One of the two points on the Celestial Sphere where the Ecliptic and the Celestial Equator
cross one another. The First Point of Aries, which is actually in Pisces, defines the
zero-point for Right Ascension.
When the Sun reaches the First Point of Aries, as it does once each year, an equinox occurs. In the northern hemisphere, this is the Vernal Equinox, before which the north pole is tipped away from the Sun, and days are shorter than nights. After the equinox, the north pole is angled toward the Sun, so that days start to become increasingly longer than nights, and moving the northern hemisphere into spring and summer. For the southern hemisphere, these effects are reversed: the Autumnal Equinox occurs, and days start to become shorter than nights.
The First Point of Aries is not a fixed point in space: it moves along the Ecliptic at a rate of
roughly one degree every seventy years. When the Equinox was first observed, thousands
of years ago, the First Point actually lay in the constellation of Aries. Due to the
effect of precession, the First Point of Aries crossed into the neighbouring
constellation of Pisces in about 70 BCE. It has taken about 2,000 years to cross Pisces, and
it will cross into the next zodiacal constellation, Aquarius, in about the year 2600.
Following its journey along the Ecliptic, it will return to Aries once again in about
Because the First Point of Aries is the zero-point for calculating coordinates on the
Celestial Sphere, its own coordinates are always fixed, regardless of its motion. They
are, of course, zero hours Right Ascension and zero degrees Declination.