| right ascension (RA) |
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One of the coordinates used to define position on the celestial sphere in the equatorial coordinate system. It is the equivalent of longitude on the Earth but is measured in hours, minutes and seconds of time eastwards from the zero point, which is taken as the intersection of the celestial equator and the ecliptic, known as the First Point of Aries. One hour of right ascension is equivalent to 15 degrees of arc; it is the angle through which the celestial sphere appears to turn in one hour of sidereal time, as the Earth rotates. See also: declination. |