Autumn is almost upon us with the days growing shorter by the minute and lush orange hues sweeping across the countryside.
Meteological autumn started earlier this month but the days will only start to get shorter after the autumn equinox later this week (there are technically two starts to autumn).
Not only that but the clocks will be going back soon too – throwing us back into darkness on the long trudge home from work, school, university, yoga class [delete where necessary].
Why we start autumn twice
Yes, our Gregorian calendar is based on lunar movements and doesn’t take into account the Earth’s rotational axis in relation to its orbit around the sun.
This means we need both a meteorological autumn based on the calendar months and an astronomical autumn based on the Earth’s orbit and proximity to the Sun.
Meteorological autumn helps us compare seasonal and monthly statistics as well as forecasting while astronomical autumn is perfect for determining when temperatures are going to start getting colder.
When is the start of astronomical autumn?
Astronomical seasons meanwhile start on a different dates each year and totally vary in length, depending on the Earth’s 23.5 degree tilt on its rotational axis as well as its orbit around the Sun.
Spring and autumn equinoxes (equal day and night) and the summer (longest day) and winter (longest night) solstices mark the start of the astronomical seasons each year. The word ‘equinox’ itself actually mean ‘equal’ (equi) and ‘night’ (nox).
The dates of these change each year as they are dependent on the Earth’s position. The Earth is closest to the Sun, known as perihelion, in early January and furthest away, aphelion, in early July.
In 2016 the astronomical autumn will start on September 22 and end on December 21 with the winter solstice.
When is the start of meteorological autumn then?
The meteorological calendar splits the seasons into four equal periods, with spring, summer, autumn and winter all lasting exactly three months of the Gregorian calendar.
This means autumn on the meteorological calendar falls on the same date every year – September 1 – lasting through October and November and ending on November 30.
Meanwhile, winter starts on December 1 (December, January, February), spring starts on March 1 (March, April, May) and summer starts on June 1 each year (June, July, August).
So which autumn definition is the better measure?
While the astronomical seasons might give a better idea of when temperatures will get colder, the meteorological calendar is a lot easier to use when comparing one season to the last.
This is due to the seasons being equal length each year compared to the wildly varying season lengths with the astronomical calendar.
The Met Office, for example, and other forecasters around the world use the meteorological definition of seasons as this ‘determines a clear transition between the seasons’.
Do the clocks change due to the equinoxes?
The clocks are going back one hour on October 30 this year and in 2017 they will change on October 29 – but this is not tied in with the equinox.
Clocks going forward one hour to British Summer Time (GMT+1) was introduced by the Government during World War One to save fuel.
The date changes each year because it is always the last Sunday in March on the Gregorian calendar rather changing in line with the date of the spring equinox.