What’s The Difference Between Meteorological and Astronomical Autumn?

Many people associate the beginning of autumn with the equinox, which takes place each year on 22nd or 23rd September. This is when day and night are almost equal in length and the days begin to feel noticeably shorter. Meteorologists, however, have a different view on when autumn begins. They would argue that 1 September marks the first day of this season. So why do we have differing views on this? And which system is correct?

The four seasons

Meteorologists divide the year into four seasons, each of which is three months long. These dates are fixed and never change. Under this system, spring takes place between 1 March and 31 May, summer runs from 1 June to 31 August, autumn occurs between 1 September and 30 November and winter begins on 1 December, ending on 28 or 29 February (depending on leap years). This system is based on weather rhythms and the Gregorian calendar.

Fixed calendar months

The main reason why meteorologists use this system is consistency. Using fixed calendar months makes it much easier to compare long-term weather patterns and analyse climate data. For example, if meteorologists want to compare average autumn temperatures over the past 50 years, it’s much easier to compare the months of September, October and November than to work with seasons that begin on slightly different dates each year. This system also helps with producing monthly and seasonal statistics as well as identifying climate changes and trends. Using fixed dates also allows meteorologists to calculate seasonal averages. Based on climate records from 1991 to 2020, the UK’s long-term average temperature during meteorological autumn is 9.71°C.

Following the Sun

Astronomers divide up the seasons in a different way, and the dates under this system vary slightly, year by year. The divisions are based on the Earth’s position as it travels around the Sun, and the dates of the seasons coincide with the autumn and spring equinox and the winter and summer solstice. The Earth’s orbit isn’t perfectly aligned with our calendar year though, which means that the autumn equinox doesn’t fall on exactly the same date every year. It’s also worth noting that, despite what many people believe, the autumn equinox isn’t actually the day when day and night are exactly the same length. Instead, they are only nearly equal. The day when daylight and darkness are precisely equal is known as the equilux, and it usually occurs a few days later. The autumn equinox tends to fall on 22 or 23 September. Very occasionally, it falls on 24 September, but this is so rare that the last time it happened was in 1931 and it won’t happen again until 2303.

The choice is yours!

So, which system is correct? The simple answer is that both are. Meteorological seasons are designed for practical purposes, whereas astronomical seasons reflect the Earth’s movement around the Sun and have been recognised for centuries. Whichever system you choose to recognise, both mark the changing of the seasons and a time to appreciate the arrival of autumn in all its colourful glory.