Autumn equinox

19th September 2019 Last updated at 15:30

As you have very likely noticed, the sun is setting earlier and the nights are getting longer. The autumn equinox falls on Monday 23 September this year and marks the day when day and night are roughly equal length.

A globe showing the day and night are equal lengths during the equinox
Equal day and night on the equinox

The word "equinox" is derived from Latin and literally translates to "equal night". On these days, everywhere on Earth experiences roughly 12 hours of sunshine and 12 hours of darkness. The Earth's axis is titled at an average of 23.5°. As the Earth travels on its year-long path around the sun it is tilted towards or away from the sun, which gives us our seasons. But, on the day of an equinox, the tilt of Earth's axis is perpendicular to the sun's rays, producing a nearly equal amount of day and night all over the world.

A graphic showing the two equinoxes in a year
Royal Meteorological Society
A graphic showing the equinoxes and solstices

Normally, the autumn equinox falls between 22 and 24 September, and this year the equinox will occur on Monday 22nd September at 08:50am (BST).

Equinox or equilux?

However, the equinox marks the point when day and night are "roughly equal". Strictly speaking, there is not exactly 12 hours of light and 12 hours of darkness on the equinox, with the amount varying by a few minutes. This is because the equinox is measured with respect to the sun's centre, not the edge of the sun. Therefore, although it appears the sun has risen, the centre is still below the horizon.

Additionally, light is refracted (able to bend) which means it begins to appear light before the sun has risen, as well as a few extra minutes of light after the sun has set. Because of these two factors, the date when day and night are exactly equal is called the equilux, which occurs a few days after the autumn equinox.

You may be asking, hasn't autumn already started? Meteorologists use 1 September as marking the first day of autumn. The main reason behind this is that it makes it simpler to compare seasonal and monthly statistics. Whereas, autumn beginning on the day of the equinox is based on astronomy, taking into consideration Earth's orbit and proximity to the sun.

Misty sunrise by a lake
TomEllmore67/Weather Watchers
A misty sunrise in London on the first day of astronomical autumn last year

Over the next 12 weeks as we approach December, the Northern Hemisphere of Earth will tilt further away from the sun. By 22nd December, the sun will have reached its lowest point in the sky during the day, marking the winter solstice - at which point, daylight hours will begin to increase once again.

The Royal Meteorological Society is the BBC's academic partner for Weather Watchers, providing educational content, all with the aim of helping you understand the weather and fully engage with Weather Watchers. Click here for more resources.