Space Facts

Right Ascension & Declination

How do you measure star positions in the sky?

Instead of "latitude," however, astronomers use something called, "declination." Like latitude, this measure uses degrees north and south of the celestial equator (a projection of Earth's equator onto the infinite sky). The degrees are divided into minutes and seconds of arc to give finer levels of measure.

Instead of "longitude," astronomers use the term, "right ascension." This is measured from the position of the sun on the first day of spring — a place in the sky called the "First Point of Aries," where the sun passes from south to north of the celestial equator. Because of precession, this point is no longer in Aries, but resides in Pisces. From this point, right ascension is measured eastward in hours, each fifteen degrees wide. These units are further divided into minutes and seconds.

Because of precession of the equinoxes (a near-twenty-six thousand year wobble of Earth's axis), right ascension is constantly changing (as is declination). In order to establish standards of measure, astronomers speak of the coordinates during a certain year — called an "epoch." This reference standard can be any year, or any point in time within a year. Typically, however, the measures are given at certain milestone epochs like, 1900, 1950, and 2000.

As an example, the epoch 2000 coordinates for the star "Vega" (Alpha Lyrae) are, 18^h 36^m 56.2^s +38° 47' 1".