Autumnal equinox: When it is and what it actually means

The word equinox comes from the Latin for “equal night” – which is exactly what it is, and happens just twice a year

Chestnuts lay at the wayside in Cologne, western Germany, on September 22, 2016. Autumn officially begins on the equinox on September 22 (FEDERICO GAMBARINI/AFP/Getty Images)

This is it: the end of summer. The nights are now longer than the day.

The autumn equinox falls on 22 September and officially marks the beginning of autumn in the Northern Hemisphere and spring in the Southern Hemisphere. But apart from that it has a long history of associations, taking on a huge significance culturally and in astronomy.

The word equinox comes from the Latin for “equal night”. And that’s exactly what happens at the beginning of autumn and spring – the days are divided neatly into a 12-hour day and night for just two days a year, after which they start splitting apart.

The event happens because the Earth is slightly tilted. That means that as it moves around the Earth, once a year, its top and bottom tilt slightly towards and away from the Sun – not only leading to the different lengths of the day but to the different amount of heat, too.

On the equinox, the sun is visible right above the planet’s equator, splitting it down the middle in a neat astronomical alignment. That leads to perfectly divided amounts of sunshine and a sunset and sunrise that happens perfectly in the east and west.

From today, the planet will tip so that people in the Northern Hemisphere gradually have shorter days than nights and cooler weather. That will carry on until the solstice or shortest day, when we’ll start moving back towards the sun and again and move back towards lengthening days.

The equinox has traditionally been less celebrated than the solstice, at least in the UK. But many cultures celebrate the event, including Ancient Greece, China and Japan among others.