Celestial Equator

Extension into space of Earth's equator

Right Ascension

Right Ascension is measured in hours (h), minutes (m) and seconds (s) and is similar to longitude on Earth.

As the Earth rotates, stars appear to rise in the East and set in the West just like the Sun. For example, the constellation Orion has a Right Ascension (RA) of 4 hours, which is where the center of the constellation appears directly overhead. The constellation Cancer has a RA of 9h (9 hrs). If you wait 3 hours, Cancer will be directly overhead (9 hrs - 4 hrs).

0 hours RA is by convention the right ascention of the Sun on March 21.

Declination

Declination is measured in degrees (°), arc-minutes (') and arc-seconds ("), and is similar to latitude on Earth.

There are 60 arc-minutes in a degree and 60 arc-seconds in an arc-minute. Declination measures how far overhead an object will rise in the sky, and is measured as 0° at the equator, +90° at the North Pole and -90° at the South Pole.