When is autumn 2020? Date and meaning behind the autumn equinox - and why two dates mark the start of the season
The summer solstice took place back in June, marking the start of the summer season.
This means the longest day of the year is now long behind us and the days are rapidly getting shorter.
But is summer officially over and has autumn now begun?
Here’s what you need to know.
When does autumn start this year?
The date that determines when autumn starts depends on which definition you are going with.
The two options are the meteorological and astronomical definitions.
The autumn equinox
This autumn equinox is when the sun is directly above the earth’s equator, and the day and night are of equal length.
This occurs as the sun passes the equator, moving from the northern hemisphere to the southern hemisphere, as the north pole gradually tilts away from the sun.
The first day of autumn this year falls on Tuesday 22 September, and autumn ends on 21 December.
There are two equinoxes that occur every year, the other of which is the Spring Equinox which marks the start of spring and takes place around 20 March.
Do equinoxes fall on the same date each year?
Equinoxes don't always fall on the same day every year, as they can occur up to six hours apart from previous years.
The autumnal equinox usually falls on either 22 or 23 September, while the spring equinox occurs on either 20 or 21 March.
The reason for these discrepancies is due to the time it takes for the earth to travel around the sun. This process takes 365.25 days, yet our calendars measure a full year to the nearest round number, which is 365.
Every four years, we have a leap year in which an extra date is added to our calendar to make up for the missing 0.25 days. The way we measure the years and our use of leap years essentially disrupts the date on which equinoxes are expected to fall.
When does meteorological autumn begin?
On the meteorological calendar, autumn always begins on 1 September and ends on 30 November.
Why is there different definitions for the seasons?
The reason there are two different definitions of each season, is due to how the seasons are measured.
Astronomical seasons are based on the position of earth in relation to the sun and this has been used to mark time for thousands of years due to observable natural and celestial occurrences.
Meteorological seasons on the other hand are calculated by meteorologists and climatologists by using the annual temperature cycle.
They break down the seasons into four groups, each lasting three months.
Meteorological spring consists of March, April, and May, whereas summer includes June, July, and August.
Meteorological autumn includes the months of September, October, and November while winter is December, January, and February.