Right Ascension & Declination

Right Ascension and Declination are a system of coordinates used in astronomy to determine the location of stars, planets and other objects in the night sky. They are similar to the system of longitude and latitude used to locate places on Earth.

Right Ascension (RA)

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 5 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, the vernal equinox.

Declination (Dec)

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.