Understanding Celestial Coordinates
The celestial coordinate system, which serves modern astronomy so well, is firmly grounded in the faulty world-view of the ancients. They believed the Earth was motionless and at the center of creation. The sky, they thought, was exactly what it looks like: a hollow hemisphere arching over the Earth like a great dome. The stars? "They're fireflies," explains Timón in The Lion King, "stuck to that big, uh, blue-black thing up there."
The celestial dome with its starry decorations had to be a complete celestial sphere, early skywatchers realized, because we never see a bottom rim as the dome tilts and rotates around the Earth once a day. Part of the celestial sphere is always setting behind the western horizon, while part is always rising in the east. At any time half of the celestial sphere is above the horizon, half below.
Even today this is how the cosmic setup actually looks. Never mind that we're on a moving dust mote orbiting a star in the fringe of a galaxy. In astronomy, appearances and reality are more different than in any other area of human experience. Perhaps for this reason, astronomers are quite comfortable living with both as long as the two are kept in their proper relationship. The celestial sphere, with its infinitely large radius, appears to turn daily around our motionless Earth, from which we use telescopes to examine wonders painted on its inside surface.
From Earth to Sky
Whenever you want to specify a point on the surface of a sphere, you'll probably use what geometers call spherical coordinates. In the case of Earth, these are named latitude and longitude.
Imagine the lines of latitude and longitude ballooning outward from the Earth and printing themselves on the inside of the sky sphere, as shown at right. They are now called, respectively, declination and right ascension.
Directly out from the Earth's equator, 0° latitude, is the celestial equator, 0° declination. If you stand on the Earth's equator, the celestial equator passes overhead. Stand on the North Pole, latitude 90° N, and overhead will be the north celestial pole, declination +90°.
At any other latitude let's say Kansas City at 39° N the corresponding declination line crosses your zenith: in this case declination +39°. (By custom, declinations north and south of the equator are called + and rather than N and S.) This is the declination of the bright star Vega. So once a day, Vega passes overhead as seen from the latitude of Kansas City.
Lines of both right ascension and declination stay fixed with respect to the stars. That's why they can be permanently printed on star maps. (This does mean that the one-to-one connection between right ascension and longitude is broken the moment after you imagine the lines ballooning out from Earth and printing themselves on the sky; the two systems rotate with respect to each other.)