The science and significance of the autumnal equinox
In the language of science, an equinox is either of two points on the celestial sphere where the ecliptic and the celestial equator intersect. For the rest of us, it's one of two times a year when the Sun crosses the equator, and the day and night are of approximately equal length.
At the autumnal equinox in Sept., the Sun appears to cross the celestial equator, from north to south; this marks the beginning of autumn in the Northern Hemisphere.
The vernal equinox, also known as “the first point of Aries,” is the point at which the Sun appears to cross the celestial equator from south to north. This occurs in March, marking the beginning of spring in the Northern Hemisphere.
On the date of either equinox, the Sun is above the equator and night and day are of approximately equal length; the word equinox is often used to refer to either of these dates.
The equinoxes are not fixed points on the celestial sphere but move westward along the ecliptic, passing through all the constellations of the zodiac in 26,000 years. This motion is called the precession of the equinoxes. The vernal equinox is a reference point in the equatorial coordinate system.