An Etymological Dictionary of Astronomy and Astrophysics

1. One of the two points on the → celestial sphere where the → celestial equator intersects the → ecliptic, that is when the apparent → ecliptic longitude of the Sun is 0° or 180°.
   

2. Either of the times at which the center of the Sun’s disk passes through these points. → autumnal equinox; → vernal equinox.
   

At equinox, the length of the day and the night are equal all over the globe. The equinox is not a fixed point; it moves due to → precession and → nutation. If only precession is considered, we deal with the → mean equinox of date. If nutation is also taken into account, then we are concerned with the → true equinox.

Etymology (EN): M.E., from O.Fr. équinoxe, from M.L. equinoxium “equality of night (and day),” from L. æquinoctium, from æquus, “→ equal” + nox “→ night” (gen. noctis). In Gk. isimeria “equal day,” from isos “equal,” → iso-, + hemera “day.”

Etymology (PE): From hamug, → equal, + -ân suffix denoting time and place.