What Are Celestial Coordinates?

In this time exposure, stars circle the north celestial pole as Earth turns. The stars trace out arcs of constant declination, the sky equivalent of latitude. Bright Polaris, the North Star, lies within 1° of the north celestial pole — where Earth's axis intersects the celestial sphere.

© 2001 Edwin L. Aguirre & Imelda B. Joson

Newcomers to astronomy can get thrown for a loop when they first encounter declination and right ascension, the terms astronomers use to define coordinates in the sky. Why are the positions of stars that are light-years away in the depths of space stated in a system that's tied to latitude and longitude here on Earth? What are celestial coordinates and do I need to know them for everyday sky gazing?

What Are 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 Earth is at the center of the celestial sphere, an imaginary surface on which the planets, stars, and nebulae seem to be printed. On the celestial sphere, lines of right ascension and declination are similar to longitude and latitude lines on Earth. When a telescope's right-ascension axis is lined up with the Earth's axis, as shown here, the telescope can turn on it to follow the rotating sky.

S&T / Steven Simpson

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 horizo