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It is Right Ascension and it is one of the important parameters describing a star's position (or planet etc.). The RA of a star is how many hours minutes and seconds etc. it crosses the prime meridian after the reference point, call the First Point of Aries, which obviously has an RA of zero.
The prime meridian for any observer is the plane defined by the two poles and the observer., and it is the north-south plane for each observer wherever they might be. So when a star is due south it crosses the meridian (some stars cross the meridian to the north of overhead).
Stars, planets etc. can have an RA between 0 and just under 24 hours. The RA of a planet changes as it moves round the ecliptic. The RA of a star changes very slowly as it moves in space. The RA of all stars also changes because of precession which causes a slow continuous movement of the First Point of Aries (it goes round a full circle in the 24,000 years).
The first point of Aries is the line or direction defined by the intersection of two planes: the plane of the Earth's equator and the plane of its orbit round the Sun. This crossing obviously defines a line in two opposite directions, which are RA = 0 and RA = 12 hours. The First Point is defined by the Sun's ascending node, which is where it is on March 21, crossing the equator from south to north.
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ra ra ra ra ra ra ra ra ra ra ra ra ra cheater cheater cheater cheater cheater cheater cheater cheater cheater cheater cheater cheater ra ra ra ra ra ra ra ra ra ra ra ha ha ha ha ha ha ha ha ah ah ah ah ah ar ar ar ar ra ra ra ra ra t(._.t)
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