Autumn is already beginning to make its presence felt, with the evenings getting darker, the leaves starting to change and summer slowly becoming a distant memory.
But while many of us think of September as the beginning of autumn, the astronomical season doesn’t actually begin until the autumn equinox.
This year, the autumn equinox takes place on Wednesday 23 September 2026 at 1:06am BST.
So, what exactly is the autumn equinox, and does it really mean that day and night are the same length?
What is the autumn equinox?
The autumn equinox happens once a year in September and marks the beginning of astronomical autumn in the Northern Hemisphere.
Earth is tilted on its axis as it travels around the Sun. This tilt is responsible for the changing seasons and the big difference in daylight hours we experience between summer and winter.
At the equinox, the Sun crosses the celestial equator and the Northern and Southern Hemispheres are illuminated approximately equally.
There are two equinoxes each year — one in March and another in September.
For us in the Northern Hemisphere, the September equinox marks the beginning of astronomical autumn. In the Southern Hemisphere, the same event marks the beginning of spring.
Are day and night exactly equal?
Despite the name, not quite.
The word “equinox” comes from Latin words associated with “equal night”, and you’ll often hear that the equinox is the day when we get exactly 12 hours of daylight and 12 hours of darkness.
In reality, there is still slightly more than 12 hours of daylight on the day of the equinox.
This is partly because sunrise and sunset are measured using the upper edge of the Sun rather than its centre, while the Earth’s atmosphere also bends sunlight slightly when the Sun is close to the horizon.
The date when daylight and darkness are closest to being equal is known as the equilux.
For the UK in 2026, the Royal Observatory Greenwich says the equilux falls on 25 September, a couple of days after the equinox.
Why are the evenings getting darker so quickly?
If you’ve noticed a big change in the evenings recently, you’re certainly not imagining it.
Around the autumn equinox, we’re at the time of year when daylight is disappearing particularly quickly.
After the equinox, the nights continue to become longer than the days as we head through October, November and towards the winter solstice in December.
For somewhere as far north as Enniskillen, the difference between our long summer evenings and short winter days is particularly noticeable.
Does autumn start on 1 September or 23 September?
Technically, both dates can be correct — it depends on which definition of autumn you’re using.
Meteorological autumn begins on 1 September and runs through September, October and November. Dividing the year into three-month seasons makes it easier for meteorologists to compare weather and climate statistics.
Astronomical autumn, however, is based on Earth’s position in its orbit around the Sun. In 2026, astronomical autumn begins with the autumn equinox on 23 September.
So if you’ve already been saying autumn has arrived since the beginning of September, you haven’t been wrong!
What happens after the autumn equinox?
From this point onwards, the nights will continue getting longer and the days shorter.
That continues until the winter solstice on 21 December 2026, which marks the shortest day and longest night of the year.
After that, the process slowly begins to reverse and the daylight starts increasing again.
And while the autumn equinox may signal the astronomical end of summer, there is still plenty of autumn ahead of us before those shortest days arrive.
