The equinox is when the centre of the sun is directly above the equator and day and night are roughly equal in length.
This happens twice a year, in March (the vernal or spring equinox) and September (autumn equinox).
As the calendar year and the tropical year changes each time, the September equinox can happen any time between September 21 and 24.
Equinox comes from the Latin words ‘aequi’, meaning equal, and ‘nox’, meaning night.
As the Earth orbits the sun, it is on a tilt at a fixed angle. Half of the year, the North Pole is also tilted slightly towards the sun, resulting in longer days for the Northern Hemisphere.
When is the autumn equinox 2021?
This year the autumn equinox, also known as the autumnal equinox, is occurring on September 22 at approximately 7:21pm. For the last five years it has fallen on either September 22 or 23.
The equinox may be taken to mark the end of astronomical summer and the beginning of astronomical autumn in the Northern Hemisphere, while marking the end of astronomical winter and the start of astronomical spring in the Southern Hemisphere.
According to the ancient Greeks, the autumn equinox symbolises the return of the goddess Persephone to the darkness of the underworld, where she is reunited with her husband Hades.
During the ancient times, they didn’t have clocks to measure minutes of the day and night time, but could calculate the sun’s position geometrically using a sundial.