change appears to be beyond even the interest of most radio observations.


What's the big deal with WD 1856+534 b? A few days ago (september 2020) a planet candidate was announced orbiting white dwarf WD 1856+534. Some media outlets announce it as the First Possible ‘Survivor’ Planet around a white dwarf, while others even say it is the first planet around a white dwarf. Some add that if further confirmed, (...) shows that some planets could survive the destruction of their sun-like stars. I can't find the source, but I even recall reading an article saying that the host being a stellar remnant was what made this finding relevant.
However:

the first confirmed exoplanets (1992) orbit a stellar remnant (PSR B1257+12),
there are other planet candidates around white dwarfs, and
it is not confirmed that the planet survived the death of its host star (as opposed to being formed or captured later).

Another highlight given is that it is still to be explained how such a planet survives being so close to its star, but do we know the planet is beyond its Roche limit?
I am puzzled: what exactly makes this planet's finding so special/a first? Is it just de degree of certainty about one of the previous assertions? (First around a WD, first survivor.) Is it something else? Or was it just a case of recency bias that made the discovery more visible?

I'll address the three sub-questions individually, as a way of fully answering the title question.

While there are indeed three planets orbiting PSR B1257+12, note that it's a pulsar, the compact remnant of an energetic event involving the system's progenitor. However, that event would likely have destroyed any planets that originally orbited the star, meaning that the three planets we see today likely formed afterwards (see e.g. Podsiadlowski 1993). By contrast, it seems that WD 1856+534 b may have formed prior to its parent star becoming a white dwarf, after which its orbit was disrupted and it moved into a tighter orbit.

While there are other candidate exoplanets orbiting white dwarfs, some consist of planetesimals, asteroids or other debris (e.g. WD 1145+017), some have only been detected indirectly (e.g. WD J0914+1914), and some are being actively disrupted by the star (e.g. WD 1145+017). On the other hand, WD 1856+534 b seems to be somewhat stable and is quite substantial $(M\sim10\text{-}15M_J$), likely falling into the giant planet regime.

It's certainly true that we don't know that the planet formed before the progenitor became a white dwarf, and it definitely can't be ruled out. The authors (Vanderburg et al. 2020) don't explicitly discuss a capture model, but they do attempt to make a statistical estimate of how close a nearby star is likely to have come within the past ~6 billion years, i.e. since the star became a white dwarf:
$$D\sim\left(\pi vt_{\text{cool}}n\right)^{-1/2}$$
with $v$ the mean local stellar velocity, $t_{\text{cool}}\approx5.85\;\text{Gyr}$ the cooling age of the white dwarf, and $n$ the local stellar number density$^{\dagger}$. Assuming $v\approx60\;\text{km s}^{-1}$, they find that in the last ~5.85 billion years, the closest a nearby star would be likely to come to the system is roughly 600 AU. If we consider the capture model, an encounter at that distance seems unlikely to transfer a planet to a comparatively tight orbit.
That said . . . we obviously can't rule out that sort of interaction. The same holds for a bunch of other proposed scenarios which call for "either
finely-tuned or a priori unlikely initial conditions", as Vandenburg et al. put it. So, sure, capture is possible. It just would presumably require a fairly close approach. The authors also don't (I think) explicitly address the idea of formation after the white dwarf evolved to its present state.

All of that said, yes, this isn't an enormous leap into unexplored territory, but it does seem that we have solid evidence of a massive exoplanet in a stable, close orbit around a white dwarf, and models involving it forming prior to the progenitor's evolution to a white dwarf phase appear to hold water.

$^{\dagger}$ As the system is only $\sim25\;\text{pc}$ away, they make safe assumptions about the values using data from Gaia in the solar neighborhood, i.e. $n\approx0.1\;\text{pc}^{-3}$.


Orbiting and landing on non-sperical objects If we orbit a non-spherical asteroid or moon at a sufficient distance I believe that we can consider it to be a point mass. Therefore we can take up a conventional orbit.

Assuming our lander craft is of insignificant mass compared with the body we wish to orbit and land on, how do we cope when we get near to the surface?

Presumably an orbit will become more and more chaotic the nearer we get. As we try to land, unless we somehow synchronise with the body's rotation things will be equally tricky.

What implications did this have for Rosetta's Philae and what implications will it have for much larger and maybe more irregular bodies?

Is there some rule-of-thumb method to get an approximate answer or do spacecraft simply have to make minute by minute adjustments?

Note

I understand mathematics to a reasonable level but really I'm more interested in the practicalities of such landings. To what extent can they be pre-calculated and to what extent must they be adjusted on the fly.

For bodies small enough to be non-spherical, the mass is relatively low (as an example, Rosetta's bounces took a long time - it touched the surface at 15:34, 17:25 and 17:32 GMT comet time) but the centre of mass is still the point the lander will orbit.

So a chaotic orbit is not a problem here

What will be problems are:

trying to arrange for the lander's velocity to match that of the landing site. For example if the body is spinning rapidly and an outcrop spins round into the path of the lander that could end badly
staying on the surface - Rosetta tried to use harpoons. Another proposed solution was a low powered rocket pointing upwards
while small bodies won't have an atmosphere as such, outgassing and dust can cause problems

I'm assuming 1 is the one you are most interested in here. The solution Rosetta favoured was to pick a site that was on one of the outer surfaces, rather than on the 'neck' of the comet, as this offered the cleanest route to landing. A landing on the neck would have necessitated flying into a channel on a spinning body, and that would make the calculations much harder.


Dark matter and vacuum How different would space have been if there was no dark matter?
Can space be considered vacuum even when occupied with abundant dark matter?

Non-baryonic dark matter (ie dark matter made of some particle that doesn't interact with light) had an important role in forming the large scale structure of the universe. The early universe was dominated by light radiation, and this would have prevented the early clumps of matter from growing. But dark matter isn't affected by radiation, so it can form clumps, which then attracts the normal matter towards it. Without dark matter, galaxies wouldn't have been able to form. You can read about structure formation on Wikipedia.

The second question is just about definitions. Generally, when we talk about a "vacuum" we mean a space with few molecules of gas or other atoms. This is because other particles, such as photons or dark matter particles don't contribute to pressure. A vacuum is a place with a very low pressure, not necessarily a place with no particles at all. So you can talk about a vacuum without needing to make a fuss over dark matter. However the actual density of dark matter is still very low: about the same as one hydrogen atom for every 3cm³. Even in the best vacuums that we can create on Earth, and even in interplanetary space, regular matter dominates.


Would you expect this planet to be hotter or colder than earth? I have a lab where given the mass of a star being $1.23$ times the math of our sun, some planets wobble of central start is $273$ days, and the velocity of 79 m/s. I found that the mass of the planet is $5.3\times 10^{27}$ kg and is approximately $.82$ AU away from its star. Here is a screen shot of my excel file:

Now I am asked would we expect this planet to be hotter or colder than earth?

I believe this question is rather silly since we have no idea the composition of the planet or other related factors to make any conclusion. From what I found is based on the mass and radius it is probably a water world, and since it is pretty close to its star I would say that it is probably hotter than earth.

Anyways, any suggestions or thoughts on this matter would be appreciated.

Yes, it is absolutely silly to predict an atmospheric/surface temperature without knowing it's composition or density. And we don't understand the formation process of atmospheres well enough to exclude any type of atmospheres around different stars.

There is however one more or less objective quantity that you can calculate: The equilibrium temperature. That is the temperature that the planet would have without an atmosphere.

This is essentially given by the planetary distance (which you have) and the stellar irradiation (which you have still to find), and the planetary albedo (a free parameter between 0 and 1).


Is there a way to know if there are/are not solar-system-like systems of objects without a star? There's an extremely high amount of variation in All of Known Existence. We do see many many stars, with an extensive amount of evidence for planets orbiting around stars. However, is there are way to determine whether or not there's something like a 'dark solar system'? That's the only thing I can think of to call it.
This would be made out of a central body of mass, where the majority of material is located, with smaller planet-like objects in orbit around it, not unlike Pluto and its satellites.
Well, what about nearly-massive-enough-to-be-stellar objects? Why would there not be more of these - and the inevitable satellites involved in something of that mass - than there are solar systems?
I'm also assuming these would be made out of the same material that nearly everything else is, and I would expect them to be in abundance, so, could this explain the dark matter mystery? Assuming these dark systems are made out of the same - or mostly the same - material, wouldn't that also mean that spectral analysis wouldn't be able to determine if light from stars passed through the dark systems?
Intuition tells me someone has already debunked this, but I haven't been able to find an answer one way or the other. (yet)

Brown dwarfs (ie sub-stellar objects that are too small to support hydrogen fusion) do exist and some brown dwarfs have been found to host planets.  An example of this is the brown dwarf 2M1207.  It is a very dim object, about 25 Jupiter masses. It is still quite hot, so not completely "dark", and it hosts a planet with a mass of 3-10 Jupiters.
There are doubtless other similar bodies, and smaller rogue planets with moons, though such bodies would be very hard to observe. Light would rarely pass through such a system (the gaps between systems are just too great) but if it did, the gravity of the system could cause light to be focussed.  This is called "microlensing" and is an observed phenomenon.
The idea that such bodies could explain dark matter is a good one, and it has even been given a name "MACHO" for "Massive Compact Halo Objects".  The trouble with the MACHO hypothesis is that it doesn't fit with our understanding of how matter forms after the big bang. If matter behaves how we observe to behave, then there is no way that the big bang could have created enough normal matter to explain the observed amount of mass, this means that the excess mass must be in some other form, perhaps as a "WIMP" (Weakly Interacting Massive Particle) And yes the names were chosen to mirror each other.
We seem to live in a universe dominated by WIMPs and not MACHOs.


Direction of movement sun with respect to the NGP When viewed from above, i.e., looking down from the North Galactic Pole towards the South Galactic Pole, does the sun rotate clockwise or counterclockwise around the Galactic center?

Backer and Sramek 1999
observed that relative to more distant radio sources, the galactic center
Sgr A*
had a heliocentric proper motion
of about -6 milliarcseconds per year in galactic longitude.
Reid et al. 1999
reported a similar result.
Looking from NGP to SGP,
galactic longitude
increases counterclockwise, a decrease in longitude is a clockwise motion, and the Sun revolves clockwise around the galactic center.


Official registry of lunar placenames? What's the definitive source for names of geographic objects on the moon?

Reason: As a computer graphics project I've rendered a scene based on LROC terrain data. I want to know what I'm looking at.

The most detailed atlas I can find online is this one:

http://planetarynames.wr.usgs.gov/Page/Moon1to1MAtlas

...but it's not brilliant and there are lots of small but prominent craters and ridges which aren't labelled. It may be the some of these simply don't have names, but it's also plausible that there are simply too many to list in that atlas. How can I find out for sure?

The resource you cite, in addition to this full list of crater's names are the official, definitive lists of named craters. Note that you have the coordinates of each crater in the complete list; I guess you could reuse these informations for your project. As for the atlas, it is specified in the "introduction" of the atlas that the list is up-to-date with the approved nomenclature of lunar craters.

The IAU (the International Astronomical Union) is the official organisation in charge of naming celestrial objects et al. You can rely on it.