Autumn 2023: has the Autumn equinox begun? What is it, when did it start in the UK - and when will it end?

Equinoxes and solstices are key dates in the calendar, used to define the transitional periods between the seasons.

The Autumn equinox is now upon us signalling the change from summer to autumn.

There are actually two ways of calculating the first day of autumn, depending on whether you follow the meteorological or definitions of the seasons.

But, whichever one you follow, both happen in September.

Meteorological autumn begins first, on Friday 1 September, followed by astronomical autumn on Saturday 23 September.

An equinox happens when the amount of daytime is pretty much equal to the amount of night time with the word equinox coming from two Latin words - equi which means equal and nox meaning night.

The moment when the amount of daytime is completely equal to the amount of night time actually occurs a few days after the autumn equinox which is called the equilux.

What is the autumn equinox?

The autumn equinox can occur at any time between 21 and 24 September, and marks the start of autumn - if you follow the astrological definitions of the seasons.

This year, the autumnal equinox will occur on 23 September at around 6.50am UK time.

Equinoxes mark the moment the Earth’s equator passes directly through the centre of the sun’s path. On each of these days, the planet should get 12 hours of daylight and 12 hours of darkness, although this is complicated somewhat  by the Earth’s atmosphere and weather conditions affecting the way we see sunlight.

What’s the difference between meteorological autumn and astronomical autumn?

Meteorological autumn

The meteorological autumn date is the easiest to work out. It is based on the simple principle that the year is split into four seasons, and that each of these seasons are made up of three full months, as per the Gregorian calendar.

This means that every year autumn begins on 1 September and lasts until 30 November, with winter then starting on 1 December. In this instance, summer ends on 31 August.

Astronomical autumn

The astronomical season is less straightforward to work out as it can change every year. This is because it starts on the date of the autumn solstice, also known as the autumn equinox or the September equinox, which comes later in September but can vary slightly from year on year.

In 2023, the autumn solstice falls on Saturday 23 September, which is the same date that it fell on last year. In this instance, summer will end on Friday 22 September. It can, however, occur on any date between 21 and 24 of the month, although 22 September is the most common date for it to occur.

The astronomical autumn then lasts until another changeable date, the winter solstice, the shortest day of the year, which is set to take place on Friday 22 December this year. The number of daylight hours on the shortest day will be 7 hours, 49 minutes and 42 seconds. That’s 8 hours, 48 minutes and 38 seconds shorter than the summer solstice, when daylight hours are at a maximum.

When will autumn end?

There are also two ways of calculating the last day of autumn. According to the metrological measure of the seasons, the last day of autumn this year will be Thursday 30 November, with winter then starting on Friday 1 December.

But, the astrological measure of the seasons states that the last day of autumn 2022 will be on Thursday 21 December, followed by the first day of winter on Friday 22 December.