You might have felt over the last three weeks that autumn has already begun and you would be partially correct to believe that.
By the meteorological and calendar standards, summer weather officially waved goodbye to us on September 1.
But an astronomer at Royal Observatory Greenwich told Express.co.uk there is a second date on which the first day of autumn falls.
The date is based on the Earth’s orbit around the Sun and the position of the Sun in the sky.
When is the Autumn Equinox this year?
This year autumn officially takes over the reign from summer early tomorrow – September 23, just before 3am UK time.
The equinox, sometimes referred to as the Vernal Equinox, will officially mark the start of the astronomical autumn.
Every year the equinox falls on one of three days towards the end of September.
Tom Kerss, astronomer at the Royal Observatory Greenwich, told Express.co.uk: “Every year, the autumn equinox falls between the September 22 and 24, but the time and date varies because the Earth’s orbit takes 365.25 days and our calendar is rounded down to 365 – with a corrective extra day every four years.”
What is the Autumn Equinox?
According to Mr Kerss, the Autumn Equinox occurs when the Sun passes over the Earth’s equator.
This only happens twice a year – once during the Spring Equinox and once during the Autumn Equinox.
The astronomer said: “This year’s autumnal, or vernal, equinox falls on September 23 in the early hours of the morning.
“At 1.54am GMT (2.54am BST) the Earth will reach a point in its orbit at which the axis of its rotation – the line connecting the poles – does not point preferentially towards or away from the Sun.
“At this moment neither hemisphere is ‘leaning’ towards the Sun, but after the 23rd, the Northern Hemisphere will begin to point away from the Sun, bringing ever longer nights and shorter days.
“Astronomers consider this moment the beginning of autumn in the Northern Hemisphere, and thus the beginning of spring in the Southern Hemisphere.”
What is the difference between an equinox and a solstice?
Each year there are two equinoxes and two solstices – each tied to one of the four seasons.
These are the Summer Solstice, Winter Solstice, the Spring Equinox and the Autumn Equinox.
Mr Kerss explained: “The next milestone in the Earth’s orbit occurs this year on December 21, a date known in the Northern Hemisphere as the Winter Solstice.
“The solstices mark the beginning of summer and winter and occur when one of the Earth’s poles is as close to the Sun as it can be, and the other as far as it can be.
“Therefore, we have two solstices and two equinoxes every year, each separated by about three months.
“They signal the start of astronomical seasons, although our meteorological sense of when the seasons begin varies according to the climate and weather.”