It marks the astronomical beginning of autumn in the northern hemisphere and spring in the southern hemisphere. This year, the precise moment of the equinox falls at on 23 September. It is commonly said the equinox is when day and night are equal, because the name derives from the Latin for “equal night”, but in practice that is not quite true.
An equinox occurs when the sun crosses the celestial equator, the imaginary projection of Earth’s equator into the sky. As Earth moves around its orbit, the sun’s apparent position shifts north and south of the celestial equator. At the equinox it crosses from one side to the other.
But even then, day and night are not precisely equal. This is because the sun appears as a disc rather than a point of light, and the bending of its rays by our atmosphere means we can still see it even when it is just below the horizon. Together, these effects make the measured daytime several minutes longer than 12 hours at the equinox.
In the UK, the date on which day and night come closest to being equal will be a couple of days later, around 25 September. This is called the equilux, for “equal light”.
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