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right ascension

astronomical equivalent of longitude

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Right ascension is the astronomical equivalent of longitude, used to locate stars and other celestial objects by measuring their position east to west across the sky. Astronomers use it as a key part of coordinate systems for mapping the universe and pinpointing where to find specific objects in the night sky.

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Right ascension and declination as seen on the inside of the celestial sphere. The primary direction of the system is the March equinox, the ascending node of the ecliptic (red) on the celestial equator (blue). Right ascension is measured eastward up to 24 along the celestial equator from the primary direction.

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.