What is the autumn equinox and when is it in 2021? Date and time of this year's September equinox

The September equinox marks the arrival of autumn in the UK and other countries in the northern hemisphere – but when is it?

Wednesday, 22nd September 2021, 11:04 am

As the summer quickly fades in Scotland and across the majority of northern Europe, Brits will be preparing to wrap up warm and turn on their radiators for the arrival of the colder, autumn season.

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According to the Met Office, the UK’s last autumn in 2020 was warmer than usual – with dry and sunny days in September quickly descending into an overcast October full of downpours.

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What is the autumn equinox and when is it in 2021? Date and time of this year's September equinox (Image credit: Owen Humphreys/PA Wire)

With the September equinox fast approaching, here’s what you need to know about the upcoming event, when it will take place and what the arrival of the autumn equinox means for the UK.

What is the autumn equinox?

Taking place twice a year in different seasons, the equinox is when the sun crosses the equator and sees day and night periods change to become equal length.

The word ‘equinox’ is rooted in Latin terms for ‘equal’ (aequus) and ‘night’ (nox).

It occurs in both March and September every year, with the dates changing slightly in accordance with the sun’s astronomical calendar.

The March equinox is the spring equinox, whereas the September equinox marks the arrival of the autumn season in Northern Hemisphere countries such as the UK and northernly European nations – with chillier temperatures, less sunlight and shorter days.

For countries in the Southern Hemisphere such as Australia, New Zealand and South Africa, the September equinox sees the arrival of spring.

The equinox is different to the solstice, which marks the shortest and longest days of the year that come with the more dramatic seasonal changes of summer and winter in the northern hemisphere.

In the UK, the clocks will also be turned back in October to mark the end of British Summer Time.

When does the September equinox fall in 2021?

The September equinox to mark the arrival of autumn will take place this year on Wednesday 22 September at approximately 8.20pm.

In Scotland, this collides with the September Weekend – an extension of one of September’s final weekends by a single or two days depending on where you live.

For instance, the September Weekend will take place this weekend from 24 to 27 September in Glasgow City Council areas while taking place earlier in Edinburgh and the Lothians.

How long does autumn last?

The autumn period lasts until winter officially arrives in the Northern Hemisphere.

This year’s winter solstice, the shortest day of the year, is set to take place on 21 December.

The number of daylight hours on the shortest day amounts to 7 hours, 49 minutes and 42 seconds - some 8 hours, 48 minutes and 38 seconds shorter than the summer solstice, when daylight hours are at a maximum.

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