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 (...
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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 ...
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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 ...
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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 ...
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10 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 - ...
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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 ...
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9 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 ...
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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) ...
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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 ...
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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 ...
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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 ...
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8 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 ...
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7 votes
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Calculating RA/dec from JPL ephemeris data

Read the fine print under the Horizons output when you select an OBSERVER table. It says (emphasis mine) R.A._(ICRF/J2000.0)_DEC = J2000.0 astrometric right ascension and declination of target ...
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7 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 \\ \...
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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 ...
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7 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 ...
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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 ...
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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 ...
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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 ...
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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 ...
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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 ...
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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 ...
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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, ...
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6 votes
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Why do J2000 coordinates change with time in Stellarium?

Apparently, Stellarium's "J2000.0" reports the coordinates of stars in J2000 frame, but at the epoch of the date you specify, instead of reporting the coordinates of the star at epoch 2000.0, which ...
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6 votes
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Is the sun flipped on the other side of the world?

Yes, the orientation of the Sun will be different from the Earth's northern and southern hemisphere, just like your example of the Moon. I would not say that a sunspot in the "northern" hemisphere ...
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  • 7,073
5 votes
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Which is the shape of the sky?

For computer software, the easiest way to take a sphere (and/or hemisphere) and flatten it into a flat shape (usually a rectangle) is the equi-rectangular projection (also known as the plate carrée), ...
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  • 1,666
5 votes
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Why is right ascension measured in hours?

In order to know when a star will be above horizon, you'll need an equation with times, not "celestial latitudes". So you end up adding RA and sidereal time. EDIT In order to find out the position ...
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5 votes

Why is right ascension measured on a 24 hour scale rather than a 23 hours and 56 minutes scale?

Right ascension is a historical oddity. To specify a point in the sky you need a coordinate system, the one which we have come to use has it's origin at the Point of Aries on the Equator and the ...
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