The nights are drawing in, temperatures are slowly dropping and leaves are starting to change colour. For some of us, it feels like autumn is imminent. But when does autumn officially start?

When is the first day of autumn?

I’m a physicist and engineer by training, so always understood that autumn was governed by the astronomical calendar. In other words, the seasons change when we reach an equinox or a solstice. So that means the first day of autumn is 22 September, and the last is 21 December.

Meteorologically speaking though, autumn is said to start on 1 September and last until 30 November. This method splits the year into four seasons of three whole months which makes meteorological observing and forecasting easier.

What’s the difference between an equinox and a solstice?

An equinox is when the sun is visible above the horizon and below the horizon for equal time – 12 hours each. This corresponds to the sun being apparently directly overhead at noon on the equator. That happens on 21 March and 21 September.

A solstice is the longest or shortest day in each hemisphere of the earth, usually reckoned from the north (that’s us). So our winter solstice is 21 December (mid-winter’s day) and the summer solstice is on 21 June.

Signs of autumn in trees and plants

A familiar sign of autumn is leaves changing colour. This is a visible signal of the physical and chemical changes going on inside the plant as it prepares for winter.

Plants depend on light for photosynthesis to work, so that they can grow, change and reproduce. Less light and heat means that plants will either slow down their metabolisms as winter approaches, or if they are annual plants, it will be time to set seed and then die.

Many perennial plants will shed their leaves in winter and go into a state akin to hibernation in animals. If they shed their leaves – making them deciduous plants – they withdraw chemicals and nutrients from those leaves into the main body of the plant, such as the trunk and roots of a tree. This also rids them of products they will no longer need when they are resting, or that would even be harmful if kept in the main body of the plant. This is the equivalent of excretion or shedding of skin, fur or feathers in an animal.

At the same time, many of these large perennials will be setting their seeds and then shedding them. Probably the best-known is the horse chestnut (Aesculus hippocastanum). Its conkers are still beloved of many children, and in Southern England, of ring-necked parakeets who eat them before squirrels can bury the same seeds as a food store!

Signs of autumn in animals

Animal populations change and move at this time of year too. Summer-visiting birds fly south to warmer climes for our winter, whilst others come from further north to stay with us. Swallows and swifts, nightingales, chiffchaffs and willow warblers are well-known examples of those that leave at the end of summer. Whooper swans and waxwings are among the more spectacular winter visitors to Britain.

Autumn records so far this year

After a relatively mild January followed by some viciously cold, wet and snowy weather in March, temperatures rose and rainfall was low. Many plants suffered from the summer’s drought conditions and I noticed the effect this had on butterflies too. Numbers appeared to be down this spring, though there was a second burst in late June and into July. I saw fewer species overall though compared to last year, when I saw every single species on the Nature’s Calendar checklists.

Blackberry records have been coming in to Nature's Calendar thick and fast this year, with 310 so far. The first was on 1 July from London. The first ripe rowan berry was also reported on 1 July. The first ripe sloe was seen on 12 July in South East Wales. The first leaf tint record came from a silver birch in Cambridgeshire on 27 June. It’s a bit soon to be sure but it seems like fruiting and leaf tint may be happening quite early due to the warm, dry summer weather.

Help us record the changing seasons

Depending on whether you take the meteorological or astronomical view, autumn has either arrived or soon will!

Join Nature's Calendar and record the signs of autumn to help scientists build a picture of how the changing climate is affecting UK wildlife. Record the last date you see a swallow for example, or look out for the arrival of the first autumn fieldfare and redwing.

Enjoy your observations and the changes around us as we slide downhill into winter. As Oscar Wilde once said, “Do not mind the snow and the frost and the ice, for winter is but spring sleeping.”

Visiting woods

Nature's Calendar

Have you seen a first fruit, changing leaf or migratory bird yet? Let us know what's happening near you and help scientists track the effects of climate change on wildlife.

Explore Nature's Calendar

Discover more about trees and wildlife in autumn