equinox

noun5

1. •
   One of two times in the year (one in March and the other in September) when the length of the day and the night are equal, which occurs when the sun is directly overhead at the equator; this marks the beginning of spring in one hemisphere and autumn in the other.

   • [T]he Months of March and September, the tvvo Aequinoxes of Our year, are the most vvindy and tempestuous, the most unsettled and unequable of Seasons in most Countries of the VVorld. — 1692
2. •
   The circumstance of a twenty-four hour time period having the day and night of equal length.

   • [D]oe but see his vice, / Tis to his vertue, a iust equinox, / The one as long as th'other: […] — 1603
3. •
   One of the two points in space where the apparent path of the Sun intersects with the equatorial plane of the Earth.
4. •
   (no longer used) A celestial equator (“great circle on the celestial sphere, coincident with the plane of the Earth's equator (the equatorial plane)”); also, the Earth's equator.
5. •
   (no longer used, rare) A gale (“very strong wind”) once thought to occur more frequently around the time of an equinox, now known to be a misconception; an equinoctial gale.

   • The passage yet vvas good, the vvind, 'tis true, / VVas somevvhat high, but that vvas nothing nevv, / Nor more than usual Equinoxes blevv. — 1687