Latin 'aequus' (equal) + 'nox' (night) — the Romans measured the year's balance point by the length of night, not day.
Either of the two times in the year when the sun crosses the celestial equator and day and night are of approximately equal length, occurring about March 20 and September 22.
From Latin 'aequinoctium,' composed of 'aequus' (equal, level, flat) + 'nox' (genitive 'noctis,' night), from PIE *nókʷts (night). The PIE root for night — *nókʷts — also produced Greek 'nyx,' Sanskrit 'nakta,' and Welsh 'nos.' The 'aequus' element comes from PIE *h₂eykʷ- (level, even). Romans measured the twice-yearly
The words 'equinox' and 'equity' share the Latin root 'aequus' (equal). An equinox is when day and night are equal; equity is fairness and equality before the law. And 'nocturnal,' 'nocturn,' and the 'nox' in equinox all trace back to the same PIE word for night (*nókʷts) that also produced