34 votes

Where is it possible to observe the Moon for 24 hours?

Lunar standstill Estimate at what latitudes it is possible to observe the Moon for at least 24 hours. This will depend on the Moon's maximum declination, which varies with a period of 18.6 years (...
Eric Duminil's user avatar
  • 1,365
31 votes

How to describe the Sun's location to an alien from our Galaxy?

This has already been done. The pioneer 10 and 11 probes have a description of the solar system's location and Earth engraved for aliens to understand (or so one hopes). The physical parameters of our ...
planetmaker's user avatar
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26 votes
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Where is it possible to observe the Moon for 24 hours?

Near the Poles is where unusual things occur. Like, in parts of Greenland, which is pretty close to the north pole, the sun never sets during summers, and the sun never rises during winter. The same ...
Ishaan Manish's user avatar
14 votes
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Coordinate system for space

For locating objects in the sky, the horizontal and equatorial coordinate systems are commonly used. These systems describe the position of some object in the sky very well, but do not explain the ...
Tosic's user avatar
  • 1,681
13 votes
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How to describe the Sun's location to an alien from our Galaxy?

If the travel was instantaneous (or say, less than a million years) it should be relatively easy to relocate the Sun from triangulation using well-known objects visible from anywhere in the Galaxy. If ...
ProfRob's user avatar
  • 152k
12 votes

Is there a map of the galaxies?

A map of all galaxies gets kind of unwieldy, like a map of all stars in the milky way or a map of every house in the country, or every grain of sand on a beach . . . you get the idea. Start here - ...
userLTK's user avatar
  • 24k
12 votes
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Coordinates in the Night Sky

The altazimuth system is directly linked to your position on the Earth. It is the “left-right/up-down” coordinates system; the azimuth being the “left-right” (with 0° for North, 90° East, etc.), and ...
Pierre Paquette's user avatar
10 votes
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How big is one arcsecond at various distances?

How big is one arcsecond at various distances? An arcsecond is a small angle, 1/3600 of a degree or about 5 millionths of a radian ($4.85\times10^{-6}$). To estimate the size of something that ...
James K's user avatar
  • 121k
10 votes
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Local Sidereal Time

LST = 100.46 + 0.985647 * d + long + 15*UT They don't explain what the two constants are (100.46 and 0.985647), could anyone explain what those constants are ...
David Hammen's user avatar
9 votes
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How to convert density from physical unit to comoving unit?

You're asking two different, but somewhat related questions: One has to do with a practical way of describing the expansion of the Universe; the other has to do with a way of dealing with our ...
pela's user avatar
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9 votes
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Where is the North of the Universe

Sort of... There is a system called the International Celestial Reference System (ICRS) which has center at the Solar System Barycenter (normally inside the Sun but not the same as the Sun's center) ...
astrosnapper's user avatar
  • 8,357
9 votes
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How can non-earth-orbiting satellite coordinates be obtained by amateurs?

Does a publicly accessible website/API exist that provides the current coordinates for such objects? Yes! There's a pre-launch planning trajectory from ISRO/JPLNAV in the Horizons system. Chandrayaan-...
PM 2Ring's user avatar
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8 votes
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Why is the symbol of Vernal Equinox ♈?

As Wikipedia explains in the article on the Zodiac, the Zodiac was originally developed with the assumption of fixed equinoxes. It is assumed this is because the Babylonians who developed the Zodiac ...
called2voyage's user avatar
  • 6,312
8 votes

Angle from 2 point on celestial sphere

There are multiple formulae for how to calculate this angle. The simplest is to construct the unit vectors: $$\begin{align} \hat{n}_i & = \left[\begin{array}{c} \cos \delta_i \cos \alpha_i \\ \...
Sean Lake's user avatar
  • 2,946
8 votes

Determining latitude and longitude in bad weather

Sunstones are believed to have been used by vikings to determine the direction to the Sun on cloudy days. That helps you point the ship in the right direction, and by experience you can estimate the ...
LocalFluff's user avatar
  • 11.4k
8 votes
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How to find Greenwich Mean Sideral Time?

To do so, I'm following what's described in this document and implementing C++ code. That is a very old document you are using. It's using the 1982 IAU precession model. There have been multiple ...
David Hammen's user avatar
8 votes
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How was the galactic plane established?

The zero of Galactic latitude (i.e. the Galactic plane) was defined by a working group of the International Astronomical Union. The observations used to do this were 21 cm radio observations of atomic ...
ProfRob's user avatar
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8 votes
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Measurement unit of coordinate systems in astronomy

In science, there are cases in which some weird units are used for historical reasons only and we would all be better off without them. One could argue that the magnitude systems is one of those cases,...
Prallax's user avatar
  • 4,431
8 votes
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Right ascension - Gaia vs Simbad

1 hour = 15 degrees. 60 min = 1 hour and 60×60=3600 sec = 1hour. So to convert: (5 + 38/60 + 42.0/3600)*15 = 84.675 There is a small discrepancy, which is due to ...
James K's user avatar
  • 121k
7 votes

Determining latitude and longitude in bad weather

When it's cloudy at sea, the one measurement one still has is approximate sunrise or sunset, which can be used to determine longitude, but the uncertainty is very large, in particular at higher ...
gerrit's user avatar
  • 2,033
7 votes
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Measuring misalignment between two positions on sky

Assuming you mean the angle between the meridian line through A and the great circle that goes through points A and B, then it goes something like this. Define vectors from the origin to A and B ...
ProfRob's user avatar
  • 152k
7 votes
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How do I plot galactic coordinates using matplotlib and astropy in python?

This code reads coordinates as equatorial (ra, dec) and transforms them to galactic (l, b): eq = SkyCoord(xarr[:], yarr[:], unit=u.deg) gal = eq.galactic The ...
Mike G's user avatar
  • 18.7k
7 votes
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Difference between J2000, FK5 and ICRS coordinate systems? Which one does the Yale Bright Star Catalog use?

The questions confuses things: J2000 is the epoch / equinox in which the catalogue is in. This defines the reference point in time for Right Ascension and Declination. See e.g. here https://en....
Christian Herenz's user avatar
7 votes

Latitude, longitude and altitude of Ckoirama Observatory; where can things like this be looked-up?

Near the pinned location, at 24.0893°S 69.9306°W, Bing Maps shows a building like the one in the University of Antofagasta photos, with signs of recent construction. It's probably a matter ...
Mike G's user avatar
  • 18.7k
7 votes
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Conventional units for astronomical object coordinates

Right ascension is usually given in hours, minutes and seconds, but declination is usually given in degrees, arcminutes (') and arcseconds ("), with one arcminute being 1/60th of a degree and one ...
HDE 226868's user avatar
  • 36.6k
7 votes

Where is it possible to observe the Moon for 24 hours?

In addition to the previous answers, another point might be added. We are dealing with the Moon. The Moon - just like the Sun - has a movement eastward (in the right ascension) during the day which ...
d_e's user avatar
  • 1,667
7 votes

Difference in results between JPL Horizons and cspice (rust-spice)

The issue is with how you have configured HORIZONS to show the output. You have apparent RA, Dec selected as output columns which includes the precession and nutation, in addition to the light time ...
astrosnapper's user avatar
  • 8,357
6 votes
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a random distribution of RA&Dec

Well, your problem is that you have a sphere. To compensate for the polar declination skew, you just calculate $$\sin^{-1}(\mathtt{rand})$$ Where $\mathtt{rand}$ is a number in an evenly divided -1 to ...
SE - stop firing the good guys's user avatar
6 votes
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Can you translate a GPS coordinate to a Galactic coordinate?

Sure you can! Galactic coordinates have the same origin as other J2000.0 systems; the solar system barycenter (center of mass). This is very close to the Sun, usually but not always inside the Sun, ...
uhoh's user avatar
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