One of the two occasions in the year when the length of the day and night are equal, which occurs when the apparent path of the Sun (the ecliptic) intersects with the equatorial plane of the Earth; this happens on a day between March 19 and 21 (spring in the Northern Hemisphere, and autumn in the Southern Hemisphere), and on another day between September 21 and 24 (autumn in the Northern Hemisphere, and spring in the Southern Hemisphere); hence, the exact time when the intersection occurs.

Misspelling of equinox.

plural of equinox / plural of equinoxe

(astronomy) The moment when the sun appears to cross the celestial equator while tilting away from the observer, taking place in September in the northern hemisphere and March in the southern hemisphere.

autumnal equinox

(astronomy) vernal equinox

The slow westward shift of the equinoxes along the plane of the ecliptic, resulting from precession of the earth's axis of rotation, and causing the equinoxes to occur earlier each year

(astronomy) The moment when the sun appears to cross the celestial equator while tilting towards from the observer, taking place in March in the northern hemisphere and September in the southern hemisphere.

autumnal equinox

September equinox, when the sun crosses the equator while heading southward.