Right Ascension

Right ascension (abbreviated RA; symbol α) is the angular distance measured eastward along the celestial equator from the vernal equinox to the hour circle of the point in question. When combined with declination, these astronomical coordinates specify the direction 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 which rises with any celestial object, as seen from the Earth's equator, where the celestial equator intersects the horizon at a right angle. It is contrasted with oblique ascension, the point on the celestial equator which rises with a celestial object as seen from almost anywhere else on Earth, where the celestial equator intersects the horizon at an oblique angle.

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. Right ascension is measured from the vernal equinox or the First Point of Aries, which is the place on the celestial sphere where the Sun crosses the celestial equator from south to north at the March equinox and is located in the constellation Pisces. Right ascension is measured continuously in a full circle from that equinox towards the east.

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