Equinox

Equinox, either of the two moments in the year when the Sun is exactly above the Equator; also, either of the two points in the sky where the ecliptic (the Sun’s annual pathway through the sky) and the celestial equator intersect. On March 20 or 21, the Sun moves north across the celestial equator, marking the vernal equinox, the beginning of spring in the Northern Hemisphere, and the autumnal equinox, the beginning of autumn in the Southern Hemisphere. On September 22 or 23, as the Sun crosses the celestial equator going south, marking the autumnal equinox, the beginning of autumn in the Northern Hemisphere, and the vernal equinox, the beginning of spring in the Southern Hemisphere.

Some astronomical coordinates—e.g., right ascension and celestial longitude—are measured from the vernal equinox. It is sometimes called the first point of Aries because it was at the beginning of that constellation some 2,000 years ago. The term is still used, though precession of the equinoxes has moved the vernal equinox into Pisces.