The September equinox will take place on Wednesday, marking the astronomical beginning of autumn in the Northern Hemisphere and spring in the Southern Hemisphere.
The precise moment of the equinox will occur at 1:05am British Summer Time on September 23, as the Sun crosses the celestial equator from north to south. The event is linked to Earth’s movement around the Sun and the changing position of the Sun as viewed from Earth.
The equinox is commonly associated with a period when day and night are of equal length. However, the two are not exactly equal on the day of the equinox because of the way sunlight is refracted through Earth’s atmosphere and the fact that the Sun appears as a disc rather than a single point of light.
These effects allow sunlight to remain visible for several minutes longer than it otherwise would, meaning daylight lasts slightly more than 12 hours at the equinox.
The actual point when daylight and darkness are closest to equal comes a few days later. In the UK, this will occur around September 25, a phenomenon known as the equilux, meaning “equal light.”
The September equinox also represents an important seasonal transition across the globe. While countries north of the equator move into astronomical autumn, locations south of the equator begin astronomical spring.
The change is part of Earth’s regular annual cycle and occurs as the Sun’s apparent path crosses the celestial equator, an imaginary line extending Earth’s equator into the sky.
For observers, the changing season will also gradually alter the appearance of the night sky as different constellations become more prominent at different times of the year.
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