What's All This Stuff About Coordinates?

Jack Kramer

Confirmed star-hoppers may be reluctant to admit that they do sometimes use the coordinate system, but the truth is that no amateur astronomer can long observe without knowing this essential aspect of astronomy. Even those with Dobsonian-mounted scopes will find it necessary to locate objects on a chart when the coordinates are all that is known. Typical examples might be a comet or an obscure deep sky object. Here are some terms.

Altitude depends on where you are situated right now: 0o is your horizon (forgetting about houses and trees) and 90o is your zenith (directly overhead). If an object is exactly 20o above our southern horizon, it would be slightly higher for someone living in Chicago and slightly lower for someone in Milwaukee. To estimate the difference, recall that Chicago's latitude is 41o52'N, Milwaukee is 43o02'N, and we are about 42o20'N.

Azimuth is also measured in degrees, starting with 0o as you look due north, then moving clockwise.


Right Ascension is easy to remember if you recall that the stars appear to move across the sky "to your right". Unlike azimuth, Right Ascension (RA) is measured in time -- hours, minutes and seconds -- that decrease as you move to the right, or in a clockwise direction. The above illustration should help visualize how this works; imagine that you're looking down at the hour circle on an equatorially-mounted telescope aimed due south. The zero hour is the point of the vernal equinox -- the east/west point that the Sun crosses on the first day of spring for the northern hemisphere. It's shown as the "vernal colure" -- a circle drawn from each celestial pole through the point where the ecliptic crosses the celestial equator.

Since I slipped in a couple more terms, we had better define them. The ecliptic is the plane of the solar system -- the band of sky through which the Sun, Moon, and planets appear to move. It runs diagonally across charts of the sky due to the fact that the Earth's axis is tilted 23.5o with respect to the orbital plane. The celestial equator is the circle drawn through the sky at the position of 0o declination. In effect, it's the Earth's equator projected against the sky.

Declination (DEC) is measured in degrees plus from the celestial equator to the north celestial pole and minus from the celestial equator toward the south celestial pole. Thus an object that's between the celestial equator and the north celestial pole (roughly the position of Polaris) is shown with a + declination. The north celestial pole lies directly above the Earth's north pole, or axis of rotation. Obviously, the south celestial pole lies directly above the Earth's south pole. An object on the celestial equator has a DEC of 0o. No object can have a DEC greater than 90o, which lies at the pole.

Right Ascension and Declination are the same regardless of where you happen to be on the Earth. The coordinates of a fast moving object, such as a comet, change rapidly, while those for planets change more slowly. The stars also are moving, which is referred-to as their proper motion. For this reason, star atlases list the date for which the positions were calculated. Modern charts are identified as being plotted for the epoch 2000. Galaxies are also moving, but this motion is so slow as to be inconsequential for our purposes here.

The best way to learn about coordinates is to actually use them. In the section of chart below, estimate the coordinates of the globular cluster shown roughly in the center.

Star Charts

If you got anywhere near RA 15hr18m40s, DEC +2.0o, you were right on the money. The globular cluster is actually a Messier object. Now that you know the coordinates, your assignment is to to get out your own star charts and use the coordinates to determine exactly which object it is. No cheating now!

... published in the May 1996 issue of the NightTimes

Published in the May 1996 issue of the NightTimes