High Quality Content by WIKIPEDIA articles! Right ascension (abbrev. RA; symbol ?) is the astronomical term for one of the two coordinates of a point on the celestial sphere when using the equatorial coordinate system. The other coordinate is the declination. RA is the celestial equivalent of terrestrial longitude. Both RA and longitude measure an east-west angle along the equator; and both measure from a zero point on the equator. For longitude, the zero point is the Prime Meridian; for RA, the zero point is known as the First Point of Aries, which is the place in the sky where the Sun crosses the celestial equator at the March equinox. RA is always zero on the equinoctial colure, the meridian of the celestial sphere which passes through the celestial poles and first point of Aries.

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