Marine Glossary

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Aries

Also called the "First Point of Aries," this is the celestial equivalent to the Greenwich meridian. Longitude lines on a star globe (called Sidereal Hour Angles, SHA) are all relative to Aries, just as earth longitudes are all relative to Greenwich. As the earth turns, the Aries meridian circles the earth once every 24 hours. In star sight reductions, to find the star's GHA from the Nautical Almanac, add the permanent SHA of the star to the GHA of Aries at the time of the sight. The meridian of Aries passes approximately through Polaris and Caph, the leading star of Cassiopeia; extended backwards from Polaris, it passes through Phecda, inside the cup of the Big Dipper.

The Aries reference meridian is no longer in the constellation of Aries, where it was originally located at the time this concept was developed in early Greek astronomy. It is now in Pisces. This point has slipped to the east by about 30º in this time period due to the precession of the earth's rotation axis ("precession of the equinoxes"). So the answer to the question, "Where is Aries?" is Pisces.

Technically the "First Point" of Aries is the equivalent of the point on earth where the equator and Greenwich meridian cross (0ºN,0ºW), which in the sky would be SHA=0 and Dec=0. Thus even though some references equate "Aries" with "First Point of Aries" this is not strictly the case. The former is a meridian line; the latter is a point on that line.