- published: 09 Feb 2011
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Right ascension (abbreviated RA; symbol α) is the astronomical term for one of the two direction coordinates of a point on the celestial sphere in the equatorial coordinate system, usually combined with declination. Right ascension's angular distance is measured eastward along the celestial equator from the vernal equinox to the hour circle of the point in question.
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 to 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. For right ascension, the primary direction is known as 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. Right ascension is measured continuously in a full circle towards the east.