What does equinox mean?
An equinox is one of the two points in the year at which the sun crosses the celestial equator, so that its centre rises and sets at roughly the same time for observers everywhere on Earth, producing days and nights of approximately equal length. The two events occur around 20 March — the vernal or spring equinox in the Northern Hemisphere — and around 22–23 September, the autumnal equinox. Because these dates mark turning points in the solar calendar, they have long been used to define the beginnings of spring and autumn, both astronomically and in cultural traditions ranging from harvest festivals to Easter calculations. The word carries a slightly formal, scientific register but appears widely in weather reporting, gardening advice, and general writing about the seasons. Its Latin roots — aequus (equal) and nox (night) — capture its defining feature: near-equal shares of daylight and darkness.
nounEither of the two times in the year when the sun crosses the celestial equator, making day and night of approximately equal length everywhere on Earth. By extension, the moment itself, or the date on which this occurs (around 20 March and 22–23 September).
- Either of the two annual moments when the sun crosses the celestial equator and day and night are of approximately equal length.
- The date on which one of these moments falls, especially as marking the start of spring or autumn.
"The autumnal equinox marks the traditional start of fall in the Northern Hemisphere."
"Farmers have long planted their crops around the spring equinox."
"Day and night are nearly equal at the September equinox."
Standard plural, pronounced with a final /ɪz/. Used when referring to both yearly events collectively or to the event across multiple years.
"Ancient monuments such as Stonehenge are aligned with the solstices rather than the equinoxes."
On the equinox, the sun rises almost exactly due east and sets almost exactly due west — a fact navigators have relied on for centuries.
Reviewed by Deb Chak, Editor. AI-assisted content curated by RJS Tech Solutions LLP.
Etymology of equinox
Equinox comes from the Latin aequinoxium, formed from aequus meaning 'equal' and nox meaning 'night', reflecting the near-equal length of day and night at these times. The word entered English in the late Middle English period, borrowed via Old French from Latin. It shares its Latin root elements with words such as equator, equilibrium, and nocturnal.
Related word forms
How equinox is actually used
Formal to neutral register; common in astronomy, calendars, and seasonal discussion. Often paired with 'vernal' (spring) or 'autumnal' (autumn), though plain 'spring equinox' and 'fall equinox' are increasingly common in everyday American usage.
Easily confused with equinox
A solstice is when the sun is at its greatest distance from the celestial equator, giving the longest or shortest day, whereas an equinox is when day and night are approximately equal.