Right ascension
astronomical equivalent of longitude
In astronomy, Right ascension (abbreviated RA; symbol α) is the angular distance of a particular point measured eastward along the celestial equator from the Sun at the March equinox to the (hour circle of the) point in question above the Earth.
When paired with declination, these astronomical coordinates specify the location of a point on the celestial sphere in the equatorial coordinate system.
An old term, right ascension (Latin: ascensio recta) refers to the ascension, or the point on the celestial equator that rises with any celestial object as seen from Earth's equator, where the celestial equator intersects the horizon at a right angle. It contrasts with oblique ascension, the point on the celestial equator that rises with any celestial object as seen from a given latitude on Earth, where the celestial equator intersects the horizon at an oblique angle.
Explanation
Right ascension is the celestial equivalent of terrestrial longitude. Both right ascension and longitude measure an angle from a primary direction (a zero point) on an equator.
In astronomy, the equinox refers to a direction in space that is coincident with the intersection of the celestial equator and the plane of Earth's orbit. To measure right ascension, 0 hours is in the direction of the Sun from Earth at the March equinox. This is currently located in the constellation Pisces. (Though defined according to the position of Earth and Sun at the March equinox, nevertheless the direction 0 hours persists throughout the year.
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