Sun
The September equinox marks the beginning of autumn for the northern hemisphere Getty

Countries around the world will mark a shift in the seasons with the equinox on 22 September - which will ring in autumn for the northern hemisphere and spring for the south. But what is an equinox?

Theoretically, it is a celestial event when all points on the surface of Earth experience 12 hours of daylight and darkness.

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As the Earth travels around the sun along its orbit, the north to south position of the sun changes over the course of the year due to the changing orientation of the Earth's tilted rotation axes. The dates of zero tilt of the Earth's equator correspond to the spring equinox and the autumn equinox.

Equinoxes occur when the plane of our Earth's equator passes the centre of the sun. At that instant, the tilt of the Earth's axis neither inclines away from, nor towards, the sun. This happens twice a year, around 20 March and 20 September, which are the only times when the subsolar point – the place on Earth's surface where the centre of the sun is exactly overhead – is on the equator and consequently, the sun is at zenith over the equator. The subsolar point crosses the equator, moving northward at the March equinox and southward at the September equinox.

mid-autumn festival
The Mid-Autumn Festival, a southeast Asian harvest festival, is marked around the time of the equinox ChinaFotoPress/Getty

This means that day length is the same - 12 hours - at all points on the Earth's surface on these days, with the exception of each pole, where it will be about to change from permanent light to dark, or vice versa.

The term equinox comes from the Latin aequus, meaning equal, and nox, meaning night. This is derived from the oldest understanding of an equinox, which suggests it is the day when daytime and night are of equal duration. This definition is inaccurate, however.

Firstly, sunrise occurs when the top of the sun's disk rises above the eastern horizon. At that instant, the disk's centre is still below the horizon. Secondly, Earth's atmosphere refracts sunlight and as a result, an observer sees daylight before the first glimpse of the sun's disk above the horizon.

Times of sunset and sunrise vary depending on the location of an observer – longitude and latitude – so the dates when day and night are closest together in length depend on location.

Cultures across the world celebrate the September equinox in a number of different ways and is linked to ancient myth and superstition. A number of harvest festivals are marked around this time, including South East Asia's Mid-Autumn Festival.