Right ascension (abbrev. RA; symbol α) 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.
Right ascension is the celestial equivalent of terrestrial longitude. Both right ascension and longitude measure an angle that increases toward the east as measured from a zero point on an equator. For longitude, the zero point is the Prime Meridian on the geographic equator; for right ascension, the zero point is 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.
Any units of angular measure can be used for right ascension, but it is customarily measured in hours, minutes, and seconds, with 24 hours being equivalent to a full circle. The reason for this choice is that the earth rotates at an approximately constant rate (see sidereal time). Since a complete circle has 360 degrees, an hour of right ascension is equal to 1⁄24 of this, or 15 degrees of arc, a single minute of right ascension equal to 15 minutes of arc, and a second of right ascension equal to 15 seconds of arc. Sidereal hour angle, used in celestial navigation, is similar to right ascension, but increases westward rather than eastward. It is important not to confuse sidereal hour angle with the astronomical concept of hour angle, which is how far west an object is from one's local meridian.
Right ascension 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 a location's meridian, then a star with RA = 20:00:00 will be in the meridian 18.5 sidereal hours later.
The tilt of the Earth's axis gradually changes over time. This effect, known as precession, causes the measured right ascension and declination of even a perfectly stationary celestial object to change noticeably over the span of decades. Therefore, equatorial coordinates are inherently relative to the year of their observation. Astronomers always specify equatorial coordinates with reference to a particular epoch. An astronomer comparing coordinates from different epochs must mathematically rotate one of the coordinate systems to match the other, or rotate both to match a common epoch.
The currently used standard epoch is J2000.0, which is January 1, 2000 at 12:00 TT. The prefix "J" indicates that it is a Julian epoch. Prior to J2000.0, astronomers used the successive Besselian Epochs B1875.0, B1900.0, and B1950.0.
The concept of right ascension has been known at least as far back as Hipparchus who measured stars in equatorial coordinates in the 2nd century BCE. But Hipparchus and his successors made their star catalogs in ecliptic 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, which allows the telescope to be aligned with one of its two pivots parallel to the Earth's axis. A motorized clock drive often is used with an equatorial mount to cancel out the Earth's rotation. 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).