Glossary term: Right Ascension (RA)

Description: Right ascension is one of two coordinates in the equatorial coordinate system (the other being declination), which astronomers use to define the positions of celestial objects in the sky. As seen from Earth, all the various positions in the sky together form what appears to be a distant sphere with Earth at its center. The points in the sky directly above Earth's equator form the celestial equator on that sphere. The point directly above Earth's geographic North Pole is the celestial North Pole, and that above Earth's South Pole, the celestial South Pole. Just like geographers define geographic longitude and latitude on Earth's surface, one can define longitude and latitude on the celestial sphere. If we were to choose a celestial object's longitude coordinate to be that of the location on Earth directly below, a star's coordinate value would change over time as the Earth turns. Instead, equatorial coordinates measure right ascension as a form of celestial longitude relative to a "meridian" in the sky that does not rotate with Earth, but instead is fixed relative to the fixed stars. That meridian, the analog of the Greenwich meridian on Earth, is defined by where it intersects the celestial equator: At the exact point where the Sun's apparent path crosses the celestial equator from the southern to the northern celestial hemisphere. This longitude is called right ascension. Its value increases towards the east. Look towards the celestial equator, and the longitude values will pass you by in the course of (roughly) 24 hours. That is why right ascension is typically stated as a time value, with 24 hours corresponding to the full 360 degrees. Declination, the second equatorial coordinate, corresponds to geographic latitude. A slight wobble in Earth's rotation axis known as precession makes the equatorial coordinate system, and with it the right ascension and declination of stars and other celestial objects, change over time, but only very slightly and very slowly.

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