One of the two times each year, around March and September, when day and night are almost exactly the same length. It marks the start of spring and autumn in many calendars.
From Latin *aequinoctium* 'equality of night and day', from *aequus* 'equal' and *nox* 'night'. The word came through Old French into English.
The equinox is literally the 'equal night'—a cosmic moment of balance between light and dark. Ancient cultures built monuments, like Stonehenge, that line up with the Sun on these days. It shows how carefully people have watched the sky long before modern science.
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