observation - Could there be a closer star to Earth than the Alpha Centauri triple star system, excluding the Sun?

In this answer you can find a calculation for how bright "Nemesis" would be at near-infrared wavelengths. This calculation assumed we were looking for a 20 Jupiter mass object with a similar age to the Sun at a distance of 1.4 light years (to fit in with the Nemesis hypothesis). The calculated magnitudes were H=14 and W2=8 (in the WISE infrared satellite system).

If we relax the assumptions and let the object be 4.4 light years away, we have to add 2.5 magnitudes to these numbers. i.e. H=16.5 and W2=10.5. Whilst the former is right on the edge of detectability in the 2MASS survey, the former is comfortably detectable in the WISE survey (limit is about W2=15.6 see http://wise2.ipac.caltech.edu/docs/release/allsky/ ) and the object would have a large parallax/proper motion. Whether this would be detectable might depend on the presence of a good "first epoch" image. In the case of the brown dwarf Luhman 16 at 6.5 light years (but which is maybe 40-50 Jupiter masses), well it showed up easily in first epoch 2MASS images (H=9.6).

Now, your question asks whether a star may have been missed. Such an object would be much brighter than the hypothesised brown dwarf above, or Luhman 16. The "industry-standard" models of Baraffe et al. (1998) suggest that a $0.075 M_{odot}$ minimum mass stellar object at 4.4 light years would have H=7.5. It is hard to imagine how such an object would have been missed, unless it has such a small proper motion that it has not moved significantly between the 2MASS survey in the 90s to the WISE survey in ~2010. This is unlikely (but not impossible).

Once Gaia results are published in 2017, it will have complete parallax data for all stars down to about V=19. This should include even the lowest mass M-dwarf stars out to around 10 parsecs (30 light years).

How many planets are there in this solar system?

In addition to Undo's fine answer, I would like to explain a bit about the motivation behind the definition.

When Eris was discovered, it turned out to be really, really similar to Pluto. This posed a bit of a quandary: should Eris be accepted as a new planet? Should it not? If not, then why keep Pluto? Most importantly, this pushed to the foreground the question

what, exactly, is a planet, anyway?

This had been ignored until then because everyone "knew" which bodies were planets and which ones were not. However, with the discovery of Eris, and the newly-realized potential of more such bodies turning up, this was no longer really an option, and some sort of hard definition had to be agreed upon.

The problem with coming up with a hard definition that decides what does make it to planethood and what doesn't is that nature very rarely presents us with clear, definite lines. Size, for example, is not a good discriminant, because solar system bodies come in a continuum of sizes from Jupiter down to meter-long asteroids. Where does one draw the line there? Any such size would be completely arbitrary.

There is, however, one characteristic that has a sharp distinction between some "planets" and some "non-planets", and it is the amount of other stuff in roughly the same orbit. This is still slightly arbitrary, because it's hard to put in numbers exactly what "roughly" means in this context, but it's more or less unambiguous.

Consider, then a quantity called the "planetary discriminant" µ, equal to the ratio of the planet's mass to the total mass of other bodies that cross its orbital radius and have periods up to a factor of 10 longer or shorter. This is still a bit arbitrary (why 10?) but it's otherwise quite an objective quantity.

Now take this quantity and calculate it for the different bodies you might call planets:

Suddenly, a natural hard line emerges. There's a finite set of bodies that have "cleared their orbits", and some other bodies which are well, well behind in that respect. Note also that the vertical scale is logarithmic: Neptune's planetary discriminant is ~10,000 bigger than Ceres'.

This is the main reason that "clearing its orbital zone" was chosen as a criterion for planethood. It relies on a distinction that is actually there in the solar system, and very little on arbitrary human decisions. It's important to note that this criterion need not have worked: this parameter might also have come out as a continuum, with some bodies having emptier orbits and some others having slightly fuller ones, and no natural place to draw the line, in which case the definition would have been different. As it happens, this is indeed a good discriminant.

For further reading, I recommend the Wikipedia article on 'Clearing the neighbourhood', from which I took the data for the image. If you don't mind skipping over some technical bits, go for the original paper where this was proposed,

What is a planet? S Soter, The Astronomical Journal 132 no.6 (2006), p. 2513. arXiv:astro-ph/0608359.

which is in general very readable.

space - What is the composition of an asteroid in percentages?

I am developing a RPG game (or so I like to tell myself) within outer space. Within the game, players would be able to mine small asteroids and collect resources, in order to make money. I'm having some difficulty determining exactly how much of the resources there would be in relation to everything else gotten from the asteroid.

This image has provided me with a good idea of what asteroids generally contain. It does not, however, show me the quantities. For example, I'm fairly certain that an asteroid would have more oxygen than palladium, but I don't know by how much.

Question: Can someone tell me (or better yet show me) the percentage of materials within an asteroid? I realize not all asteroids are the same, so categorizing or averaging is fine. The more detailed the better. The ideal information would be a list of values that I can scale with asteroid size.

extra terrestrial - Why would discovering life on another planet be important/matter to us?

I had a funny thought, and would like to pose it to you:

When I was a kid, reading any kind of astronomy or similar books would all say that there was no other life in the universe, and made it pretty clear that anyone thinking there could be was probably off his rocker.

Nowadays, it seems this has become a very important question for astronomers and other scientists, a new deep-seated belief that must now be proven. Some seem to think it would be the most important discovery of all time.

I'm wondering why?

My guess is, if we did 'discover' life on another planet, it'll make the news for a bit, and then the average person will go back to their every day life without much more thought about it.

For me, I would think this is great, only insofar as it means a possible new source of food (i.e., some delicious exotic cuisine assuming it's edible, and possibly even some place to go conquer and colonize if there was any profitability/money in doing so).

For some scientists, though, I get the sense it's more of an axe to grind with the long-dreary he-said she-said argument of science vs. religion.

Why do you think it would matter?

My question assumes any life discovered is probably not going to be very 'interesting' life, at least nothing we're going to have a conversation with.

galaxy - Is there a map of the galaxies?

A map of all galaxies gets kind of unwieldy, like a map of all stars in the milky way or a map of every house in the country, or every grain of sand on a beach . . . you get the idea.

Start here - Local Group

Source

Then Virgo Supercluster

Source

Then local superclusters

Source

And an article, even if it's a summary it's very much worth reading, with a more recent map with a "ginormous" supercluster that includes the milky way.

and here's another, more info here.

The largest of these maps is some 520 million light years across, so this is just a tiny part of the entire known universe, which (depending on how you measure) is either 27.5 billion light years across or 84 billion light years across.

It's worth pointing out, I haven't actually answered your question, and that was deliberate, cause I think the nearest 520 million light years is enough and it's all we have a really good picture of anyway (as far as I know). The farther out you go the more holes and inaccuracies there will be in the map.

There's also some closer stuff that's blocked by the Milky way so we can't see it, like the great attractor. No worries about superclusters eating our galaxy one day because dark energy expansion keeps these super-clusters from merging most of the time. Andromeda will merge with the Milky way in about 4 billion years though but we're not expected to ever merge with the great attractor, even as it pulls our galaxy towards it, the distance between us is growing.

atmosphere - Why can't moon light (reflected sun light) turn the sky blue?

The simple answer is that it does, but it's not bright enough to be visible to the naked eye. The atmosphere scatters the blue light just like sunlight.

The full moon (like the sun) fills about 1/2 of 1 degree of the sky, the entire sky being 180 degrees, give or take, so the full moon fills less than 1 part in 100,000 of the night sky, so there simply isn't enough blue light to be visible over the brighter stars even with the brightest full moon. Our eyes are very good at seeing variations in brightness, but not that good. . . . and, for what it's worth, the night sky has always appeared to have a dark bluish tint to me, but that might just be my brain playing tricks on me because logically I know it's there. I'm not sure whether it's actually visible.

With a good sized telescope, moonlight scattering acts as a form of light pollution. Telescope users know that you get better visuals when there's no moon.

Source.

How to calculate the heliocentric velocity of an object?

The heliocentric velocity $V_mathrm{H}$ of an object is its velocity wrt. the Sun. When you measure an object's velocity, you measure it in the reference frame of Earth, which revolves around the Sun with ~30 km/s (varying a bit from aphelion to perihelion), so convert to $V_mathrm{H}$ you need to know the time of the year of the observation (unless the line of sight toward your object is exactly perpendicular to the ecliptic plane), as well as the angle between the line of sight, and the line of sight toward the Sun. This involves a number of sines and cosines that you can find in e.g. Barbieri (2006).

If you further want to convert from $V_mathrm{H}$ to the reference frame in which the Milky Way's center is at rest, the Local Group is at rest, or the Cosmic Microwave Background is isotropic (the "cosmic" frame), then you use the formula you link to, adding a similar term as described above, but instead using the velocity (i.e. speed and direction) of the Sun wrt. to the given frame.

the sun - How long does a sunrise or sunset take?

As noted in http://aa.quae.nl/en/antwoorden/zonpositie.html#14 the
length of sunrise/sunset varies from approximately 128/cos(latitude)
seconds at the equinoxes to approximately 142/cos(1.14*latitude) at
the solstices.

More specifically, here's the length of sunrise/sunset at various latitudes:

Beyond 65 degrees north or south latitude, the sun does not rise or
set daily, and the length of sunrise/sunset increases significantly.

The data plotted above is the length of sunrise, but the length of
sunset is very similar.

All calculations for this program were made with this program:

https://github.com/barrycarter/bcapps/blob/master/ASTRO/bc-solve-astro-12824.c

The raw output of sunrise/sunset times:

https://github.com/barrycarter/bcapps/blob/master/ASTRO/sun-rise-set-multiple-latitudes.txt.bz2

You can verify these results at:
http://aa.usno.navy.mil/data/docs/RS_OneYear.php

The longest sunrise I found for 2015 was at 89 degrees 51 minutes
south latitude, 125 degrees east longitude. There, the sun starts
rising 20 Sep 2015 at 2352, bobbles up and down a bit (but never quite
sets), and finally finishes rising 43 hours and 21 minutes later, at
22 Sep 2015 at 1913, but see caveat at the end of this answer.

You can "verify" this by first visiting
http://aa.usno.navy.mil/data/docs/RS_OneYear.php with these
parameters:

to get:

Sun or Moon Rise/Set Table for One Year
             o  ,    o  ,                                                                              Astronomical Applications Dept.
Location: E125 00, S89 51                          Rise and Set for the Sun for 2015                   U. S. Naval Observatory        
                                                                                                       Washington, DC  20392-5420     
                                                            Universal Time                                                            


       Jan.       Feb.       Mar.       Apr.       May        June       July       Aug.       Sept.      Oct.       Nov.       Dec.  
Day Rise  Set  Rise  Set  Rise  Set  Rise  Set  Rise  Set  Rise  Set  Rise  Set  Rise  Set  Rise  Set  Rise  Set  Rise  Set  Rise  Set
     h m  h m   h m  h m   h m  h m   h m  h m   h m  h m   h m  h m   h m  h m   h m  h m   h m  h m   h m  h m   h m  h m   h m  h m
01  **** ****  **** ****  **** ****  ---- ----  ---- ----  ---- ----  ---- ----  ---- ----  ---- ----  **** ****  **** ****  **** ****
02  **** ****  **** ****  **** ****  ---- ----  ---- ----  ---- ----  ---- ----  ---- ----  ---- ----  **** ****  **** ****  **** ****
03  **** ****  **** ****  **** ****  ---- ----  ---- ----  ---- ----  ---- ----  ---- ----  ---- ----  **** ****  **** ****  **** ****
04  **** ****  **** ****  **** ****  ---- ----  ---- ----  ---- ----  ---- ----  ---- ----  ---- ----  **** ****  **** ****  **** ****
05  **** ****  **** ****  **** ****  ---- ----  ---- ----  ---- ----  ---- ----  ---- ----  ---- ----  **** ****  **** ****  **** ****
06  **** ****  **** ****  **** ****  ---- ----  ---- ----  ---- ----  ---- ----  ---- ----  ---- ----  **** ****  **** ****  **** ****
07  **** ****  **** ****  **** ****  ---- ----  ---- ----  ---- ----  ---- ----  ---- ----  ---- ----  **** ****  **** ****  **** ****
08  **** ****  **** ****  **** ****  ---- ----  ---- ----  ---- ----  ---- ----  ---- ----  ---- ----  **** ****  **** ****  **** ****
09  **** ****  **** ****  **** ****  ---- ----  ---- ----  ---- ----  ---- ----  ---- ----  ---- ----  **** ****  **** ****  **** ****
10  **** ****  **** ****  **** ****  ---- ----  ---- ----  ---- ----  ---- ----  ---- ----  ---- ----  **** ****  **** ****  **** ****
11  **** ****  **** ****  **** ****  ---- ----  ---- ----  ---- ----  ---- ----  ---- ----  ---- ----  **** ****  **** ****  **** ****
12  **** ****  **** ****  **** ****  ---- ----  ---- ----  ---- ----  ---- ----  ---- ----  ---- ----  **** ****  **** ****  **** ****
13  **** ****  **** ****  **** ****  ---- ----  ---- ----  ---- ----  ---- ----  ---- ----  ---- ----  **** ****  **** ****  **** ****
14  **** ****  **** ****  **** ****  ---- ----  ---- ----  ---- ----  ---- ----  ---- ----  ---- ----  **** ****  **** ****  **** ****
15  **** ****  **** ****  **** ****  ---- ----  ---- ----  ---- ----  ---- ----  ---- ----  ---- ----  **** ****  **** ****  **** ****
16  **** ****  **** ****  **** ****  ---- ----  ---- ----  ---- ----  ---- ----  ---- ----  ---- ----  **** ****  **** ****  **** ****
17  **** ****  **** ****  **** ****  ---- ----  ---- ----  ---- ----  ---- ----  ---- ----  ---- ----  **** ****  **** ****  **** ****
18  **** ****  **** ****  **** ****  ---- ----  ---- ----  ---- ----  ---- ----  ---- ----  ---- ----  **** ****  **** ****  **** ****
19  **** ****  **** ****  **** ****  ---- ----  ---- ----  ---- ----  ---- ----  ---- ----  ---- ----  **** ****  **** ****  **** ****
20  **** ****  **** ****  **** ****  ---- ----  ---- ----  ---- ----  ---- ----  ---- ----  2352       **** ****  **** ****  **** ****
21  **** ****  **** ****  **** ****  ---- ----  ---- ----  ---- ----  ---- ----  ---- ----  **** ****  **** ****  **** ****  **** ****
22  **** ****  **** ****  1842 1614  ---- ----  ---- ----  ---- ----  ---- ----  ---- ----  **** ****  **** ****  **** ****  **** ****
23  **** ****  **** ****       0708  ---- ----  ---- ----  ---- ----  ---- ----  ---- ----  **** ****  **** ****  **** ****  **** ****
24  **** ****  **** ****  ---- ----  ---- ----  ---- ----  ---- ----  ---- ----  ---- ----  **** ****  **** ****  **** ****  **** ****
25  **** ****  **** ****  ---- ----  ---- ----  ---- ----  ---- ----  ---- ----  ---- ----  **** ****  **** ****  **** ****  **** ****
26  **** ****  **** ****  ---- ----  ---- ----  ---- ----  ---- ----  ---- ----  ---- ----  **** ****  **** ****  **** ****  **** ****
27  **** ****  **** ****  ---- ----  ---- ----  ---- ----  ---- ----  ---- ----  ---- ----  **** ****  **** ****  **** ****  **** ****
28  **** ****  **** ****  ---- ----  ---- ----  ---- ----  ---- ----  ---- ----  ---- ----  **** ****  **** ****  **** ****  **** ****
29  **** ****             ---- ----  ---- ----  ---- ----  ---- ----  ---- ----  ---- ----  **** ****  **** ****  **** ****  **** ****
30  **** ****             ---- ----  ---- ----  ---- ----  ---- ----  ---- ----  ---- ----  **** ****  **** ****  **** ****  **** ****
31  **** ****             ---- ----             ---- ----             ---- ----  ---- ----             **** ****             **** ****

(**** object continuously above horizon)                                                      (---- object continuously below horizon)

Note that the sun rises at 2352 on September 20th, and doesn't set for
the rest of the year, verifying the sunrise start time.

Verifying the end time is a little tricker. To do this, visit
http://ssd.jpl.nasa.gov/horizons.cgi with the following parameters:

to get:

Revised : Jul 31, 2013                  Sun                                 10

 PHYSICAL PROPERTIES (revised Jan 16, 2014):
  GM (10^11 km^3/s^2)   = 1.3271244004193938  Mass (10^30 kg)   ~ 1.988544
  Radius (photosphere)  = 6.963(10^5) km  Angular diam at 1 AU  = 1919.3"
  Solar Radius (IAU)    = 6.955(10^5) km  Mean density          = 1.408 g/cm^3
  Surface gravity       =  274.0 m/s^2    Moment of inertia     = 0.059
  Escape velocity       =  617.7 km/s     Adopted sidereal per  = 25.38 d
  Pole (RA,DEC in deg.) =  286.13,63.87   Obliquity to ecliptic = 7 deg 15'        
  Solar constant (1 AU) = 1367.6 W/m^2    Solar lumin.(erg/s)   =  3.846(10^33)
  Mass-energy conv rate = 4.3(10^12 gm/s) Effective temp (K)    =  5778
  Surf. temp (photosphr)= 6600 K (bottom) Surf. temp (photosphr)=  4400 K (top)
  Photospheric depth    = ~400 km         Chromospheric depth   = ~2500 km
  Sunspot cycle         = 11.4 yr         Cycle 22 sunspot min. =  1991 A.D.

  Motn. rel to nrby strs= apex : RA=271 deg; DEC=+30 deg
                          speed: 19.4 km/s = 0.0112 AU/day
  Motn. rel to 2.73K BB = apex : l=264.7+-0.8; b=48.2+-0.5
                          speed: 369 +-11 km/s

Results

*******************************************************************************
Ephemeris / WWW_USER Fri Jan  1 21:49:19 2016 Pasadena, USA      / Horizons    
*******************************************************************************
Target body name: Sun (10)                        {source: DE431mx}
Center body name: Earth (399)                     {source: DE431mx}
Center-site name: (user defined site below)
*******************************************************************************
Start time      : A.D. 2015-Sep-22 19:00:00.0000 UT      
Stop  time      : A.D. 2015-Sep-22 20:00:00.0000 UT      
Step-size       : 1 minutes
*******************************************************************************
Target pole/equ : IAU_SUN                         {East-longitude +}
Target radii    : 696000.0 x 696000.0 x 696000.0 k{Equator, meridian, pole}    
Center geodetic : 125.000000,-89.850000,7.057E-13 {E-lon(deg),Lat(deg),Alt(km)}
Center cylindric: 125.000000,16.7540774,-6356.730 {E-lon(deg),Dxy(km),Dz(km)}
Center pole/equ : High-precision EOP model        {East-longitude +}
Center radii    : 6378.1 x 6378.1 x 6356.8 km     {Equator, meridian, pole}    
Target primary  : Sun
Vis. interferer : MOON (R_eq= 1737.400) km        {source: DE431mx}
Rel. light bend : Sun, EARTH                      {source: DE431mx}
Rel. lght bnd GM: 1.3271E+11, 3.9860E+05 km^3/s^2                              
Atmos refraction: NO (AIRLESS)
RA format       : HMS
Time format     : CAL 
RTS-only print  : NO       
EOP file        : eop.160101.p160324                                           
EOP coverage    : DATA-BASED 1962-JAN-20 TO 2016-JAN-01. PREDICTS-> 2016-MAR-23
Units conversion: 1 au= 149597870.700 km, c= 299792.458 km/s, 1 day= 86400.0 s 
Table cut-offs 1: Elevation (-90.0deg=NO ),Airmass (>38.000=NO), Daylight (NO )
Table cut-offs 2: Solar Elongation (  0.0,180.0=NO ),Local Hour Angle( 0.0=NO )
*******************************************************************************
 Date__(UT)__HR:MN     Azi_(a-appr)_Elev
****************************************
$$SOE
 2015-Sep-22 19:00 *m  128.1772  -0.3117
 2015-Sep-22 19:01 *m  127.9272  -0.3109
 2015-Sep-22 19:02 *m  127.6771  -0.3101
 2015-Sep-22 19:03 *m  127.4270  -0.3093
 2015-Sep-22 19:04 *m  127.1770  -0.3085
 2015-Sep-22 19:05 *m  126.9269  -0.3077
 2015-Sep-22 19:06 *m  126.6769  -0.3069
 2015-Sep-22 19:07 *m  126.4268  -0.3061
 2015-Sep-22 19:08 *m  126.1767  -0.3053
 2015-Sep-22 19:09 *m  125.9267  -0.3045
 2015-Sep-22 19:10 *m  125.6766  -0.3037
 2015-Sep-22 19:11 *m  125.4266  -0.3029
 2015-Sep-22 19:12 *m  125.1765  -0.3021
 2015-Sep-22 19:13 *m  124.9264  -0.3013
 2015-Sep-22 19:14 *m  124.6764  -0.3005
 2015-Sep-22 19:15 *m  124.4263  -0.2997
 2015-Sep-22 19:16 *m  124.1762  -0.2989
 2015-Sep-22 19:17 *m  123.9262  -0.2981
 2015-Sep-22 19:18 *m  123.6761  -0.2973
 2015-Sep-22 19:19 *m  123.4261  -0.2964
 2015-Sep-22 19:20 *m  123.1760  -0.2956
 2015-Sep-22 19:21 *m  122.9259  -0.2948
 2015-Sep-22 19:22 *m  122.6759  -0.2940
 2015-Sep-22 19:23 *m  122.4258  -0.2932
 2015-Sep-22 19:24 *m  122.1757  -0.2923
 2015-Sep-22 19:25 *m  121.9257  -0.2915
 2015-Sep-22 19:26 *m  121.6756  -0.2907
 2015-Sep-22 19:27 *m  121.4256  -0.2899
 2015-Sep-22 19:28 *m  121.1755  -0.2890
 2015-Sep-22 19:29 *m  120.9254  -0.2882
 2015-Sep-22 19:30 *m  120.6754  -0.2874
 2015-Sep-22 19:31 *m  120.4253  -0.2865
 2015-Sep-22 19:32 *m  120.1753  -0.2857
 2015-Sep-22 19:33 *m  119.9252  -0.2849
 2015-Sep-22 19:34 *m  119.6751  -0.2840
 2015-Sep-22 19:35 *m  119.4251  -0.2832
 2015-Sep-22 19:36 *m  119.1750  -0.2823
 2015-Sep-22 19:37 *m  118.9250  -0.2815
 2015-Sep-22 19:38 *m  118.6749  -0.2807
 2015-Sep-22 19:39 *m  118.4248  -0.2798
 2015-Sep-22 19:40 *m  118.1748  -0.2790
 2015-Sep-22 19:41 *m  117.9247  -0.2781
 2015-Sep-22 19:42 *m  117.6746  -0.2773
 2015-Sep-22 19:43 *m  117.4246  -0.2764
 2015-Sep-22 19:44 *m  117.1745  -0.2756
 2015-Sep-22 19:45 *m  116.9245  -0.2747
 2015-Sep-22 19:46 *m  116.6744  -0.2739
 2015-Sep-22 19:47 *m  116.4243  -0.2730
 2015-Sep-22 19:48 *m  116.1743  -0.2721
 2015-Sep-22 19:49 *m  115.9242  -0.2713
 2015-Sep-22 19:50 *m  115.6742  -0.2704
 2015-Sep-22 19:51 *m  115.4241  -0.2696
 2015-Sep-22 19:52 *m  115.1740  -0.2687
 2015-Sep-22 19:53 *m  114.9240  -0.2678
 2015-Sep-22 19:54 *m  114.6739  -0.2670
 2015-Sep-22 19:55 *m  114.4239  -0.2661
 2015-Sep-22 19:56 *m  114.1738  -0.2652
 2015-Sep-22 19:57 *m  113.9237  -0.2644
 2015-Sep-22 19:58 *m  113.6737  -0.2635
 2015-Sep-22 19:59 *m  113.4236  -0.2626
 2015-Sep-22 20:00 *m  113.1735  -0.2618
$$EOE
*******************************************************************************
Column meaning:

TIME

  Prior to 1962, times are UT1. Dates thereafter are UTC. Any 'b' symbol in
the 1st-column denotes a B.C. date. First-column blank (" ") denotes an A.D.
date. Calendar dates prior to 1582-Oct-15 are in the Julian calendar system.
Later calendar dates are in the Gregorian system.

  Time tags refer to the same instant throughout the universe, regardless of
where the observer is located.

  The dynamical Coordinate Time scale is used internally. It is equivalent to
the current IAU definition of "TDB". Conversion between CT and the selected
non-uniform UT output scale has not been determined for UTC times after the
next July or January 1st.  The last known leap-second is used over any future
interval.

  NOTE: "n.a." in output means quantity "not available" at the print-time.

SOLAR PRESENCE (OBSERVING SITE)
  Time tag is followed by a blank, then a solar-presence symbol:

        '*'  Daylight (refracted solar upper-limb on or above apparent horizon)
        'C'  Civil twilight/dawn
        'N'  Nautical twilight/dawn
        'A'  Astronomical twilight/dawn
        ' '  Night OR geocentric ephemeris

LUNAR PRESENCE WITH TARGET RISE/TRANSIT/SET MARKER (OBSERVING SITE)
  The solar-presence symbol is immediately followed by another marker symbol:

        'm'  Refracted upper-limb of Moon on or above apparent horizon
        ' '  Refracted upper-limb of Moon below apparent horizon OR geocentric
        'r'  Rise    (target body on or above cut-off RTS elevation)
        't'  Transit (target body at or past local maximum RTS elevation)
        's'  Set     (target body on or below cut-off RTS elevation)

RTS MARKERS (TVH)
  Rise and set are with respect to the reference ellipsoid true visual horizon
defined by the elevation cut-off angle. Horizon dip and yellow-light refraction
(Earth only) are considered. Accuracy is < or = to twice the requested search
step-size.

 Azi_(a-appr)_Elev =
   Airless apparent azimuth and elevation of target center. Adjusted for
light-time, the gravitational deflection of light, stellar aberration,
precession and nutation. Azimuth measured North(0) -> East(90) -> South(180) ->
West(270) -> North (360). Elevation is with respect to plane perpendicular
to local zenith direction.  TOPOCENTRIC ONLY. Units: DEGREES


 Computations by ...
     Solar System Dynamics Group, Horizons On-Line Ephemeris System
     4800 Oak Grove Drive, Jet Propulsion Laboratory
     Pasadena, CA  91109   USA
     Information: http://ssd.jpl.nasa.gov/
     Connect    : telnet://ssd.jpl.nasa.gov:6775  (via browser)
                  telnet ssd.jpl.nasa.gov 6775    (via command-line)
     Author     : Jon.Giorgini@jpl.nasa.gov

*******************************************************************************

The sun's angular diameter is about 32 arcminutes, so the sun's lower
limb is 16 arcminutes below the sun's center. When the center of the
sun has geometric elevation -18 arcminutes (-0.3 degrees), the lower
limb has geometric elevation -34 arcminutes. Since refraction near the
horizon is also 34 arcminutes, the sun's lower limb rises when the
sun's geometric elevation is -0.3 degrees.

In the table above, this occurs between 1914 and 1915, but my program
uses slightly more accurate data for the sun's angular diameter, and
the sun actually finishes rising between 1913 and 1914 (and closer to
1913).

You can then fly almost halfway across the world to latitude 89
degrees 51 minutes and longitude -19 degrees to see the
one-minute-shorter longest sunset, which starts at 23 Sep 2015 at 2128
and ends at 25 Sep 2015 at 1648, a length of 43 hours and 20 minutes.

In this case, you would use
http://aa.usno.navy.mil/data/docs/RS_OneYear.php to verify the ending
time of the sunset, and HORIZONS to verify the start time of the
sunset.

Polar sunrises and sunsets are considerably shorter:

• At the North Pole, the sun starts rising at 18 Mar 2015 at 2015,
  and finishes rising at 20 Mar 2015 at 0441, a length of 32 hours and
  26 minutes.




• At the South Pole, the sun starts setting at 21 Mar 2015 at 1650,
  and finishes setting at 23 Mar 2015 at 0117, a length of 32 hours
  and 27 minutes.




• At the South Pole, the sun starts rising at 21 Sep 2015 at 0508,
  and finishes rising at 22 Sep 2015 at 1400, a length of 32 hours and
  52 minutes.




• At the North Pole, the sun starts setting at 24 Sep 2015 at 0243,
  and finishes setting at 25 Sep 2015 at 1131, a length of 32 hours
  and 48 minutes.

Main caveat: Like HORIZONS and the sunrise/sunset tables above, I
assume 34 arcminutes of refraction at the horizon. That's reasonable
for most locations, but may be unreasonable close the pole, where the
longest sunrises and sunsets occur. In particular, refraction can
change rapidly at these latitudes, allowing for potentially much
longer sunrises and sunsets.

I now believe that http://what-if.xkcd.com/42/ is inaccurate, and will
ping the author to let him know.

moonlanding - Why aren't people going to The Moon any more?

I see from your comment on another post that you are a so-called skeptic, so no answer that is given will satisfy your irrational beliefs.

The simple reason is that it is very expensive; nothing to do with technological capabilities. And as there is little to be gained scientifically, then astronomers/geologists etc. would rather spend the money elsewhere.

Politicians are unwilling to ask taxpayers/voters to finance further manned (or womanned) trips to the Moon. For instance each space shuttle mission cost around a billion dollars. The cost of the Apollo programme in today's money would easily exceed 100 billion dollars.

Priorities may change when a Chinese person walks on the Moon...

Also worth noting that neither the USA or Russia have the launch capabilities of a Saturn V or similar at the moment. http://space.stackexchange.com/questions/5459/russian-manned-moon-landing-capability-today

gravity - How did LIGO verify that the gravitational waves originated 1.3 billion years ago from two specific black hole collision?

The original paper states the methodology used to determine the location of the gravity waves. The bottom line is that they were detected in two different locations. Using the difference in timing, one can estimate the location in the sky to an area of about 600 degrees squared where the origin of the wave could have been. The two signals had an offset of 7 ms. Pure speed of light differences would have been 10 ms. This can be used to effectively identify a circle in the sky where the merger might have happened. Of course, this doesn't reveal the distance.

The distance was figured by a number of things. First of all, the frequency pattern was used to estimate the mass of each black hole. Using these masses, one can find the estimated total power of the event. Using this estimate, and seeing the estimate of the amplitude from the signal, one can use the inverse squared law to estimate the distance to said event.

How high does light pollution reach into the sky?

Light pollution occurs because light from the ground refects off atoms in the atmosphere. So you can reduce light pollution either by getting away from light, or getting above the atmosphere. 50%of the atmosphere lies below 5500m, if you can get 5500 m high, you half light pollution. Getting high also improves clarity, and reduces the disturbance caused by atmospheric turbulence, which is why most major telescopes are situated on mountain tops.

universe - Why does a planet rotate and revolve?

As the planets evolve during their protoplanetary stage and accrete materials from the protoplanetary disks, which are gravitationally collapsing interstellar dust and gases, these accreted particles retain some of the angular momentum from the materials they form from and being in constant motion.

    

      ^{Generated image (virtual fly-by) from a simulation of the accretion period of the protoplanetary disk, showing preservation of
      angular momentum in the orbit around a Jupiter-size planet, as it clears its neighborhood. (Source: Frédéric Masset)}

One nice description for this angular momentum preservation, and why the planets appear to rotate faster than their surrounding protoplanetary disk goes like this:

Conservation of angular momentum explains why an ice skater spins more
rapidly as she pulls her arms in. As her arms come closer to her axis
of rotation, her [rotation] speed increases and her angular momentum remains the
same. Similarly, her rotation slows when she extends her arms at the
conclusion of the spin.

^{Source: Scientific American article on Why and how do planets rotate? (George Spagna)}

So it could be described as this axial rotation of planets resulting in conservation of the angular momentum of the materials in the protoplanetary disk, forming during the accretion period of the planetary system as the protoplanets gain in weight, and preserve this angular momentum due to inertia of their radial velocity.

star - Image sets for testing stacking algorithms?

I am looking for sets of astronomical images for testing different kinds of stacking algorithms.

The idea is simple: if one has $N$ images of the same object, the signal-to-noise ratio of the averaged image increases with $sqrt{N}$ under certain conditions.

Where could I obtain data for testing?

PS. I would propose adding the tags "image", "ccd", "research", "signal processing", and "machine learning".

Is a supernova's core temperature absolute zero just before collapse?

No, absolutely not. The core of a core-collapse supernova is one of the hottest places in the present-day universe. The temperature as the star runs out of nuclear fuel in its core is around 6-10 billion Kelvin. As it collapses, the core gets even hotter, perhaps as high as 100 billion Kelvin for a few seconds, before neutrino cooling starts to become effective.

We know that the temperatures are getting as high as this because at least two neutrino instruments on Earth detected roughly "thermal neutrinos" with energies of $sim 10$ MeV coinciding with SN1987A in the Large Magellanic Cloud (see for instance Mirizzi et al. 2015).

orbit - Day Length on Double Planets

Yes, if the planets are tide locked to one another, their orbital period and sidereal day are the same. That's assuming their orbital plane about each other isn't much inclined to their orbital plane about the star.

From the Wikipedia article on orbital period:

$T_d=2pi*sqrt{a_p^3/(G*(M_1+M_2))}$

Where G is gravitational constant, $M_1$ is mass of 1st body and $M_2$ mass of 2nd body. $a_p$ is semi major axis of elliptical orbit of the planets about each other.

For the planet's year you'd use a very similar equation.

$T_y=2pi*sqrt{a_s^3/(G*(M_0+M_1+M_2))}$

Where $a_s$ is the radius of their orbit about the sun and $M_0$ is the mass of the sun. If you like, you can leave off $M_1$ and $M_2$ as their masses are probably neglible in comparison to the sun's mass.

I am guessing by "conventional day" you want the time an inhabitant sitting on his front porch would measure between one high noon and the next.

Solar day = sidereal day * (1 + (sidereal day/year))

For example our solar day is about 1/365 longer than a sidereal day.