Online Astronomy eText: The Sky
Astronomical Coordinates
(also see The Celestial Sphere)
Spherical coordinate systems
On the Earth (Terrestrial)
Latitude (measured N and S from the Equator)
Longitude (measured E and W from the Prime Meridian — politically decided in early 1900's as going through Greenwich, England)

Note to self: NEED DIAGRAMS SHOWING/COMPARING: Terrestrial coordinate system; Terrestrial vs Equatorial; Terrestrial vs Ecliptic; Ecliptic vs Equatorial; Horizon; Horizon vs Equatorial; Galactic?

In the Sky (Celestial)
Several coordinate systems, each named by the circle which corresponds to the Equator in the Earth-based system.

The Equatorial system is based on the Celestial Equator (and the Celestial Poles)
Circles parallel to the Equator are like parallels of latitude on the Earth, and we measure N and S from the Equator to the ‘parallel’ that a star is on to measure its DECLINATION (from zero at the Celestial Equator to N or S 90 degrees at the Celestial Poles).
Circles perpendicular to the Equator are like meridians of longitude on the Earth and we measure from the 'Prime Meridian' of the sky to the 'meridian' that a star is on to measure its RIGHT ASCENSION. EXCEPT — we don’t measure E and W, but only TO THE EAST, and we measure it in time units, not degrees.
We measure right ascension to the East in TIME units so that as the stars move to the West they can serve as a clock. If a star with a right ascension of 6h 45m is on “The Meridian” (the arc running from the North point on the horizon through the Celestial Pole, through the Zenith, through the South point on the horizon), it is 6:45 on a star clock. If a star with a right ascension of 12h is on the Meridian, it is 12:00 on a star clock. And if a star with a right ascension of 18h 40m is on The Meridian, it is 18:40 on a star clock.
IN THIS SYSTEM every star has a particular declination and right ascension and we could, on a globe (that is, a celestial globe) plot the positions of all the stars in the sky, and use that to see where they are relative to each other.
These numbers — RA (right ascension) and Dec (declination) — are almost constant for a given star, because the stars are so far away that any motion that they have relative to us (or vice-versa) is too small to see without tremendous effort over times as short as a human lifetime. As a result, the positions of the stars relative to each other seem absolutely fixed to a casual observer (leading to the term the ‘fixed’ stars).
(There are very small changes in these coordinates over long periods of time, because of proper motion and precession. In 2009 a 'quick and dirty' summary of these motions was added to The Many Motions of the Stars. The discussion of precession, although lacking diagrams, detail, and historical context, serves as an introduction to the topic which will be fleshed out at a later date.
However, there are seven objects — the Πλανητες (‘planetes’), or “wanderers” — which MOVE relative to the stars in ‘short’ periods of time: the Moon, the Sun, Mercury, Venus, Mars, Jupiter, Saturn (see The Wanderers)

Comparison of Spherical Coordinate Systems

 System Name Terrestrial Equatorial Ecliptic Horizon Based On Earth rotation Earth rotation Earth orbit Gravity "Poles" North PoleSouth Pole North Celestial PoleSouth Celestial Pole North Ecliptic PoleSouth Ecliptic Pole ZenithNadir "Equator" Equator Celestial Equator Ecliptic Horizon N/S Angle(from "Equator") Latitude0 to 90 degreesN/S = +/- Declination0 to 90 degreesN/S = +/- Celestial Latitude0 to 90 degreesN/S = +/- Altitude0 to 90 degreesup/down = +/- E/W Angle(along "Equator") Longitude0 to 180 degreesE or Wfrom Prime Meridian Right Ascension0 to 24 hoursE onlyfrom Vernal Equinox Celestial Longitude0 to 360 degreesE onlyfrom Vernal Equinox Azimuth0 to 360 degrees*Eastwardfrom North Point
*(Azimuth can also be measured from 0 to 180 degrees, either E or W from the North Point on the Horizon)