The astronomical equinox takes place this week, occurring at 01.05 BST on 23 September. This celestial event marks the beginning of autumn in the northern hemisphere while simultaneously marking the start of spring in the southern hemisphere.
While the equinox is commonly understood to be the moment when day and night are equal in length, the reality is more nuanced. The equinox actually occurs when the sun crosses the celestial equator, an imaginary line representing the projection of Earth’s equator into the sky. As Earth orbits the sun, the sun’s apparent position shifts north and south of this celestial equator, and the equinox marks the moment it crosses from one side to the other.
Despite the term’s derivation from Latin meaning “equal night,” daytime and nighttime are not precisely equal length during the equinox. The apparent diameter of the sun and the refraction of its light through Earth’s atmosphere extend the visible daylight hours. These optical effects combine to make the measured daytime several minutes longer than 12 hours at the equinox.
In the UK, the date when daylight and darkness come closest to perfect equality occurs slightly later, around 25 September. This moment is referred to as the equilux, a term meaning “equal light.”
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