The equinox takes place this week, signifying the astronomical beginning of autumn in the northern hemisphere and spring in the southern hemisphere. The exact moment of the equinox arrives at 01.05 BST on 23 September.
While the equinox is commonly understood to be when day and night have equal length—the name itself deriving from Latin meaning “equal night”—the reality is more nuanced. An 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 line, and at the equinox, it transitions from one side to the other.
Despite this astronomical definition, day and night are not precisely equal in duration during an equinox. The sun appears as a disc rather than a point, and atmospheric refraction bends its light rays, allowing it to remain visible even after it dips below the horizon. These combined effects extend the measured length of daytime to several minutes beyond 12 hours at the equinox.
In the UK, the moment when day and night duration come closest to being truly equal actually occurs a couple of days after the astronomical equinox, around 25 September. This specific occurrence is referred to as the equilux, derived from the phrase “equal light.”
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