In non-scientific terms the Autumnal Equinox marks the start of Autumn.
Equinoxes occur twice a year, in Spring and Autumn around 21 March and 23 September.
These are the only times during the whole duration of the year when the Earth is not titled at a 23.5 degree angle away from the plane of the ecliptic resulting in equal day and night on both sides of the Earth's hemisphere, hence the word 'equinox' meaning 'equal night'.
The illustration show Earth's orbit around the sun and the beginnings of the seasons
As well as being an important astronomical event the beginning of Autumn also means we get to take out our wooly jumpers, start thinking about our Christmas festivities and more importantly enjoy the seasonal bloom of autumnal colours around us.
Autumn
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Autumn Leaves, seen in Wakefield this morning. Many parts of the country will see a good deal of dry weather today, with some bright or sunny spells and a few showers here and there. (credit:Michiko Smith /Ross Parry Agency)
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The pink sky over the meadow on September 20, 2015 in Somerset, England. (credit:Barcroft Media)
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A beautiful view of sunrise over the Watchet Harbour Marina on September 11, 2015 (credit:Barcroft Media)
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Christine Mountney, Manager of Capesthorne Hall, Macclesfield, Cheshire, looks out of an office window amongst a sea of red leaves. (credit:Peter Byrne/PA Wire)
(05 of26)
Pennine Region Hot Air Balloon Association, Naburn Lock, York, Yorkshire, Britain 19 Sep 2015 (credit:Charlotte Graham/REX Shutterstock)
(06 of26)
Bright Start to the day over the farm fields of West Lancashire. Burscough, Lancashire, UK. 23rd September, 2015. (credit:Mar Photographics/Alamy Live News)
(07 of26)
The beautiful view of sun bursting through misty fog in woodland on the hills above Huish Champflower on September 20, 2015 in Somerset, England. (credit:Barcroft Media)
(08 of26)
Tu Hwnt ir Bont Tearooms in Llanrwst, Wales, where the cafe's roof is covered in Virginia creeper and has started to change from green to red, as the autumnal colours start to show. (credit:Peter Byrne/PA Archive)
(09 of26)
The autumn sun rises over the north sea in front of The Couple by artist Sean Henry at Newbiggin-by-the-Sea, Northumberland, as mixed conditions and colder nights are in store for Britain following the arrival of the autumn equinox. (credit:Owen Humphreys/PA Wire)
(10 of26)
Rays of sunshine beam through the clouds on the North Sea as a ship passes through near the mouth of the Tyne this morning. (credit:Owen Humphreys/PA Wire)
(11 of26)
Sheep graze in early morning sunlight in Wiltshire fields near Stonehenge, as September is set to turn soggy with the remnants of a tropical storm due to hit the south-east coast of Britain. (credit:Ben Birchall/PA Wire)
(12 of26)
The sunset over the River Trent at Gunthorpe, Nottinghamshire. (credit:Neil Squires/PA Wire)
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The stormy sunrise seen at Dunster Beach sand bar on September 19, 2015 in Somerset, England (credit:Barcroft Media)
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The autumn sun rises over the north sea in front of The Couple by artist Sean Henry at Newbiggin-by-the-Sea, Northumberland, as mixed conditions and colder nights are in store for Britain following the arrival of the autumn equinox. (credit:Owen Humphreys/PA Wire)
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A general view as the sun rises over Stonehenge, Wiltshire (credit:Ben Birchall/PA Archive)
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Sunset on the Steart Marsh peninsula from the mouth of the river Parrett, across Bridgwater Bay National Nature Reserve, toward Hinkley Point C on September 09, 2015 in Somerset, England. (credit:Barcroft Media)
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A water-skier at sunset on the River Trent at Gunthorpe, Nottinghamshire. (credit:Neil Squires/PA Wire)
(18 of26)
Worthing starts its day with clear skies and the odd light cloud as the sun rises, which makes for a beautiful start to the day on what promises to be a fine day along the south coast. Tuesday 23rd September 2015. UK Weather © Photovision images news / Alamy live news.
(19 of26)
The sun rises over a hillside covered with hay bales on the first day of Autumn or the Autumn Equinox on September 23, 2015 in Saltburn, England. An equinox occurs when the sun is directly over the equator and the north and south poles of the Earth are not tilted towards or away from the sun, but are aligned giving the same amount of daylight in both of the hemispheres. The word equinox is derived from the Latin, meaning equal night. (Photo by Ian Forsyth/Getty Images) (credit:Ian Forsyth via Getty Images)
(20 of26)
The beautiful orange sunset clouds seen from Pitsford Hill in Somerset, England (credit:Barcroft Media)
(21 of26)
A view of sunset on the path from the back of Steart Marsh peninsula from the mouth of the river Parrett (credit:Barcroft Media)
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A field of straw bales pictured with spectacular clouds is seen on a farmland near Williton on September 11, 2015 in Somerset, England. (credit:Barcroft Media)
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A colourful rainbow pictured at the Quantock Hills on the way to Somerset coast, England. (credit:Barcroft Media)
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A majestic view of the Doniford Bay seen before dawn on September 18, 2015 in Somerset, England. (credit:Barcroft Media)
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The beautiful orange sunset clouds seen from Pitsford Hill on September 19, 2015 in Somerset, England. (credit:Barcroft Media)
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Autumn Leaves, seen in Wakefield this morning. Many parts of the country will see a good deal of dry weather today, with some bright or sunny spells and a few showers here and there. (credit:Michiko Smith /Ross Parry Agency)