How long does a sunrise or sunset take? From the time that the sun appears on the horizon, or meets it on its setting, to the time that it is fully visible, or no longer visible on its setting, how much time passes?
Secondly, is there a place in the world where a sunrise/sunset occurs over a period of a few days? Meaning, that from the time it begins to appear over the horizon until it is fully visible, a period of a few days pass without night intervening (and the same for the opposite with sunset)?

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.


What is the orientation of planetary orbits? I'm working on a planetary motion simulator. I've been working through the equations anomaly, eccentricity, etc. The one thing I'm curious about is if all the ellipses are oriented the exact same way (the direction of the periapsis of all planets are pointing the same way), or is it different for each planet?

What would this be called? I've only seen eccentricity and inclination.

No, the planet orbits do not have their major axis aligned.

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

You have Inclination and Longitude of the ascending node defining the plane, and Argument of periapsis defining the ellipse orientation in that plane.


How many arms does the Milky Way galaxy have? How many arms does the Milky Way galaxy have and how is it determined?

This is actually a really, really tough question.
Look at this diagram:

Purple: Norma Arm and Outer Arm.

Green: Scutum-Centaurus Arm

Pink: Carina-Sagittarius Arm

Cyan: 3 kpc Arm and Perseus Arm
So we can slightly modify this picture by saying that there are four arms, and calling them by the following names:
Norma-Outer Arm
This arm has one end at the center of the Milky Way; this end is called the Norma Arm. It's actually quite small. However, as you continue outward along the lanes of gas, dust and stars, the Norma Arm becomes the - wait for it - Outer Arm.
Scutum-Centaurus/Crux-Scutum Arm
This arm also emanates from the center (well, duh!) and is distinguished by large numbers of clusters of red supergiants near the center (where it is referred to as the Scutum Arm) of the Milky Way. The Scutum-Sagittarius Arm is one of the Milky Way's two major arms.
Carina-Sagittarius Arm
The defining feature of this arm is that is has many H II regions, where ionized gas is plentiful. It is though that stars can form there. However, H II regions are also present in many other parts of the galaxy. Observations from the Spitzer Space Telescope seem to support these theories and prior observations.
The Spitzer results are, I think, very important for understanding our galaxy and the things in it. It confirmed the existence of all four arms, but also confirmed that the Norma-Outer Arm and Sagittarius Arm are relatively minor compared to the other two.
Near/Far 3 kpc Arm and Perseus Arm
The Near and Far 3 kpc Arms (which really are just one arm) are very close to the galactic center - about 3,000 parsecs (hence the name). The Far Arm was only discovered recently, while the Near Arm was first observed about 50 years ago. These two mini-arms are expanding outward at an enormous rate. The combined 3 kpc Arm then becomes the Perseus Arm, which extends out to about 3.5 times the distance from the center of the 3 kpc Arm. It is the other major spiral arm of the Milky Way.

Okay, so that's simple enough. The Milky Way has four spiral arms, right? Well . . . sort of. The discovery of the "New Outer Arm"1 shook things up, because that would mean that the Norma-Outer Arm reaches nearly all the way around the Milky Way. That's strange and borderline inexplicable. Well, not really. But it's very interesting.
This image gives you a good idea of what it might look like:

By the way, someone at one point wondered, "Which are we in?" That, too, doesn't have a simple answer. Current observations state that we're in the Orion-Cygnus Arm, a/the midget of the galaxy (depending on whether or not there are other "mini-arms"). It's tiny - 1.1 kpc wide and 10 kpc long - and is between the Carina-Sagittarius Arm and the Perseus Arm.
Here's what it might look like:

Putting it all together, Robert Hurt made a very famous impression of what the Milky Way might look like:

NASA gives a good explanation of it here.
If you want a good overview on everything, check out this blog post, which is reasonably well-sourced. Another post explains the four-arms-vs.-two-arms issue:

Like the proverbial blind men describing an elephant, the two groups of scientists are examining very different parts of the Milky Way. The Spitzer study detected hot objects visible in infrared. The CfA study used radio telescopes which can also detect colder objects such as supernova remnants, very young star formation regions and huge clouds of hydrogen gas. So it seems as though the older established star formation regions are mostly concentrated in two spiral arms, but that the very new star formation regions and the hydrogen clouds from which they form are also developing in two additional arms.
If you were hovering above the Milky Way in a spacecraft using binoculars or a regular optical telescope, you would see two main arms. But if you also had a radio telescope with you, it would detect two more.

The other group of scientists referred to here used the Very Long Baseline Array to image the Milky Way with radio waves; their observations support the four-main arms model.
So at the moment, I think the answer is four, though two arms are more distinct that the other two.

1Dammit, paywall! Thank goodness for arXiv.


Simulating location of the moon I'm researching high altitude photography and want to know a schedule for taking pictures of the moon.

Preferably I want a picture of the (full) moon on the horizon.

How would I go about scheduling such an event from a high altitude location?

The US Naval Observatory has a web page which lets you print a table of moonrise/moonset times for any location, together with twilight times. If you use this approach, you would print the data and then examine it for a suitable date. I don't know of a computing resource where you would put in your requirements it would report the appropriate date and time.


Why would astronomers want to eclipse the Sun? I've heard that some astronomers are building an artifical solar-eclipse ship, that should help us to dim the diffraction of the Sun.

Why don't we just wait till night? What is the function of the eclipse-ship really?

As Conrad Turner notes, that talk is about a device for blocking the light from another star (one you think might have planets orbiting around it). It's not for blocking the light of the Sun!

If you try to look at a planet orbiting around another star, the glare from the star makes it very hard to see the planet. By placing a specially shaped device in front of the telescope (in this case, thousands of kilometers in front), between the telescope and the star you're interested in, you block most of the light from the star, but not the light from any planets near that star.

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


Tallest cliffs, by falling duration If you threw something over a certain cliff on a moon of Uranus, it would take over ten minutes to hit bottom.

By this measure, local fall time rather than absolute height, what are the near contenders to the prize for tallest cliff on a body in hydrostatic equilibrium in the solar system?  Throwing the object horizontally at a few meters per second is ok.  Not ok is throwing it beyond a tiny body's Hill sphere.  Cliffs, not spheres.

Simplifying assumptions about atmospheres and exospheres are ok.

Let's apply @uhoh's answer's equation to a few bodies.  Numbers come from the obvious Wikipedia pages unless otherwise stated.  G = 6.6741e-11, in kg-m-s.

Verona Rupes on Miranda.
f = 20 km / 236 km, ρ = 1200 kg/m^3.  So t = 711 s.

Enceladus has rifts 1 km deep.
f = 1 km / 252 km, ρ = 1600.  So t = 133 s.

Those are the two bodies with smallest mean radius, of all solar system bodies in
hydrostatic equilibrium.

Let's try a much smaller eccentric body, just to check.
The nucleus of the comet Wild 2 has cliffs "hundreds of meters tall" (Ulivi's "Robotic Exploration of the Solar System," part 3, p. 228).
f = 200 m / 6400 m, ρ = 600.  So t = 610 s.

So Miranda's Verona Rupes beats even a comet.  Compared to Miranda, any other round body is much bigger (f is much smaller) and significantly denser (ρ is bigger), so to beat this it would need truly brobdingnagian cliffs, which flybys would have seen by now at least with radar if not with visible light.

I could grind through rest of that table of bodies in h.eq., but it's a safe bet that Verona Rupes wins by a long shot, Enceladus is a distant second, and everything else is a giant hardly differentiated peloton.


Temperature of the intracluster medium (ICM) I've read once in an article that the temperature of the medium within galaxy clusters (the intracluster medium or ICM) is extremely high.

What does that temperature really mean? Particle velocity?
Why is that so?
Is that medium the "Void"?
Is it in conflict with the CMB temperature (that's about 3 K)?

The answer to your first question is "Yes, the temperature referred to is the 'normal' temperature, reflecting the average kinetic energy of the gas particles".

The answer to your second question is a bit more complex:

Cooling function

Gas cools by various processes, with an efficiency depending on the temperature, the density, and the composition of the gas. At "low" temperatures, most cooling happens because particles collide and their kinetic energy is used to excite or even ionize atoms. The atoms subsequently recombine or de-excite, emitting radiation which can carry energy away from the system.

Most of the gas in the Universe is hydrogen, which cools most efficiently around $10^4\,\mathrm{K}$. At higher temperatures, helium becomes the major contributor. Additionally, various metals contribute at various temperatures depending on their density and ionization state. At very high temperatures ($\gtrsim10^7\,\mathrm{K}$) where the gas is highly ionized, cooling mainly happens through bremsstrahlung, i.e. deceleration of charged particles by other charged particles.

The figure below (from Mo, Bosch & White 2010) shows the cooling function for various metallicities (with my own annotations):

Collapse of gasous halos

So what makes the intracluster medium (ICM) different from the interstellar medium (ISM)? In the expanding Universe, overdensities try to contract. These overdensities can reach hydrostatic equilibrium only if radiative cooling is small. If the cooling timescale $t_\mathrm{cool}$ is much smaller than the free-fall timescale $t_\mathrm{ff}$, it can collapse and form stars. From the virial theorem, which gives the relation between potential and kinetic energy of the system, you can calculate the relation between the two timescales. It's a bit too much math for this post, but not difficult. I recommend reading chapter 8.4 in Mo, Bosch & White (2010). Their Fig. 8.6 shows a cooling diagram with the locus of $t_\mathrm{cool} = t_\mathrm{ff}$ in the density-temperature plane:

Above the locus, cooling is effective, and the cloud can collapse. The tilted dashed lines are lines of constant gas mass. Now you see that even for solar metallicity (the $Z = Z_\odot$ line), clouds of gas masses larger that $\sim10^{13}\,M_\odot$ are unable to cool. This is the fundamental difference between a galaxy and a cluster. They are both overdensities that resisted the expansion, but the gas in the cluster is not able to cool and form stars.

Voids

When people talk about "voids", they usually refer not to the ICM — which is the hot and dilute gas between the galaxies of a cluster — but to the immense regions of even more dilute gas which are… well, void of galaxies. Take a look at this image (from here). Here, I marked clusters with green and voids with purple. I also marked some filaments with cyan.

The cosmic microwave background

As for your last question, the ICM doesn't interact much with the CMB, so they're not in thermodynamic equilibrium, and there's no conflict. A small fraction (<10%) of the CMB photons does interact. This doesn't change the state of the gas, but it tends to polarize a fraction of the CMB, and this we can use to study the ionization history of the Universe.

EDIT: I forgot about the Sunyaev-Zel'dovich effect, which is the interaction of hot electrons with CMB photons. Rob Jeffries discusses this in his answer.


How to communicate Fine Structure Constant to aliens? Sorry if this is not the right question for this channel, I just need to know and there is no good place to ask.
It is said the Fine Structure Constant α is one of the most fundamental constants in nature. Unlike the likes of the speed of light, α is dimensionless. I have heard an argument that when we encounter intelligent aliens sending the α at them is the best way to let them know how much we know. It was said that we should of course send the constant in the binary numeral system.
My question is: how do you communicate a fraction (α = roughly 1/137) in binary so that even aliens can decode it?

Send two pulses of whatever (light, radio) you are using for communication, one a factor of $\alpha$ longer than the other.


Is there any possible connection between Gamma Ray Burst and Fast Radio Bursts? The physical nature of Fast Radio Bursts (FRBs), a new type of cosmological transients discovered recently, is not known. It has been suggested that FRBs can be produced when a spinning supra-massive neutron star loses centrifugal support and collapses to a black hole.

Here we suggest that such implosions can happen in supra-massive neutron stars shortly (hundreds to thousands of seconds) after their births, and an observational signature of such implosions may have been observed in the X-ray afterglows of some long and short Gamma-Ray Bursts (GRBs).

Under this picture, a small fraction of FRBs would be physically connected to GRBs. Is there any possible connection between Gamma Ray Bursts and Fast Radio Bursts ??

There is a tentative detection of a GRB simultaneously with an FRB,  DeLaunay et al 2016. If it is true, it would mean that at least some FRBs have the same origin as some GRBs.

Theoretically many possibilities were suggested before and after this tentative, detection. For now they all remain quite speculative, and more observations are required (and planned)
For some selection of models, it could be interesting to look into citations to this paper or its references.


Can't pulsars and stars be used for gravitational wave measurement? Suppose that we collected photons from a distant star, and they arrive regularly at 15 photons every microsecond onto a CCD, when a gravity wave bends space time, wouldn't the regular 15 photons graph wobble as a result of the gravitational wave?

Why does the LISA observatory have to compare laser timings, if pulsars can be as precise as atomic clocks, can't it just compare a bunch of pulsar signals? A single satellite that records 50 pulsars would move compared the the pulsars and would have a frequency of 50Hz ticks with pulses which can be graphed at kHz frequencies.

How LIGO, LISA, etc. Detect Gravitational Waves

The point of instruments like LIGO and LISA is to measure time-varying changes in the distance within different arms of the instrument. In the case of an arm oriented in the direction of an incoming gravitational wave (GW), the length of the arm will increase and  decrease, while an arm oriented perpendicular will remain unchanged.

The method for measuring this is to split a laser beam, send each half down the different arms, and then recombine them and look at the interference pattern. If the arms have identical lengths, then the beams will destructively interfere. If, however, one of the arms becomes longer (or shorter), the recombined beams will be out of phase, and you will no longer have perfect destructive interference.

The key to being able to measure this sort of thing accurately is two things:

Arm length, so the beam traveling down the altered-length path has time
to accumulate a large enough phase shift for the interference pattern to
change enough to be measured. Longer arms lead to larger phase shifts,
which is why LISA aims to have arms that are 2.5 million km in length.
The wavelength of the light: the shorter the wavelength, the smaller the
actual shift in arm length needs to be to make the beam shift appreciably
in phase. If the recombined beams differ in phase by 0.001%, then the change
in the interference pattern will be very hard to detect and measure. But if they differ
in phase by 50%, then the difference will be obvious. Shorter wavelengths
make for larger phase shifts.

If you want think of the interferometric measurement as a kind of timing measurement (that is, we are, in a sense, measuring the difference in time it takes light to travel down the two arms), then optical light is vastly better than radio for "timing" measurements.

Pulsar Timing

As you pointed out, there are projects that use pulsars to try to detect gravitational waves (e.g., pulsar timing arrays).

These work by measuring time delays in the arrival of radio pulses from millisecond pulsars. However, it's important to note the limitations: in order to measure the arrival time accurately, you have to observe a pulsar over a period of several minutes (or more) and add up all the pulse arrivals to get a single, accurate value. (You are not measuring changes between the arrival of one single pulse and the next.) You then repeat this measurement several weeks or months later to get another arrival time. Since you're taking measurements weeks or months apart, you can only detect variations on that same time scale, which is why pulsar timing arrays are hoping to detect GWs with periods of months to years (e.g., from binary supermassive black holes). The kinds of GWs that LIGO detects -- with periods of fractions of a second -- are completely beyond what pulsar timing arrays can do.

We cannot use optical light from stars, because we know of no star that produces the exquisitely regular periodicity of a millisecond pulsar.

As a final comment, your scenario of photons that "arrive regularly at 15 photons every microsecond onto a CCD" has the problem that Poisson noise (a.k.a. "shot noise") means that you wouldn't actually get 15 photons every microsecond; you'd get around 15 every microsecond (a typical sequence might look like this: 16, 15, 18, 15, 14, 14, 15, 23, 16, 12, ...). With that much variation, it's really hard to detect subtle variations in intrinsic signal strength.


Is the American Astronomical Society 230 years old? The BBC News item Light shed on mystery space radio pulses is interesting enough in its own right. But I noticed that it is reporting on news from the 231st meeting of the American Astronomical Society.

These kinds of meetings are usually annual; for example the 120th Annual meeting of the American Physical Society will be in 2019.

Were there enough astronomers in the US in 1787 to have an American Astronomical Society and it's first meeting?

If it turns out that the answer is that meetings were more frequent than annual, why would that be?

The first meeting of the AAS was held in September 1899 in Williams Bay, Wisconsin. The meetings were held once a year initially. The frequency varied somewhat for the next 60 or so years. The meetings appear to have settled down to twice a year about 40 years ago, one in January and another in late spring.

Source: https://aas.org/meetings/past-aas-meetings


What would happen if a Black Hole and White Hole Collided? Given our understanding of both black holes and white holes, what would be the outcome if they were both to suddenly collide?

Black Holes:

  A black hole is a region of spacetime from which gravity prevents
  anything, including light, from escaping. The theory of general
  relativity predicts that a sufficiently compact mass will deform
  spacetime to form a black hole. The boundary of the region from which
  no escape is possible is called the event horizon.

White Holes:

  In general relativity, a white hole is a hypothetical region of
  spacetime which cannot be entered from the outside, although matter
  and light can escape from it.

From Wikipedia:

  In quantum mechanics, the black hole emits Hawking radiation and so can come to thermal equilibrium with a gas of radiation. Because a thermal-equilibrium state is time-reversal-invariant, Stephen Hawking argued that the time reverse of a black hole in thermal equilibrium is again a black hole in thermal equilibrium. This implies that black holes and white holes are the same object

The excerpt above was an argument made by Stephen Hawking, explaining that a white hole and a black hole are essentially the same object. With this in mind, we can assume that the collision will be between two black holes. The result will then be the radius of the black hole added to the radius of the white hole.


Why are asteroids with circular orbits rare? Regarding this excellent question by Swike: Why are asteroids with zero orbital inclination rare?, I recently proposed that orbits with zero inclination are rare as a natural result of the statistical distribution of orbital revolution axes extended onto the celestial sphere.
In the below graph we see a similar sparsity of orbital eccentricities near zero. If asteroid orbital eccentricities were normally distributed about zero, wouldn't we expect them to be densest here?  What is the explanation?

Orbital eccentricities cannot be normally distributed around zero, since the minimum eccentricity is zero.
Thus, in the same way that the Maxwell-Boltzmann distribution for particle speeds is zero at zero speed and peaks at finite speed, then no asteroids have exactly zero eccentricity and we would expect a peak at higher eccentricities.
The analogy can be extended further by asking what distribution might be expected for a "thermal distribution" of eccentricities - a distribution where the energies of the orbits follow the Boltzmann distribution and have been allowed to randomise by some unspecified dynamical or formation processes (e.g. Geller et al.
2019 ). In those case the density of eccentricities should go as
$$n(e) \propto e^2$$
and there should be more high eccentricity objects than low eccentricity objects.
Of course asteroids are not expected to be represented by this random distribution, since their dynamical histories likely have them being perturbed away from very low eccentricity orbits induced by the drag of a gas/dust disc in the early solar system.
Nevertheless, any initial perturbation moves them away from zero eccentricity and subsequent random perturbations might be expected to make them head towards the thermal distribution. I would guess that objects in the top end of the thermal distribution must also then just get selected out - falling into the Sun, hitting planets or getting ejected out of the main asteroid belt.


What is the organic material in Saturn's rings? The phys.org articleSurprising chemical complexity of Saturn's rings changing planet's upper atmosphere about the chemical complexity of Saturn's rings, quotes a member of the INMS (Thomas Cravens) as saying,

  What the paper is describing is the environment in the gap between the inner ring and upper atmosphere, and some of the things found were expected, such as water," Cravens said. "What was a surprise was the mass spectrometer saw methane—no one expected that. Also, it saw some carbon dioxide, which was unexpected. The rings were thought to be entirely water. But the innermost rings are fairly contaminated, as it turns out, with organic material caught up in ice. (italics and boldface are mine).

What is the nature of this organic material?

Does he simply mean a few carbon-based molecules like methane, propane, and butane, or maybe only methane,  or is it a much more interesting cocktail of chemicals than that?

It also links to Chemical interactions between Saturn’s atmosphere and its rings published in Science, but the article is paywalled.

It is indeed a much more interesting cocktail of chemicals.
Before plunging into Saturn, Cassini collected some invaluable data about composition of its atmosphere and how the complex ring system works.
Cassini experienced some "heavy ring rains" in Saturn's innermost D ring, which turns out to be dust grains. The probe tried to use the Ion and Neutral Mass Spectrometer (INMS) to analyse the chemical content of this rain even though it was designed to analyse gases but the rain hit Cassini at such tremendous velocity that INMS instantly vaporized. It was able to collect some data about its composition which include molecular hydrogen, water, butane and propane and trace amount of  methane, ammonia, carbon monoxide, nitrogen and carbon dioxide. A second study was done when Cassini used Cosmic Dust Analyzer(CDA) and inspected 2,700 grains of dust and found that, while most of the material (around 95 percent) was water ice, there was also trace amount of silicates.
The material flowed from the ring downpour was huge. Cassini measured 22,000 pounds (10000 kg) of material per second falling from rings. Most of the chemical slurry is being purged from the D-ring and in turn the C-ring is somehow replenishing the D ring (which is playing a role in the composition of the planet's ionosphere and atmosphere).
Cassini also predicted the composition of E-rings by inspecting the ice grains ejecting from Enceladus. New kinds of organic compounds have been detected in the plumes bursting from Saturn's moon Enceladus. Powerful hydrothermal vents eject material from Enceladus' core, which mixes with water from the moon's massive subsurface ocean before it is released into space as water vapor and ice grains. The newly discovered molecules, condensed onto the ice grains, were determined to be nitrogen and oxygen-bearing compounds. The organic material found were categorized in three types: (i) Amine (ii) Carbonyl  (iii) Aromatics
Additional information of E-ring came from the compositional profile of ice particles from Saturn’s moon Rhea, from a series of spectra recorded on Cassini’s Rhea flyby (R4) in 2013. No striking change in composition is observed but the ice grains had a varying size distribution. Sodium salts and organic compounds were more frequent in relatively large ice grains, whereas pure water ice particles become more abundant in smaller E ring grains.
Source:

https://www.nasa.gov/feature/jpl/nasas-cassini-reveals-new-sculpting-in-saturn-rings
https://www.smithsonianmag.com/smart-news/saturns-rings-rain-organic-compounds-its-atmosphere-180970475/
https://www.sciencealert.com/saturn-ring-rain-heavy-chemical-composition-organic-compounds-cassini
https://www.nasa.gov/feature/jpl/new-organic-compounds-found-in-enceladus-ice-grains
https://www.space.com/saturn-moon-enceladus-organic-compounds-in-plumes.html
Khawaja, Nozair. (2017). Organic compounds in Saturn's E-ring and its compositional profile in the vicinity of Rhea. (link)


Are there many faint meteors that are too faint to see with the naked eye? I remember one time a while back looking at the night sky through night vision goggles.  One interesting thing I noticed is that there appeared to be many very faint meteors in the sky practically all the time.  I assume this is dust burning up in the atmosphere, or possibly an unusual effect produced by the night vision goggles themselves.  I more suspect that it is dust burning faintly in the atmosphere because it was seen only in the sky.  My main question is: Is space dust constantly entering the atmosphere as small meteors at a fairly high rate?  If I remember right, it was a few every second, which is much more frequent than normal "brighter" meteors, even during a meteor shower.

The answer is yes. The mass (and hence brightness) of meteorites follows some sort of power-law relationship such that there are many more small particles than big lumps of rock. This means that there are indeed many more faint meteorites than bright ones, though I am struggling to find any detailed study that goes below the magnitude range you can see with the naked eye. This paper by Kresakova (1966) seems thorough and representative. It suggests that the number of meteorites goes up by a factor of 3 for each unit increase in astronomical magnitude (i.e. decreasing brightness).

The paper referred to by Barry Carter in his comments is Cook et al. (1980), which finds that log of the cumulative number of meteors is proportional to about half the astronomical magnitude. i.e.
$$\frac{d \log \phi}{dm} \simeq 0.5 $$
This means that the number of meteorites increases by a factor of $10^{0.5}$ for each unit increase in magnitude - i.e. also about 3.

It is a separate issue as to whether you see more with night vision goggles on. I guess you would because these would have light amplification properties that would enable you to see fainter objects - that's the whole point of them.


Is there a possibility of finding a system where a star is orbiting a planet? Is it possible for a star to orbit a planet IF the planet is bigger than the said star?
Or is it possible even if the star is bigger than the planet and still the star is orbiting the planet?
I have done some research with no results.

Technically stars and planets orbit a common barycenter. In case of single star systems the barycenter is usually within the star. However, the barycenter between the Sun and Jupiter is outside the Sun (though very close to it). Otherwise, colloquially, when we say something orbits another body, the body that's "being orbited" is usually the most massive one. Since stars are more massive than planets, it's the planets that orbit them.
A gas giant needs more than 12.5 Jupiter masses to start deuterium fusion, thus becoming a brown dwarf star. That doesn't mean it will. From about 15 Jovian masses on I guess it definitely will, so there's a grey zone between 12.5 and 15 Jovian masses where a more massive gas giant might not fuse deuterium (therefore being a sub-brown dwarf planet) and a less massive could (thus being a brown dwarf star). This is the only case where you might say a star orbits a planet though in fact they'd orbit a barycenter that would be outside either body.
And since black holes count as stars, you may want to consider primordial black holes stars which are less massive than gas giant planets, so if they orbit a gas giant that would be another case of a star orbiting a planet. But we're unsure whether such PBHs exist.
There might also be planets close enough to a stellar black hole that are big enough so that the barycenter is closer to the planet but I'm unsure whether the black hole's tidal forces wouldn't tear the planet apart in such case.


Why black holes have positive energy? How could Hawking get the conclusion "[...] and black holes have positive energy. That’s why empty space is stable. Bodies such as stars or black holes cannot just appear out of nothing. But a whole universe can."? It would make sense to me, if a black hole has negative energy.

Quote from book The Grand Design, page 180:

  If the total energy of the universe must always remain zero, and it costs energy to create a body, how can a whole universe be created from nothing? That is why there must be a law like gravity. Because gravity is attractive, gravitational energy is negative: One has to do work to separate a gravitationally bound system, such as the earth and moon. This negative energy can balance the positive energy needed to create matter, but it’s not quite that simple. The negative gravitational energy of the earth, for example, is less than a billionth of the positive energy of the matter particles the earth is made of. A body such as a star will have more negative gravitational energy, and the smaller it is (the closer the different parts of it are to each other), the greater this negative gravitational energy will be. But before it can become greater than the positive energy of the matter, the star will collapse to a black hole, and black holes have positive energy. That’s why empty space is stable. Bodies such as stars or black holes cannot just appear out of nothing. But a whole universe can.

To understand this, you have to understand what it means to have negative or positive energy.

Following the example given by the text, the gravitational attraction of the Earth and Moon is a system with negative energy. This is because it requires an energy input to undo the system. One must exert energy to separate them, thus requiring energy input to the system to break it apart.

The matter comprising the Earth however is all positive energy. You can get an energy output from this matter and thus it must be positive. An example of energy output might be a gravity assist around the Earth, radioactive decay of nuclear materials in the Earth, or matter-antimatter annihilation of Earth particles. Any process that involves a matter interaction producing energy in the form of heat/light/whatever takes that energy out the Earth's matter and represents an energy output. Note that energy in the matter may take many forms such as heat, motion, or simply the intrinsic energy comprised in matter (via the famous $E=mc^2$ equation)

The black hole, much like the Earth, is simply comprised of matter (albeit in some unknown form at the singularity) and as such energy can be extracted from this matter, making it a system with positive energy.


Why does higher surface gravity broaden spectral lines? It's always made intuitive sense to me that a higher surface gravity leads to broader spectral lines from a star, but yesterday I realized that I don't have a physical explanation for why this is so.  Why is this so?

In order to be in hydrostatic equilibrium, high surface gravity atmospheres have higher pressures at the same sort of temperatures. That means the density of atoms and ions is higher.

If the particle density is higher then there is more chance of interactions between the particles. In particular, collisions between atoms perturb their energy levels and reduce the lifetimes of atoms in particular states. The natural frequency width is proportional to the reciprocal of the radiative lifetime. If this lifetime is truncated due to frequent collisional (de)excitation, then the lifetime is shortened and the frequency width increased. This is known as collisional broadening.


Are many exoplanets synchronously tidally locked like Mercury? The only tidally locked planet in the Solar system is Mercury. But it is synchronously tidally locked 3:2, because of the relatively high eccentricity of its orbit, so doesn't turn the same side towards the Sun. I wonder if this is common for exoplanets?

Many discovered exoplanets are close to their star and must be tidally locked. Has any of them been determined to be synchronously tidally locked, like Mercury? Is Mercury a rare freak in this respect, or a representant of a common phenomenon?

GJ 581d and GJ 667c are candidates for showing spin-orbit resonance.
This paper mentions GJ 3634b and 55 Cnc b as two further candidates.

Spin-orbit resonances different from 1:1 are expected to be rather common among rocky planets orbiting close to a star. Direct observational evidence is difficult to obtain; results are based on model calculations.


Can we detect exoplanets with perpendicular plane of ecliptic? If I understand correctly, all the current exoplanet search is based on planets transitioning the distant star.

If the plane of ecliptic of that star happens to be perpendicular to line of sight from Earth to that star - or sufficiently inclined, so that orbits of all its planets as seen from Earth "circle" the star without ever transitioning it - does that mean we're completely unable to detect them?

There are (at least) three main methods to detect planets that orbit so that the orbital plane is at right angles (or thereabouts) to our line of sight.

The first of these is direct imaging. This is already responsible for the discovery of a number of planets. However, in order to be visible against the glare of their host star, the exoplanets need to be in wide orbits and be massive. It also favours the detection of giant planets around young stars, since these planets are still contracting, are still warm, and emitting their own infrared light. A list of about 20 directly imaged exoplanets can be found here; arguably some of these should more rightly be called brown dwarfs.

A second method is the so-called astrometric detection of exoplanets. This works because the star and planet orbit their common centre of mass. This means that the star executes an orbit with the same period as the exoplanet, but with a semi-major axis that is smaller by the ratio of the planet mass to the star mass. This motion of the star can be detected, in a similar way to which parallax is measured, by precise measurement of position with respect to background objects. The method is most sensitive to wider binaries and massive stars, but is limited by the requirement to see appreciable motion of the star within the duration of the observational dataset. In practice then, this means it is difficult to detect planets with orbital periods of decades, unless one is prepared to wait for some time!

There may only be one accepted exoplanet discovery by this method so far (HD 176051b), but the Gaia satellite should be capable of detecting many thousands of exoplanets with masses at or above that of Jupiter and with orbital radii of about 1--10 au (see Perryman et al. 2004).

Thirdly, gravitational microlensing is quite insensitive to the plane of the orbit. This works when a foreground object gravitationally lenses and magnifies the light from a background star. An exoplanet is marked by a "blip" in the lensed light curve. There are also about 20 planets detected by microlensing. The disadvantage of this technique is that the event is a non-repeatable one-off, so not much more can be learned about these systems.

Notably, all three of these techniques are less sensitive to planets that have small mass (as indeed are transit experiments and the radial velocity technique). However, whilst small planets (in terms of mass) have been found by microlensing, it seems unlikely that the other two techniques will discover Earth-like planets for quite some time.


How are solar systems found again after being discovered? Discovering a solar system that cannot be seen by the naked eye on earth seems like an easy task with telescopes (e.g. the hubble telescope). I'm guessing, that our sun is the reference point for the bearing of the telescope at the point of discovery of a solar system.

But how do you find this solar system again? F.e., after discovering a "new" solar system, a year passes by, and there is need to know in which "direction" this solar system is in relation to ours.

A bit off-topic, but a reason as to why this question might be important: In science fiction there is often the possibility to travel faster than light, reaching other solar system quickly - or for the sake of this question lets assume, we would be able to travel there too in a timely manner. How would they/we know where to go, since the universe is ever expanding.

I think you're overestimating how quickly stars appear to move as viewed from Earth.  Once a telescope like Kepler identifies a solar system around a star, we know where that star is.  Stars have coordinates associated with them that astronomers can use to know where to point their telescopes to see it.  For example, look at the star Betelgeuse.  Look in the info box on the right, the right ascension and declination are the coordinates to find it.  On the timescale of one Earth year, or even a few hundred Earth years, the stars are mostly fixed, and that coordinate to find them still works.  Sure they move slightly, but not enough to lose track of it, or even enough to dramatically change how the constellations appear to us.  Even if one does somehow move dramatically, it would take decades to centuries, and astronomers would be tracking it and updating the coordinate information.

Think about it this way.  The constellations used in the sky today are named after many ancient Greek and Roman mythological characters.  This isn't something we decided recently, they were named hundreds or thousands of years ago and they're still recognizable in the same form they were then.  If stars moved across our sky as quickly as you appear to think, those constellations would all have to be renamed.  You need timescales of tens of thousands of years to make the sky unrecognizable, and even then it would have more to do with the precession of the equinoxes.


Do brown dwarfs have stripes? Brown dwarfs (BD) are often depicted with stripes.

        brown dwarf                              Jupiter

Pictures of BD resemble Jupiter but brown dwarfs have even more stripes.
Question:
How do we know brown dwarfs have so many stripes?
I suppose it can differ depending on BD's temperature and mass. Probably only cold BD can have stripes.
Were the stripes observed? (by photometry, by transit or occultation, etc)
Or are the stripes result of theoretical modeling? Or just "artist impression" because of lack of information?
If result of modeling - do the stripes represent analogs of Hadley cell athmospheric circulation? Are they high and low pressure bands?

Do brown dwarfs have so many bands because of higher gravity?

A number of brown dwarfs have had 'surface maps' created using the light from those stars.
In 2013, observations of 2MASS J22282889–4310262, a brown dwarf 35 light years away, were published.  These were made using the Hubble and Spitzer space telescopes and were able to show changing light patterns and distinct layers of material at different altitudes in the atmosphere.  This provided evidence for textures but not necessarily banding in the atmosphere.  I  believe the Hubble has carried out other observations of brown dwarfs with the aim of determining their surface details.
In 2014 Luhman 16 A and B, a brown dwarf binary system, 6.5 light years from Earth, were imaged using the VLT CRIRES spectrograph.  Luhman 16B was found to have patches perpendicular to the plane of rotation.  Luhman 16A was found to be featureless at the frequencies observed.
Earlier this year, a paper was published describing how the VLT had been used again, this time employing polarimetry, to detect Jupiter-like banding on Luhman 16A.
The paper here describes modelling based on observations that indicate Jupiter-like cloud bands (page 13).
So there does seem to be mounting evidence that brown dwarfs have stripes.  The accuracy of the artists' impressions is something we'll probably find out in the future.

UPDATE - January 2021
A new study of Luhman 16 A and B by Apai, Nardiello and Bedin has been published.  They applied a 'novel photometric approach' to TESS data and found that we are viewing them at angles close to their equatorial planes and that they look 'strikingly similar to Jupiter'.  The paper at the Astrophysical Journal is here but is paywalled.

Last update, I promise!
This is an image from the Apai, Nardiello and Bedin paper that was reproduced at Centauri-Dreams.org.  The preprint of the paper is available for free download at Arvix.


Are white dwarf stars supported by proton degeneracy as well? In general, fermions form a degenerate gas under high density or extremely low temperature. It's clear that white dwarf stars are supported by electron degeneracy pressure. However, there are still a significant number of protons in a white dwarf. Under those high densities, do the protons form a degenerate gas?

In addition, are white dwarfs supported by proton degeneracy pressure as much as they are by electron degeneracy pressure?

Straightforwardly no.

For a start there are almost no free protons inside a white dwarf. They are all safely locked away in the nuclei of carbon and oxygen nuclei (which are bosonic).
There are a few protons near the surface, but not in sufficient numbers to be degenerate.

Let us assume though that you were able to build a hydrogen white dwarf that had equal numbers of free protons and electrons.

The density at which the electrons become degenerate is set by the requirement that their Fermi (kinetic) energy exceeds $kT$. The Fermi energy is given by
$$E_F = \frac{p_F^{2}}{2m} = \left(\frac{3}{8\pi}\right)^{2/3} n^{2/3} \left(\frac{h^2}{2m}\right),$$
where $n$ is the number density (which would be the same for protons and electrons), but $m$ is the mass of a proton or electron, which is different by a factor 1800.

Thus for a given white dwarf temperature, the electrons become degenerate at number densities a factor of $(m_p/m_e)^{3/2} = 78,600$ times lower than do protons.

Even if we were to compress a hypothetical hydrogen white dwarf to the point where the protons were also degenerate (which for a typical white dwarf interior temperature of $10^{7}$ K, would need mass densities considerably in excess of $10^{12}$ kg/m$^{3}$), the (ideal) degeneracy pressures would then be given by
$$ P = \frac{h^2}{20m}\left(\frac{3}{\pi}\right)^{2/3} n^{5/3}$$
and so we immediately see that the degeneracy pressure due to protons would be $\sim 1800$ times less than that due to the same number density of electrons.

If we push up to higher densities then both the protons and electrons will become relativistically degenerate. In this case the pressure becomes independent of the particle mass. However, the Fermi energies of the particles would now be high enough ($>1$ GeV !) to block beta decay and instigate neutronisation. The protons and electrons start combining to form neutrons and an n,p,e fluid is formed where the neutrons vastly outnumber the protons and electrons. This in fact prevents the protons ever becoming relativistic, even at neutron star densities, and the contributions of electrons to the pressure is always orders of magnitude higher than for the protons.

A handwaving way of understanding this is that the degeneracy pressure depends on the product of the momentum and velocity of the particles. In turn, the momentum of the fermions depends on how much they are compressed through the uncertainty principle. For a given particle number density, the separation $\Delta x$ is the same for protons and electrons and therefore the uncertainty principle says that the $\Delta p \sim \hbar/\Delta x$ of the momentum is also the same. This is another way of saying that the Fermi momentum does not depend on the mass of the fermion; however, for a given fermion momentum, the velocity clearly does! Therefore the degeneracy pressure must be lower by approximately the mass ratio of the fermions


Distance between Earth and where The Big Bang's happened? The universe is around 13.8 billion years old.
We know how fast the light is.
We also know how fast our galaxy moving around inside observable universe.

Is it possible to calculating distance between Earth and the place where The Big Bang happened ? If so how far away our planet ? and is there a something special about this place ? (A black hole or something else really crazy?)

The distance to where the big bang theoretically happened would be "0" (and this should be true for any point within the universe).  The entire universe was theoretically produced in the big bang, with every point in space being "inside" the big bang.  The whole universe is very special, it is amazing what is out there (including black holes, earth, stars, planets, nebulae, etc...).

Whether or not there is even a "center" to the big bang / the universe, however is another related interesting question.  Current models seem to indicate that there is no center.  It would be interesting if there is a true center of the universe, and would be interesting to know if there is anything special about the center if it exists.

The question of whether or not there is a center of the universe is addressed in the following links:

What is in the center of the universe?

http://www.spaceanswers.com/deep-space/is-there-a-centre-of-the-universe/


Instrumental magnitude to "real" magnitude - Photometry with not enough data? I have a lot of data taken in R, B, V, Luminance and Halpha-bands which I want to analyse photometrically (one target in the frame).

The frames are already reduced (with flats, darks etc.) and have a correct wcs-header. I also already have the instrumental magnitude of my target and other stars in the frame (using APT).

My question now is, how do I calculate the real magnitude of my targets?

The most simple way would be comparing the other stars' instrumental magnitude and real magnitude and then applying the (logarithmic/linear?) transformation to the instrumental magnitude of my target. But I don't know how exact that would be and how I could calculate the error of the photometric solution (by applying the same transformation to the error of the instrumental magnitude APT gave me?).

For more exact photometry I don't have enough data, aka air mass or calibration fields.

Do you have any ideas or solutions?

The easiest method of photometry is by comparing the measured instrumental intensity of the unknown target with one or more other known targets in the same field and then converting the difference into magnitudes.  This is called differential photometry, and is easier because you don't have to worry about things like atmospheric extinction.  The more comparison stars you use, the better your result.  There are tools that will automatically do this analysis and calculate your error bars.  I use MaximDL.

Visit http://www.aavso.org and use the Variable Star Plotter (VSP) tool to generate a chart for your star of interest.  This will give known magnitudes of comparison stars in the same field.  You can use those to determine the magnitude of your target.  While you're there, check out some of the FAQs and documentation on variable star observing, and submit your results to the online database.


How can cosmological bubble universes "collide" with each other? In Eternal Inflation Theory, there exists the idea of multiple "bubble" Universes that grow and according to this theory, can "collide" with each other. The paper suggests that you can search for evidence that our Universe collided with other "bubble" Universes, by looking for an imprint on the CMB radiation.

However, the concept of a Universe existing as a bubble that moves relative to other universes makes no sense:

How can you have a space "outside" of ours that allows the Universes to
move relative to each other?
In what dimension are the other "Universes" moving relative to ours?
If time started in our Universe at the Big Bang, then in what time dimension (outside of ours) do collisions between Universes occur?

PBS Spacetime looks at this idea here: https://www.youtube.com/watch?v=lrS0rxX-UhA

Edit: A winning answer will answer all three points above.

If time started in our Universe at the Big Bang, then in what time dimension (outside of ours) do collisions between Universes occur?

In inflationary cosmologies, time did not start at the Big Bang. There are many models, but in at least some of them, there is an "inflating" universe which extends infinitely in past (and possibly future) time, constantly creating new space within itself. Within that inflating universe, at any point with some low probability per unit of time, a phase transition (like water freezing to ice) can start to happen, fundamentally changing the nature of space and time at one point. That effect propagates out (again like supercooled water freezing) creating a growing bubble of this changed space-time, which in an abuse of language, we call "a bubble universe".

In what dimension are the other "Universes" moving relative to ours?

Of course there is no reason why this needs to happen just once in the whole of the infinite inflating universe. Indeed, it will happen infinitely often. If it happens at two points in space and time which are close enough together, the two bubbles may grow into one another (before they are pulled apart by the inflationary expansion of the space between them. This is (again perhaps by an abuse of language) called a "collision". Nothing is really moving, except the edges of the bubbles.
If I can suggest, the real answer to your question is that, in this branch of cosmology, "universe" does not mean quite what you think it does. Instead of referring to the totality of existence, it refers to one of these non-inflaying "bubbles" in inflating space-time.


How many stars are in the Sculptor Dwarf Galaxy? The Sculptor Dwarf Galaxy is a small satellite Galaxy of the Milky Way. There are many such satellites galaxies and I'm trying to compile basic data on them.

However, I can't find an estimate for the number of stars in the Sculptor Dwarf (and many other satellite galaxies too). I'm disappointed that Wikipedia doesn't have this basic information, and that makes me suspicious that estimating the number of stars in other galaxies is not very easy or reliable for current astronomy.

Googling these things has resulted in nothing but a bunch of pop science sites...which is no surprise except that these sites don't seem to have the number of stars either. This lack of basic data is surprising and only adds to me worries that no reliable estimates exist.

Can anyone find a link to this info? It would also be very helpful to point me to a database that has the number of stars for all galaxies in the local group, if such a database exists.

Estimates of star numbers are difficult to obtain. Red dwarfs are too dim to be seen as individual points so star counts cannot be obtained. The best guesses come from obtaining the mass of the galaxy and then guessing how much of that mass is made of stars. It is usually a low proportion, between 1 and 3%. You then guess how big the stars are on average to obtain a rough estimated of the star count.

Sources

Dwarf galaxies tend to have a high mass to light ratio meaning that more of their mass is in the form of dust, gas and "dark matter". Sculptor has particularly high ratio, of about 160$\Upsilon_\odot$, and a mass of about about 200 million times the sun.

If 1% of that mass is in the form of stars, but the stars are on average half the mass of the sun then Sculptor would have about 4 million stars.  The error bars on that number are very large: The mass could be as much as 320 million suns, and the other values are guessed by me (about 3% of the milky way is in stars, but the milky way has a much lower mass to light ratio)

Perhaps the final reason that this number is not easily available is that it is of relatively little use. While the size of a galaxy can be measured spatially or by mass, and the nature of the stars is of interest, the number of them is of less use.


Location of Big Bang Can I point to somewhere in the sky where the Big Bang happened? A pathway so to speak on where we (and our progenitor elements) came from?

There is no such unique point, because a key element of the Big Bang model is called the "cosmological principle," which means that every point in the universe is taken as essentially equivalent to any other.  The idea there is that as the universe ages, it changes, but it changes in the same ways everywhere so there is nothing going on that distinguishes one region from another.  This is a simplifying assumption that is supported by the fact that we see no boundary to the universe, and we see no evidence that anywhere else is different from here, the differences we see appear to be due only to the fact that we are seeing farthing into the past as we look farther away.  This assumption means you could pick any point you want, point to it, and say "the Big Bang started there," and you would be no more right nor wrong than if you pointed anywhere else.


How do or could radio telescopes contribute to exoplanet studies? Current and upcoming telescopes which find exoplanets and take spectra of their atmospheres seem to be optical or infrared. What about radio telescopes applied to exoplanets? They are larger and more easy to use with interferometry. Why aren't radio frequencies as useful as optical/IR for exoplanet studies?

Radio telescopes are frequently used to observe the births of stars and their planetary systems. The longer wavelengths are able to penetrate from beneath the envelope of gas and dust that shrouds any attempts to view these events at optical or infrared wavelengths. Unfortunately, the smallest angular resolution of a telescope goes as $\lambda/D$, where $D$ is the telescope diameter and $\lambda$ the observation wavelength. As radio waves are $10^5$ to $10^{6}$ times longer than optical waves, then one needs enormous telescopes to get similar resolving power. This is attempted/accomplished by constructing arrays that can act as a single aperture.

Whilst interferometric techniques to gain spatial resolution are certainly possible, if one wants to directly image exoplanets then optical/IR is still the way to go because this is where most of the intrinsic light from an exoplanet is found.
It is possible that planets orbiting stars may emit radio waves in their own right, through charged particles accelerated in their magnetospheres. Jupiter, for instance, is a strong source of radio waves in our solar system, but would be very difficult to spot at the distances of exoplanets. Some searches have been attempted, but nothing yet found (e.g. George & Stevens 2007).

Of course transit observations do not require superb angular resolution, but they do require lots of signal and precision. Levels of precision that are routinely achieved by optical telescopes (especially in space - e.g. Kepler) are not possible with radio telescopes, and the parent stars themselves are weak, variable radio sources, which makes studying any transit signature very difficult. In fact the main use of transits has been the other way around - to search for intrinsic planetary emission by looking for a drop in flux when the planet is eclipsed by the star (e.g. Lecavelier Des Etangs et al. 2011).  Radio emission might be enhanced in hot Jupiters if the exoplanetary magnetic field interacts with the stellar magnetic field.
This paper does claim to have detected a weak exoplanetary signal from a hot Jupiter, identified by its disappearance when eclipsed by the parent star. However, the result was not repeatable.

The SETI programme targeted a set of Kepler stars with evidence for a planet (or planet candidate) in the "habitable zone". The initial searches for narrow-band radio emission (at 1.1-1.9 Hz) appears to have concentrated on those systems containing multiple planets.

The (null) results for $\sim 100$ system were published by Siemion et al. (2013). The observations were not sensitive enough to detect the sort of "radio chatter" emitted from our planet, but might have detected some sort of deliberate signalling beacon.

A slightly more imaginative approach is to observe Kepler multiple planet systems when two planets are in conjunction as seen from the Earth. There is then the chance of seeing communications from one planet to the other. (e.g. http://adsabs.harvard.edu/abs/2014ebi..conf..5.2K ).


Why is it okay to watch a sunset but not an eclipse? People watch sunsets all the time.  You don't see people using special "sunset glasses".

Yet with an eclipse, warnings are posted everywhere not to view it without special glasses.

Why is this?

During a sunset, the Sun is lower in the sky than during most of the day - much lower. Therefore, light from the Sun travels through about 120 miles of dense atmosphere, compared to the roughly 2 miles it travels through from straight up. Here's a rough sketch (not to scale) to demonstrate this. It is clear that $B>A$:

Light scatters in the atmosphere; in fact, one type of scattering is why the sky is blue. The longer travel distance means that there is much more scattering of ultraviolent light, which in turn means that the light you see is less intense.

Okay, you say. But doesn't the eclipse still block a lot of light? Well, unless there's 100% coverage - totality - there's still plenty of light coming from the uncovered part of the Sun, and that matters. The uncovered part is as bright as it normally is, and looking at that part is just as dangerous with or without the eclipse.

There's one more thing to consider, which is that people watching a sunset don't look at the Sun; they look at the clouds and sky around the Sun. If you look directly at the Sun, your eyes will be damaged, no matter what's happening, an eclipse or a sunset.


What should I expect to see with my telescope with what EP's? Recently got a Celestron 130 SLT (first "real" scope) and have a question. I have the following:

130mm F/5 newtonian, 650mm focal length scope

9 and 25mm factory EP's (more on that later)

Svbony 2x barlow

Very, very dark skies (100 miles from any cities, out in the desert, light pollution maps show "zero light pollution" on the maps anywhere I view from once I'm 5 minutes from home). I don't have expensive glass, I don't know what I'm doing... but at least I have this going for me :-)

4500 elevation with low mountains on either side, 10-25 miles away  (don't know that it matters)

20/20 uncorrected vision (if that makes a difference)

I am curious what I should expect to be able to see with my scope, and at what magnification I should see them at? I plan to buy 3 EP's, and want to know the lowest and highest I should buy to see what I "should" be seeing. I don't want to pursue things that I won't be able to make out clearly anyways, or buy the wrong EP's for seeing those things. I see anywhere from 25 to 60 X your scope size, which is a wide range of mags.

I have beautifully-dark skies, so I'm not sure if the EP has to do with light pollution also, or solely "seeing".

Will I be able to make out cassini's division? Will I be able to complete the messier list?

I have nothing in-person to compare my scope to, so I'm relying on random internet information, which tells me my max magnification is anywhere from 150 to 250, and varying features and objects that I will be able to see. Any input will be greatly appreciated. I plan to start the lunar observing program soon (LOVE the moon, mostly because of my spaceflight interest and occupation), and have only looked at the moon so far, along with learning my constellations.

Thanks!

(PS, whatever EP's I buy, I am willing to spend up to 80 a piece, on 3 total new ones (or 2, the 25mm seems ok)

A simple rule for the maximum magnification is: 2x the aperture in mm. In your case, that makes it 260x. But that's more like a guideline, since it's affected by so many factors. Speaking in general, trying to exceed it almost never provides any benefits, with some rare exceptions (high quality instrument, perfectly collimated, under a very stable atmosphere, observing close double stars - is one possible exception that comes to mind).

So eyepieces shorter than 2.5 mm (after dividing it by the barlow magnification, if any) would likely not be beneficial for you. Combined with your 2x barlow, that means eyepieces no shorter than 5 mm (+/- 1 mm, depending on some factors). In practice, such short eyepieces are pretty rare anyway.

There is also a longest usable focal length for eyepieces, which is the shortest usable focal length times 12, more or less, or about 30 mm in your case (plus or minus some change). Any longer than that and the exit pupil (the diameter of the ray of light exiting the scope) would be greater than the pupil in your eye, leading to loss of light and diminished performance. That's the theory. In practice, there's just not much benefit going much longer than that, as you will find out if you try.

So stick to the 30 ... 2.5 mm interval, give or take 10 ... 20%. These limits are not set in stone, but you will rediscover them on your own, given enough time.

A minimum of 3 eyepieces, and a maximum of 6 or so, should cover most use cases. But a lot of people just end up accumulating eyepieces over the years, or keep trading them.

Look into eyepieces with a wider apparent field of view. A typical Plossl gives you about 50 degrees AFoV. But the view is more interesting, and the scope is easier to use, in a wider AFoV.

As an example and a place to start, I've used the Explore Scientific 82 degree series, which provide 82 degrees of AFoV. Large objects like the Moon or some nebulae or clusters look spectacular in such a wide eyepiece.

It's not mandatory that you strive for a bigger AFoV, but it can be worth it. Also, lenses such as the ES82 series could be used even with high end telescopes, if you ever decide to upgrade.

130 mm of aperture is definitely enough for a lot of fun.

Jupiter will show off its equatorial belts. On a good night you will begin seeing swirls in between the belts - I did that with 150 mm of aperture when seeing was good, and you're within a rounding error from it. You will also see the 4 big moons of Jupiter, but only as little dots. Use medium-high magnification, don't push it too high, because Jupiter is low contrast and it washes out easily at high mag - just try and see what works best.

Give it time and allow your eye to adapt. I believe your scope can track, so enable tracking, relax, breathe calmly, and just take the image in over time. In a couple minutes, your brain/eye system will begin to adapt, and you'll start to see more details. Allowing some time to pass is especially important with low contrast objects like Jupiter, where details are so close to the eye's performance limits.

Saturn will show you the Cassini division in its rings. You will also begin to see some equatorial belts on the planet, but just barely. High magnification is useful, but not necessarily the highest.

Mercury and Venus will show phases like the Moon, but no details beyond that.

Mars can be observed for a few weeks every 2 years, at oppositions; there's a big opposition this year (2018) in the summer. It requires the best seeing (the most stable atmosphere), excellent collimation, and the shortest eyepiece (highest magnification) your telescope will take. Whichever polar cap is pointing at you should be visible, as a tiny white dot sitting on the edge. I have seen the Hellas Basin as a large, bright, white spot when it was full of frost or fog (not sure which), but it melts away quickly. Syrtis Major is probably easiest to see, as a dark, almost black line.

http://www.nakedeyeplanets.com/mars-oppositions.htm

When seeing (air turbulence) is bad, Mars just doesn't work; it looks like an orange pill with no details. Either take a road trip to whichever part of the country has better seeing at that time, or use lots of patience.

Uranus and Nepture can definitely be seen in 130 mm. At high magnification, you will see them as tiny disks, clearly (but just barely) bigger than any star nearby. Uranus is a pretty blue-green shade, Neptune is a deep blue, almost indigo.

The Moon will provide endless entertainment, because there's so much to see. Zoom into a feature using high magnification, or zoom out and observe the whole thing at low magnification - it's fun either way.

For the planets and the Moon, what's important is the so-called seeing, which is the inverse of turbulence:

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

When seeing is good (turbulence is low) then you can push the magnification all the way up to the limit. When seeing is bad, you have to reduce magnification, or else the image is soft and foggy. Mars only works well with the best seeing; it is possible that seeing might be consistently bad for the whole duration of an opposition - or it could be consistently good. It's weather basically, so it does whatever it wants to do.

Use the Clear Sky Charts site to get a seeing forecast:

http://www.cleardarksky.com/csk/

On that site, select a location closest to your home. Use the Seeing line in the chart for an estimate. Keep in mind this is weather forecast, so it's not always 100% accurate, but usually it works well enough.

When seeing is good, observe planets. When seeing is bad, observe DSOs (deep space objects, like galaxies, nebulae, star clusters).

Light pollution does not matter at all for the planets or for the Moon. You could observe these objects from the biggest city and they look about the same.

Another important thing for planets is collimation. Read your scope's manual and practice what they preach. A scope out of collimation may severely underperform. It could mean the difference between seeing and not seeing the Cassini division. See here for some examples:

http://www.astrophoto.fr/collim.html

There's a bewildering variety of tools and techniques for collimation. Start with the simple stuff (the manual) since it's better than nothing. Then keep reading. Here's a good page:

http://www.skyandtelescope.com/astronomy-resources/how-to-align-your-newtonian-reflector-telescope/

Or see here:

http://garyseronik.com/?s=collimation

A laser collimator should in theory be one of the easiest and most precise tools, but beware of lasers that are not centered. Plug the laser in the focuser, and turn it (spin it) in place. Watch the laser spot on the primary mirror. If the spot moves in a circle as you keep turning the laser, it's not centered. Some lasers have adjustment screws for centering, some don't. High quality lasers like the Howie Glatter series are very precisely centered from the workshop and don't require adjustments.

https://www.collimator.com/

https://www.catseyecollimation.com/

I do own more complex collimators like the CATSEYE system, but in 99% of cases I just use a Howie Glatter laser with a 2" TuBlug to do a quick check-up before each session. Any laser is fine as long as it's precision-centered.

Collimation is something you'll keep learning for a long time, so don't worry, do what you can at first, and slowly refine your technique over time. But keep your scope collimated. Once you're comfortable with the technique, it only takes 5 minutes.

Air currents in your instrument can also affect the planets and the Moon. These are due to your scope being at a different temperature from the air (because it was stored indoors). The effect is like observing under bad seeing conditions.

http://garyseronik.com/beat-the-heat-conquering-newtonian-reflector-thermals-part-1/

http://garyseronik.com/beat-the-heat-conquering-newtonian-reflector-thermals-part-2/

The cure is easy: take the scope outside 1 hour before you begin your session, and let it "breathe". A scope at thermal equilibrium with the air has no internal convection.

Your scope is probably not big enough to require a fan on the primary mirror, for forced convection, but you can definitely experiment with it if you like. At your aperture size this is low priority. Fix everything else first (collimation).

DSOs (deep space objects) should be visible in your aperture, especially given the dark sky you're enjoying. More aperture is better, but the level of light pollution is also important. You are very lucky to have such a great location for DSOs.

Pretty much the whole Messier catalog, over 100 objects, should look great in your scope under a dark sky. Among the easiest objects, my favorites are: M31 (the Andromeda galaxy), M13 (the Hercules Globular Cluster), M27 (the Dumbbell Nebula), M57 (the Ring Nebula), M42 (the Orion Nebula).

But since you live under an ideal dark sky, you could just plug a low magnification eyepiece in it and slowly browse the sky. When you see something interesting, stop and switch to somewhat higher magnification. Chances are you'll stumble upon many DSOs this way. I do this with binoculars (low magnification) when I go to Death Valley, and it's amazing what you can discover randomly this way.

Different DSOs require different magnifications, from low to high. You'll just have to experiment and see what works best in each case.

Seeing, collimation, thermal equilibrium don't mean a whole lot for DSOs, because they are fuzzy and low detail to begin with. But a very dark sky helps a lot.

You could also observe the Sun. Get a white light filter, the kind that fits on the top of your telescope, covering the whole aperture. Filters made from the Baader Solar Film are best, they are very good performers and tend to be cheap. Either buy the Baader foil and make the filter yourself, or purchase a complete filter.

https://www.astrozap.com/scripts/prodList.asp?idCategory=31

https://astrosolar.com/en/information/how-to/how-to-make-an-inexpensive-filter-cell/

Just make sure the size of the filter fits the diameter of the OTA (the main tube) of your telescope - NOT the aperture size, but the external diameter of the OTA.

This will allow you to see sunspots. You can also observe the transit of Mercury over the solar disk, which happens once every several years. I think the next one is in 2019. You can also observe solar eclipses this way.

If you need to use the finderscope when observing the Sun, then make a small filter for it, too. I made some cute little solar filters for my finderscopes from Baader film and white plastic tubes used for sprinkler systems. Cheap, but effective.

https://astrosolar.com/en/information/how-to/how-to-make-your-own-objective-solar-filter-for-your-camera-or-telescope/

Obviously, never point the scope at the Sun without the full aperture filter. And make sure the filter doesn't fall off. Or else all sorts of mayhem may ensue, up to and including someone losing an eye, or the house burns down, etc. Not kidding. Unfiltered, the solar energy coming out of the scope is like a plasma torch.

Buy a book called Turn Left At Orion. It will get you started with how to find and observe DSOs, complete with instructions on which time of the year is best for each object, how much magnification (this is a bit subjective), etc. It's a very good book, I highly recommend it.

https://www.amazon.com/Turn-Left-Orion-Hundreds-Telescope/dp/0521153972/?tag=amz-track-20

I would go with either a sturdy hardcover edition, or the spiral bound one - something that works well out in the field, not some snowflake paperback.

A planisphere is a good thing to have. You only use it 5 or 10 minutes each night, but it will quickly get you oriented in the sky when you do.

https://www.amazon.com/Guide-Stars-Ken-Graun/dp/1928771017/?tag=amz-track-20

When reading the book or the planisphere, use a red light at the minimum intensity that allows you to read. This will preserve your night vision. There are many red flashlights out there made for astronomers, that allow you to dial down the intensity. This is only needed for DSOs. Night vision does not matter for the planets or the Moon.

EDIT:

It's probably beneficial to get a small sky atlas too. Let's say you read in the news that a supernova has been discovered in a nearby galaxy, and it can be seen by amateurs. You could point the scope in its general direction, but you'll need a detailed local map of the region. The one point of light that is seen in the sky but is not seen on the map is the supernova. It also helps when observing other transients such as comets and asteroids (again, they stand out because they are not on the map). Finally, with some smaller DSOs it makes it easier to identify the object from the stellar background (and they should be marked on the map because they are permanent objects).

https://www.amazon.com/Sky-Telescopes-Pocket-Atlas/dp/1931559317/?tag=amz-track-20

You could also use various digital maps on a portable device, but I recommend that you master the paper maps first.

Once you own a handful of eyepieces, a barlow, etc, you should keep them together in a carrying box. Many astronomers use a pluck foam case, and they just shape the foam to hold each and every component. I've never used the dividers that come with the case, since optical components are small, lightweight, rather fragile, and should be surrounded only by foam.

https://www.harborfreight.com/18-in-x-6-in-x-13-in-black-aluminum-case-62271.html

Another item I've found useful is a tiny folding table. It doesn't have to be huge, just big enough to hold a book, the sky atlas, etc. It's better than making a pile of stuff around you on the ground.

A cheap, collapsible barstool or something like it would help with long observing sessions. Definitely beats hunching over the scope for hours. Just make sure it's the right height. Big dobsonian owners tend to make or buy so-called observing chairs, which are adjustable, but those are probably overkill for you since your eyepiece doesn't move around that much. Your scope has a geometry similar to my SCT, which doesn't really require a true observing chair, just anything to sit on that's the correct height. Now if you had a big dob then that would be different.

A pretty common improvisation is an empty box or frame that's 3 different sizes in all directions (length, width, height). That allows some height adjustment, and it's pretty cheap to procure or make. Between sessions you could use it to store the telescope or something.


Weird Blob near Polaris? I need help identifying a blob.

Last night I took a picture of Polaris, and there is a weird blob in it that I can't identify.  Star charts and other pictures on the 'net do not show this blob.  I took three pictures (with slightly different positions) and the blob has the same shape and position (relative to Polaris) in each.  The blob does not exist in other pictures that I took (pointing in completely different directions).

Taken with a Canon t5i, 300mm lens, 3 minute exposure on a motorized equatorial mount.  Viewing conditions were terrible, with lots-o-moon and smoke from forest fires.  From Colorado.

Polaris is the brightest star, slightly left and down from center. Yildun, the next star in the handle of the little dipper, is near the upper left corner. The blob in question is very near, and slightly up and to the left of Polaris.

Edit:

Simple Lens Flare:  I don't think it's lens flare (LF).  With LF, you can draw a straight line through the photo where the flare, the bright object, and the center of the photo all line up. Also, the LF tends to match the shape of the bright object.  I don't think it's LF because the blob doesn't line up with the center of the photo and a bright object, and there are no bright objects that have the same shape as the blob (with that weird tail to it).  Also, I have three photos with the same blob. In each case, Polaris (the brightest point in the photo) moves slightly because the camera moved slightly but the blob moves exactly with Polaris and not somehow relative with the center of the frame.

Under careful analysis, there IS lens flare, but it is where you'd expect it and not where the blob is. If you draw a line though Polaris and the center of the frame, there is a flare on the opposite side of the center and the same distance away from the center as Polaris is.

Schmutz in the camera:  I don't think it's dust/dirt in the camera/sensor.  If that were the case then I would have seen it in other photos from the night, and I didn't.  Also, the blob would have stayed in the same position in the frame and not moved with Polaris as it did in the 3 different photos that I have of it.

Not-So-Simple Lens Flare:  Maybe.  Although I cant imagine how this would happen unless some lens elements were not radially-symmetric and/or elements were not lined up on a common axis.  But I don't know everything about camera design so maybe this is possible?

There's a new blob in the sky:  Possible.  Although I think the odds of "user error" are much higher.  Maybe someone else wants to take a new photo of Polaris?  (wishful thinking)

That looks like a lens effect. Polaris is the brightest object and there is an internal reflection in the lens or the camera.

It's a nice photo under difficult conditions. Can you see NGC 188, an odd open cluster?