What and when is the autumnal equinox?

When is the autumnal equinox, what date does the autumnal equinox fall on and when does summer end? 

The autumnal equinox occurs in September, and in the Northern Hemisphere the date marks the end of Summer and beginning of Autumn when the days will start getting shorter and the nights longer.

In 2018 the autumnal equinox will occur on 23 September at 2.54am

The Earth is tilted on its axis meaning that as it orbits the Sun, the Sun illuminates the Northern or Southern Hemisphere more depending on where the Earth is along its orbit. But at two points in the year the Sun will illuminate the Northern and Southern Hemispheres equally – these are known as the equinoxes.

Diagram of the seasons

Find out more about equinoxes and solstices

Autumnal equinox facts:

  • Because of time zone differences, the equinox can be on different days for different locations.
  • Saturn also has equinoxes but because it takes almost 30 years to orbit the Sun, they only happen about every 15 years.

Saturn Equinox
Saturn Equinox, NASA


The Harvest Moon

The Harvest Moon is the name given to the full Moon that occurs nearest to the autumnal equinox. Most years the Harvest Moon falls in September but this year it will occur on October 5th. This full moon won’t be anything out of the ordinary – it’s just a term that arose in the Northern hemisphere to identify the full Moon the marks the start of autumn.

More aurorae

For those located at high Northern Hemisphere latitudes, the autumnal equinox paves the way for increased chances to see the aurora borealis display. According to NASA, the equinoxes are prime time for Northern Lights and geomagnetic activities are more likely to take place in the spring and autumn, than in the summer or winter.

Photograph of Aurora over a Glacier Lagoon © James Woodend, Astronomy Photographer of the Year Earth and Space and Overall Winner 2014
Aurora over a Glacier Lagoon © James Woodend, Astronomy Photographer of the Year Earth and Space and Overall Winner 2014

Autumn equinox customs

Pagan: Mabon

Mabon is a modern Pagan ritual marking the autumnal equinox. The ritual gives thanks for a plentiful harvest and recognises the need to share the Earth’s fruits in the coming Winter months. It is the second of the three Pagan harvest festivals, which include Lammas/Lughnasadh and Samhain.

Japan: Higan  

In Japanese culture, the autumn equinox  is celebrated with the tradition of Higan and September 23 will be taken as a national holiday. It is a time to remember deceased relatives, as well as mark the passing of the seasons.

Higan officially lasts for seven days - beginning three days prior to the equinox and ending three days after it.

When is autumn?

There are three main different ways of defining autumn - astronomical, meteorological and phenological.


Astronomically, the four seasons centre around the equinoxes and solstices. However, there is disagreement between those who see the equinox/solstice as the start of the season and those who hold that it represents the middle of the season.


By contrast, meteorologists tend to divide seasons into periods of three whole months based on average monthly temperatures, with summer as the warmest and winter as the coldest. On this basis, for most of the northern hemisphere the autumn months are usually September, October, and November.


The third way of defining autumn is to use what are known as phenological indicators. These cover a range of ecological/biological signs such as the leaves falling off the trees and the migration of birds to warmer climates. These events of course are greatly influenced by weather and climate, and so changing climate could cause autumn to start earlier or later than the standard astronomical or meteorological definitions.

Photograph of Moon Behind the Trees © Emily Jeremy, Astronomy Photographer of the Year Young Commended 2014
Moon Behind the Trees © Emily Jeremy, Astronomy Photographer of the Year Young Commended 2014

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