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From the Latin aequus  'equal' + nox 'night', the term refers to the two points on the zodiac (0° Aries; 0° Libra) or the two times of the year when daytime and nighttime are equal in length. 

The vernal equinox is when the Sun reaches 0° Aries, which in the northern hemisphere  marks the first day of spring as the sun moves north across the equator to instigate the period where daytime become longer than night-time (vernal comes from the Latin word ver, meaning 'spring'.) 

The September equinox (0° Libra)  occurs when the sun crosses the equator going south (so this marks the first day of autumn in the northern hemisphere and the first day of spring for people living in the southern hemisphere). 

Contrast with solstice: either of the two moments in the year when the sun's apparent path is farthest north or south from the equator.

» Glossary of terms