Geographical

We find our way around on Earth by using latitude and longitude. Latitude is the distance N or S of the equator in degrees, where the equator is 0, and the N and S poles are +90 (N) and -90 (S) degrees, respectively). Longitude is measured from 0 to 180 degrees E(+) and W(-) of the Greenwich Meridian. The system is defined by the equator and the Greenwich Meridian which continues onto the Pacific side of the globe as the International Dateline (Longitude 180).

Horizon

Your local coordinate system is an altitude/azimuth system, where the 0 point of azimuth is due N, and E, S and W are 90, 180 and 270 degrees respectively. The altitude of an object in the sky is the distance above the horizon in degrees, from 0 to 90. Since the Earth rotates on its axis and revolves about the Sun, the horizon coordinates of a celestial body are constantly changing. The system is defined by the N compass point and the horizon. The Local Meridian is often mentioned when using this coordinate system, it is an imaginary line going from pole to pole, passing through the zenith directly over your head. Telescopes set up for terrestrial use are on altazimuth mounts, (like a machine gun or a searchlight) so you train left or right, or point up and down. Following a celestial object across the sky as the Earth rotates is cumbersome, involving simultaneous adjustments to both axes to keep the target in view.

Equatorial

This is the coordinate system used by astronomers to locate objects in the sky. Its coordinates are Declination (Dec) and Right Ascension (RA). Declination is the easy one, it is identical to latitude, that is, if you you live at latitude 26N as I do, a star with a Dec of +26 will pass directly overhead once a day.

RA is a bit more complicated. The zero point of RA is the Vernal Equinox, that point in the sky where the Sun’s path (the Ecliptic) crosses the Equator on its way N in late March. RA is measured E, towards the rising sun, but unlike all the other coordinates mentioned in this essay, it is measured in hours, minutes and seconds of Sidereal Time (a sidereal day is the amount of time it takes for the earth to spin once on its axis relative to the stars. It is about 4 minutes shorter than a solar day, the amount of time it takes for the earth to spin relative to the Sun). RA is sometimes called Hour Angle. So for example, the star Betelgeuse’s coordinates on the celestial sphere are RA 5h 55.9m, Dec +7d 25′ (Epoch 2000.0). So if you lived at 7d 25′ N latitude, Betelgeuse would pass directly overhead once every day.

Because of the slow wobble of Earth’s axis due to precession of the equinoxes, RA and Dec slowly change, but for most applications, you can ignore this shift. When doing precise work it may be necessary to do a correction to the coordinates for precession. Positions on the sky and chart grids are usually marked with the Epoch (date) they are calculated for, (as in my Betelgeuse example above) so you can correct the position for precession, if necessary. Star chart coordinate grids are usually Epoch 1950.0, or Epoch 2000.0, although some older charts may be gridded for earlier Epochs.

Astronomical telescopes are usually mounted on equatorial mounts, where the RA axis of rotation is set to your latitude, pointed at the N celestial pole and parallel to the Earth’s axis of rotation, and the Dec axis is at right angles to it. Once you get a star’s Dec (N-S) locked in, you only need rotate about the RA axis (E-W) to keep the star in the field as the Earth rotates.

Navigators use the Equatorial system as well, with one modification. They express RA as Greenwich Hour Angle (GHA) which is measured (in degrees measured W, not time measured E) between the Greenwich meridian and the star. (Unlike RA, GHA is constantly changing as the Earth rotates). It makes it easier for the kinds of calculations navigators do. However, they use Dec in the same way the astronomers do, as synonymous with latitude. The Nautical Almanac is published yearly with all positions corrected for precession, so the navigator need not concern himself.

Ecliptic

In ancient times, when astrologers were mostly concerned with where the planets were on the Zodiac, the Ecliptic coordinate system was preferred. The Ecliptic (path of the Sun amongst the stars) formed the “equator”, and the ecliptic poles at right angles to the plane of the solar system defined the axis of rotation. I don’t believe this system is used by astronomers any more, although it may have some application for working with solar system objects and in spacecraft navigation. The Ecliptic is tilted at an angle of about 23.5 degrees to the Earth’s Equator, and crosses it at the spring (Vernal) and fall (Autumnal) equinoxes in March and September.

Galactic

The galactic equator, centered on the Milky Way, defines this coordinate system, with distances measured in degrees of longitude E from the galactic nucleus (0 to 360 degrees). The center of the Galaxy is now defined as the radio source Sagittarius A, presumably, the residence of the supermassive black hole lurking in the galactic nucleus. High precision is not an issue, because Galactic coordinates aren’t used to locate objects, but to describe distributions of objects. For example, open clusters are generally found at low galactic latitudes, globular clusters are randomly distributed, and galaxies avoid the galactic plane and are more numerous at high galactic latitudes.
If you want to look out of the galaxy and avoid as much as possible of the clutter and obscuration at low latitudes (as in the Hubble Deep Field imagery), you look towards the galactic poles where dust and stars are sparsest. Remember, the galactic poles are at right angles to the galactic equator and 90 degrees away from it, they are not directly “above” the galactic nucleus.

The galactic equator is tilted at an angle of about 63 degrees to Earth’s equator.