Right ascension (abbrev. RA; symbol α = Greek letter alpha) is the astronomical term for one of the two coordinates of a point on the celestial sphere when using the equatorial coordinate system. The other coordinate is the declination.
RA is the celestial equivalent of terrestrial longitude. Both RA and longitude measure an east-west angle along the equator; and both measure from an arbitrary zero point on the equator. For longitude, the zero point is the Prime Meridian; for RA, the zero point is known as the vernal equinox point (also known as the First Point of Aries), which is the place in the sky where the Sun crosses the celestial equator at the March equinox.
RA is measured in hours, minutes, and seconds, eastward from the vernal equinox (sometimes it is also given in degrees). Being closely tied with sidereal time, it is both a unit of time and of angle. An hour of right ascension is equal to 15 degrees of arc, a minute of right ascension equal to 15 minutes of arc, and a second of right ascension equal to 15 seconds of arc. An alternative measure, used in navigation, is Sidereal Hour Angle. The main difference being that RA is measured Eastward, and SHA is measured Westward.
RA can be used to determine a star's location and to determine how long it will take for a star to reach a certain point in the sky. For example, if a star with RA = 01:30:00 is at your meridian, then a star with RA = 20:00:00 will be in the meridian 18.5 sidereal hours later.
The concept of right ascension has been known at least as far back as Hipparchos who measured stars in equatorial coordinates in the 2nd century BC. But Hipparchos and his successors made their star catalogs in ecliptical coordinates, and the use of RA was limited to special cases.
With the invention of the telescope, it became possible for astronomers to observe celestial objects in greater detail, provided that the telescope could be kept pointed at the object for a period of time. The easiest way to do that is to use an equatorial mount for the telescope, which allows the telescope to rotate at the same rate as the earth. As the equatorial mount became widely adopted for observation, the equatorial coordinate system, which includes right ascension, was adopted at the same time for simplicity. Equatorial mounts could then be accurately pointed at objects with known right ascension and declination by the use of Setting circles. The first star catalog to use right ascension and declination was John Flamsteed's Historia Coelestis Britannica (1712, 1725).