What Is Right Ascension and How Do Astronomers Determine a Star’s Right Ascension Using the Celestial Equator?

Right ascension, or RA for short, is the astronomical term for the location of an object on the celestial sphere when using the equatorial coordinate system.

On the terrestrial globe we use longitude lines to express east-west location on Earth.

The prime meridian and the International Date Line are the imaginary vertical lines splitting Earth into two halves.

Longitudinal degrees increase from the prime meridian (0°) to the east and west until they reach the International Date Line, opposite the prime meridian, which is 180°.

The celestial globe substitutes right ascension (RA) for longitude, with the vernal equinox taking the place of the prime meridian.

There are, however, some minor differences. RA is measured in hours (h), minutes (m), and seconds (S); (1 hour = 15°). The hours increase all the way around the celestial globe from Oh to 24h, starting and ending at the vernal equinox.

The right ascension of the star Betelgeuse is 05h 55m 10S RA, or 5 hours, 55 minutes, 10 seconds around the celestial globe to the right of the vernal equinox.

The symbol for right ascension is “α”.