Starwatch: Night and day reach same length after autumnal equinox

After sun crosses celestial equator, true moment of equal light and darkness approaches

Welcome to autumn! The northern hemisphere’s autumnal equinox took place on 22 September. This is the day on which the sun crosses the celestial equator, moving from the northern celestial hemisphere to the southern.

The celestial equator is the projection of the Earth’s equator up into the sky. So, on the equinox, the sun is shining directly above Earth’s equator, and this creates nearly equal hours of daylight and darkness across the globe. This is reflected in the name. The word “equinox” comes from the Latin words aequus, meaning equal, and nox, meaning night.

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