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 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. 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 RA, but increases westward rather than eastward. It is important not to confuse SHA with the concept of hour angle as it is usually used in astronomy, which is how far west an object is from one's 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.
