# Celestial Coordinates

What is the standard coordination system used to describe the location at a celestial object approximately due east and at an angle of 10 degrees above the horizon?

## 2 Answers

The coordinate system for an object in the sky measured relative to the horizon is known as altitude-azimuth. The altitude is the angle above the horizon (10 degrees in your case), and the azimuth is the angle around the horizon, usually measured from North. (Due east would be 90 degrees azimuth in your example.)

• Good answer. I am now wondering if there a standard format to use for the co-ordinates, such as stating the Altitude before the Azimuth, abbreviated, as in the following example: Alt 10°, Azi 90° ? Jan 16, 2018 at 12:59
• I am not aware of any standard format. As long as the reader is familiar with English, then the abbreviation should work fine. If they are not familiar with English, the abbreviation would be harder for them to look up. Jan 16, 2018 at 18:35

There isn't one, because there cannot be one: which celestial object you see towards the east at 10 degrees above the horizon depends on:

1. Your position on Earth

2. The time of day

3. The time of year

Because of Earth's rotation and motion around the sun, the positions of celestial objects change.

Among themselves, stars keep (for our purpose here) their relative position, and astronomical coordinate systems make use of that fact. The celestial sphere on which we project the stars rotates roughly around the North Star.

Angular distance from the North Star (or more correctly, from the celestial equator) is called declination.

As you know from Earth's coordinate system, the position of the Null Meridian is arbitrary. On Earth, it's in London, in the sky, it's in the constellation pisces. This is called right ascension.

More information: wikipedia.

Note also that, depending on the application, other coordinate systems may be used in astronomy (such as galactic or heliocentric).

• Thank you for your answer, it makes good sense. I suppose the question I need to be asking now is how can I co-ordinate the bearing between a human being and a celestial object? Jan 15, 2018 at 15:28
• To check what's where in the sky, you can use a planetarium software such as stellarium. If you want to do it on a larger scale and write your own software for it, have a look at wcslib or other libraries made for that purpose.
– Alex
Jan 16, 2018 at 16:58