Glossary of Astronomical Terms/first point of Aries

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The first point of Aries is one of the two places in the heavens when the great circle of the ecliptic crosses the celestial equator. This is also the spring equinox in the Northern Hemisphere of Earth.

Because of the precession of the Earth's axis, the celestial equator moves gradually and the first point of Aries moves as well, at a rate of approximately 1 degree every 70 years. Approximately 2000 years ago, the first point of Aries actually was in the constellation Aries. It is currently in the constellation Pisces and is moving in the direction of the constellation Aquarius.

The first point of Aries is the prime meridian for right ascension. By definition, its coordinates are always 0 hours R.A., 0 degrees declination. This means that astronomical coordinates should, to be accurate, refer to the date when the measurements were taken.