The First Point of Libra, also called the autumnal equinox point, is one of the two points on the celestial sphere where the celestial equator intersects the ecliptic. It is defined as the position of the Sun on the celestial sphere at the time of the autumnal equinox. It was named after the constellation in which it occurred in ancient times - Libra. However due to precession, the point gradually moves around the ecliptic. It is currently in the constellation Virgo. The other such point, at the vernal equinox, is the First Point of Aries.
More technically, the equinox happens when the Sun is at one of two opposite points on the celestial sphere where the celestial equator and ecliptic intersect.
Firstpoint of Aries and firstpoint of Libra.
The equinoxes are now the points where the equator intersects the ecliptic and the solstices the points on the ecliptic farthest away from the equator.
The points where ecliptic and equator intersect are called the equinoxes.
In the northern hemisphere the March equinox (or "firstpoint of Aries") is called the vernal equinox; the September equinox ("firstpoint of Libra"), the autumnal.
The first was attained by the omission from the calendar of the ten superfluous days, so as to bring things back to their proper position.
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