The September equinox brings the astronomical start of autumn to the northern hemisphere and spring to the southern hemisphere. The Guardian’s Starwatch column explains why the date is not always the day of exactly equal daylight and darkness.
The event is defined by the Sun crossing the celestial equator. Local sunrise and sunset times also depend on location and the way daylight is measured.
Two hemispheres, different seasons
The September equinox marks the northern hemisphere's transition into astronomical autumn and the southern hemisphere's into spring. The event follows Earth's geometry relative to the Sun. It does not mean temperatures suddenly change on that date, and the everyday experience of a season still depends on location and weather.
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