equinox
n. countablen. one of the two days in the year when day and night are exactly the same length. This happens in March and September when the sun is directly over the equator.
n. the moment when the sun crosses the celestial equator, resulting in a day and night of approximately equal duration globally. Occurs twice annually, around March 20 and September 23.
The spring equinox marks the first day of spring.
Ancient civilizations often built stone monuments aligned with the sun's position during the autumn equinox.
During the equinox, the Earth's axis is tilted neither toward nor away from the sun, creating a brief period of symmetry in the distribution of light across both hemispheres.
PIE word *nókʷts From Middle English equinox, equinoxe, equynox (“one of the two periods in the year when the day and night are of equal length, equinox; either the zodiac sign Aries or Libra, in which the sun crosses the celestial equator”), from Old French equinoce, equinoxe (modern French équinoxe), or from its etymon Medieval Latin ēquinoxium, ēquinoctium, from Latin aequinoctium (“equinox”), from aequus (“equal”) + nox (“night”) (ultimately derived from Proto-Indo-European *nókʷts (“night”)) + -ium (suffix forming abstract nouns). The Latin word, ultimately adopted in Middle English and modern English, displaced Old English efnniht (modern English evennight). The rare alternative plural form equinoctes treats equinox as if it were a Latin word; the plural of Latin nox (“night”) is noctēs.