It’s officially Autumn, but what is an equinox, and how is it celebrated?

What causes the autumn equinox?

The Earth takes one year to orbit the Sun – 365 cycles of day and night. In the summer, the days are much longer. You wake up in the light, and the Sun remains in the sky until after nine o’clock in the evening. Meanwhile, in the winter, it’s dark when you wake up, and the Sun has long-set by the time you go to bed.

This is because, rather than rotating perpendicular to its orbit around the Sun, the Earth is tilted on its axis. About 4.6 billion years ago, our solar system was merely a cloud of matter – gas and dust. Over time, the gravity of these particles caused the cloud to collapse in on itself, and start to spin. The Sun formed in the centre, and with a new source of gravity, matter collided, forming protoplanets. When these pre-planets slammed into each other, matter from both bodies combined to create even bigger planets. This is how the Earth is thought to have formed, and the way in which the collisions occurred is thought to have caused the axis tilt.

All of this means that for half the year – during summer – the Northern Hemisphere is nearest the Sun, getting light for longer. Whilst in the other half – during winter – the Northern Hemisphere is furthest from the Sun, resulting in fewer hours of daylight.

At two points in the Earth’s orbit, the Sun appears to cross the celestial equator, a projection of the Earth’s own equator into space. It’s at these points when the hemisphere nearest the Sun swaps to being furthest away. We call these points equinoxes, meaning “equal night” in Latin, because when the alignment occurs, day and night are both 12 hours long.

When does the autumn equinox occur?

The autumn equinox, due to the misalignment between the calendar year and the tropical year, can occur at any time from 21 to 24 September. This year, it happened at 8.50 am on Monday 23 September. If you watched the sunrise on that day, you will have seen the Sun come up due East, and the sunset will have been due West.

People who use the astronomical system to define the seasons use the autumn equinox to mark the transition from summer to autumn. An alternative system uses the months of the Gregorian calendar to divide the year into meteorological seasons, and this puts the start of Autumn at 1 September, 22 days behind the astronomical definition.

Autumn becomes winter on another important astronomical event: the winter solstice, which this year will be on Sunday 22 December. Solstices occur twice a year when the Earth’s axis is tilted most closely towards the Sun. The longest day of the year occurs in the hemisphere tilted towards the Sun, whilst the other hemisphere endures the longest night. There is debate over whether the winter solstice marks the start or the middle of winter, as well as conflict with the meteorological system. The coldest temperatures during the day are usually felt some time after the shortest day, which is why it typically snows in January or February, and the chances of a White Christmas are rather remote.

How is the autumn equinox celebrated?

For centuries, people all over the world have celebrated the autumn equinox. In Japan, Buddhists recognise the equinox as symbolising the transitions of life, because the Sun sets due West – the direction of the land of the afterlife. In the week around the equinox, known as Ohigan, people visit the graves of their ancestors, leaving flowers.

Pagans celebrate Mabon, the ‘second harvest’, on the autumn equinox. It is a time to be thankful for abundant crops of the summer months, to share with people less fortunate, and to prepare for the darker, colder nights ahead in winter. Celebrations involve gathering crops from the fields, and gardens, and from orchards, whose apple trees represent wisdom and guidance.

In the UK, we traditionally celebrate harvest festival on the full moon closest to the autumn equinox, known as the Harvest Moon.

This year, crowds gathered at Stonehenge to celebrate the autumn equinox. Pagans and Neo-Druids were amongst the hundreds of people who visited the site to watch the sunrise. Visitors to Stonehenge were allowed to go right up to the stones, and to be inside the stone circle, which is usually roped off to the public.

After the Autumn equinox

Autumn has arrived. Horse chestnut and London plane trees are lining the streets with their crisp, golden leaves. Sunset is noticeably earlier in the evening, and the air has taken on a warm and earthy, almost smoky smell. Wildlife takes advantage of the abundance of autumnal fruits. Acorns, rosehips, and crab apples are squirrelled away so animals can build up their fat reserves for the impending Winter. Jackdaws and rooks fly in flocks to their evening woodland roosts. Stags and bucks develop antlers to fight rival males. Misty mornings reveal an intricate network of spiderwebs coated in dew.

Now it’s over to you. Go for an early-morning walk and watch the sunrise in the Peaks. Celebrate the start of autumn, and look forward to a season of warm colours, crunchy leaves, and pumpkin spice lattes.

Featured image: George Tuli

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