Starwatch: Autumnal equinox in northern hemisphere, spring in the south

People watch the sun setting in Northolt, London, on the evening before the 2025 autumnal equinox. Photograph: Stephen Chung/Alamy

The September or autumnal equinox takes place this week. 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 01.05 BST 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.