The right ascension is the angular distance between two imaginary lines in the sky that run north-south. The Prime Meridian is the terrestrial equivalent of this line, which is used as a reference for equatorial coordinates of astronomical objects. The right ascension is based on a significant point in the sky, unlike longitude, which is defined arbitrarily. The right ascension, like longitude, changes slightly over time.
Despite the fact that the Earth’s surface is a three-dimensional sphere, a unique point on the surface can be specified with only two pieces of information. However, this assumes that a coordinate system has been established. Latitude and longitude are the most commonly used coordinate systems. The angular distance north or south of the equator is known as latitude. Longitude is the angular distance east or west of a north-south line that is arbitrarily defined; the Prime Meridian, which passes through Greenwich, England, is commonly used.
Other stars appear to be fixed in relation to each other because they are so far away. The celestial sphere is a sphere with “fixed” stars. A point on the celestial sphere, like a point on the Earth’s surface, can be specified with only two coordinates. In fact, when projected onto the celestial sphere, the Earth’s equator is frequently used for convenience. The right ascension is the last piece of information needed to identify a celestial point.
Any north-south line through the celestial sphere, like the Prime Meridian, could be used to define the start of the right ascension. However, there are two distinct points in the sky. Because of Earth’s tilt, the plane of the equator and the plane of the solar system are not the same. These two planes intersect to form a line that runs through the Earth’s center, pointing in opposite directions to two points in space. One of these points, known as the First Point of Aries, is used to define the right ascension’s zero point.
The First Point of Aries was named after its location in the constellation Aries when it was discovered thousands of years ago. The First Point of Aries would still be in the same constellation if the Earth’s spin axis remained constant. The Earth’s spin, like that of a spinning top, has precession. The First Point of Aries moves about one degree in the sky every 70 years due to the changing orientation of the Earth’s spin axis. As a result, the line to begin the right ascension moves with it.
Because of its convenience, an equatorial coordinate system is used in astronomical mapping. However, the right ascension’s transitory nature poses a problem in this coordinate system. As a result, equatorial coordinates that use the right ascension are always associated with a date. Declination is the astronomical equivalent of latitude that does not change with time.