Autumn equinox 2026: When is it and what happens?

Astronomical autumn begins this week, with the autumn equinox taking place in the early hours of Wednesday.

The equinox will occur at 01:06 BST on 23 September, according to Royal Museums Greenwich.

It is a precise moment in time when the Sun crosses the celestial equator and neither hemisphere is tilted towards or away from the Sun, meaning day and night are roughly equal in length across the world.

And while the equinox itself cannot be seen, the day has cultural and spiritual significance for some people. Gatherings take place at sites including Stonehenge, where people come together to mark the occasion and watch the Sun rise.

What is the difference between the equinox and equilux?

The word "equinox" comes from Latin and means equal night.

At the equinox, the daylight measurement is taken from the point when the centre of the Sun is at the horizon, and is 12 hours sunrise to sunset and 12 hours sunset to sunrise.

However, it is the top edge of the Sun which is visible first at sunrise and last at sunset.

Because of light refraction - light being bent by the atmosphere - there is still light for a time when the Sun is not directly visible. This adds extra time to daylight hours.

In autumn, this means that the time when day and night are actually equal happens after the equinox.

This is known as the "equilux", which means equal light.

This year, that happens in the UK on Friday 25 or Saturday 26 September, depending on your latitude.

Where in the UK are days getting shorter fastest?

Although the equinox happens at the same moment everywhere, the change in hours of daylight is not the same across the UK.

The further north you go, the bigger the difference between the long days of summer and the short days of winter.

That means places in northern Scotland are losing daylight more quickly at this time of year than parts of southern England.

Around the autumn equinox, the change is particularly noticeable as sunrise gets later and sunset earlier.

Does the equinox mean colder weather is on the way?

The autumn equinox may mark the start of astronomical autumn, but it does not necessarily mean there will be a sudden change in the weather.

As the Sun gets lower in the sky and the days shorten, temperatures generally fall as we head further into autumn.

So far this September, UK mean temperatures have been above average overall, according to the Met Office.

There is still some warmth in the forecast this week, particularly in the south-east of England, before rain moves across the UK on Wednesday, followed by fresher conditions on Thursday.

Towards the end of September, changeable weather is expected, with temperatures likely to remain slightly above average overall. However, clearer and cooler nights could bring some fog and even frost in places.