Related to or occurring at the time of the equinox, when day and night are approximately equal length; also refers to the celestial equator.
From Latin 'equinoctialis', derived from 'equinoctium' (equinox), which combines 'equi-' (equal) and 'nox' (night). The term evolved in astronomical contexts to describe phenomena tied to the two times yearly when the sun crosses the celestial equator.
The equinoctial line (the celestial equator) was crucial for early navigation—sailors could use it to determine their latitude at sea. The word contains the secret of the equinox itself: 'equi' means equal and 'nox' means night, so an equinox literally means when night is equal to day!
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