Right ascension (RA) 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 a zero point on the equator. For longitude, the zero point is the Prime Meridian, for RA, 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.
RA is measured eastward from the March equinox. Any units of angular measure can be used for RA, but it is most of the time it is measured in hours, minutes, and seconds, with 24 hours being a full circle. The reason for this is that the earth rotates at an approximately constant rate. 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 is 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 RA, but increases westward rather than eastward. Don't confuse SHA with the concept of hour angle as it is usually used in astronomy, which is how far west an object is from your local meridian.
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 a location's 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 BCE. 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 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.
