The right ascension is the angular distance from an imaginary line that runs north-south through the sky. This line is used as a reference for equatorial coordinates of astronomical objects; its terrestrial analog is the Prime Meridian. Unlike longitude, which is defined arbitrarily, the right ascension is based on a significant point in the sky. Also unlike longitude, the right ascension changes slightly with time.
Even though the Earth’s surface rests upon a three-dimensional sphere, it requires only two pieces of information to specify a unique point on the surface. This assumes there is an established coordinate system, however. The most frequently used coordinate system uses latitude and longitude. Latitude is the angular distance north or south of the equator. Longitude is the angular distance east or west of an arbitrarily defined north-south line; the Prime Meridian, which runs through Greenwich, England, is usually used.
Other stars are so far away that they appear fixed relative to each other. This sphere of “fixed” stars is called the celestial sphere. Like a point on the Earth’s surface, a point on the celestial sphere can be specified with only two coordinates. In fact, the Earth’s equator, when projected onto the celestial sphere, is frequently used for convenience. This leaves one more piece of information necessary to identity a celestial point—the right ascension.
Like the Prime Meridian, any north-south line through the celestial sphere could be used to define the beginning of the right ascension. There are two unique points in the sky, however. The plane of Earth’s equator and the plane of the solar system are not the same because of Earth’s tilt. These two planes intersect to form a line that runs through the center of the Earth; this line points to two points in space, in opposite directions. One of these points, called the First Point of Aries, is used to define the zero point for the right ascension.
The First Point of Aries was so named because when it was observed thousands of years ago, it was located in the constellation Aries. If the Earth’s spin axis remained steady, the First Point of Aries would still be in the same constellation. Like a spinning top, though, the Earth’s spin has precession. The changing orientation of Earth’s spin axis causes the First Point of Aries to migrate about one degree in the sky each 70 years. The line to begin the right ascension, therefore, moves along with it.
An equatorial coordinate system is used in astronomical mapping because of its convenience. The transitory nature of the right ascension presents a problem in this coordinate system, however. For this reason, a date is always attached to equatorial coordinates that use the right ascension. The astronomical equivalent of latitude, which does not change with time, is called declination.