First Point of Aries

Location of the First Point of Aries

The First Point of Aries is the point in the sky where the Celestial Meridian, the Celestial Equator and the Ecliptic all meet. It is presently in the southwest of Pisces, moving slowly towards its neighbouring constellation, Aquarius.

The First Point of Aries

Astronomers use the astrological symbol for 'Aries', depicting the head and horns of a Ram, to indicate the point where the Ecliptic crosses the Celestial Equator, known as the First Point of Aries.

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 23,000 years.

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.

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