Today is the autumnal equinox - a day when there are exactly 12 hours of dark and 12 hours of light, officially marking the first day of autumn in the Northern Hemisphere.
In the United States, however, it's known as the "fall" season - a word that also originated in Britain, but which has fallen out of favour in this country.
The older of the two words is autumn, which first came into English in the 1300s from the Latin word "autumnus".
It quickly replaced the term "harvest", which had previously been used to describe the season, because it was the time when crops were usually harvested.
However, in poetry and literature, many writers used the phrase "the fall of the leaves" to describe the season. This was shortened to fall in the 1600s.
It was around this time that England's empire was fast expanding, and the English language started to be used in the "New World" - including North America.
As time went on, the English spoken in America and the English spoken in Britain diverged, and this gulf widened even further following the US Declaration of Independence in 1776.
The type of English spoken in the US at the time became part of the country's early national identity - and "fall" became the adopted word to describe the autumn season.
Spring forward, fall back
The phrase "spring forward, fall back" came into use much later, after daylight saving time began to be implemented in the early 1900s.
It is a mnemonic device, to remind us to turn the clocks forwards an hour in the spring, and backwards an hour in the autumn (or fall).
It works in both the US and the UK because, as well as "fall back" referring to the autumn season, it also means to topple backwards - evoking the movement of the clocks.
What is the autumnal equinox?
An equinox occurs when the sun passes directly over the Earth's equator, resulting in a day and night that are equal in length.
There are two equinoxes every year - one around March 22 and one around September 22.
In the southern hemisphere the seasons are flipped, and the September equinox marks the start of spring.
When is the autumnal equinox?
This year, the autumnal equinox takes place on Monday, September 23.
The exact moment of equinox, when the sun is directly over the Earth's equator is 08:50 BST that day.
After this point, the days will get shorter and the nights will get longer until the winter solstice on December 22, when the pattern will reverse.
Why does the equinox happen?
The equinox occurs because of the tilt of the Earth in relation to the sun. This is what causes the seasons.
During the summer time, the Northern hemisphere is tilted towards the sun. So we get longer days as more light falls on this part of the surface.
In the winter time it's the Southern hemisphere that gets the majority of the light.
Equinoxes occur when the axis of rotation of the Earth is exactly parallel to the direction of motion of the Earth around the sun.
This means that day and night length is exactly 12 hours at all points on the Earth's surface.
Equal night
The word equinox is Latin for "equal night".
Meteorologists use it as the official turning point in the seasons because, although it can vary from year to year, it allows for the most accurate record-keeping.
It is celebrated around the world - notably at the Aztec monument Chichen Itza in Mexico.
During the equinox a sliver of light makes its way down the monument at sunset, drawing in crowds of onlookers.
Google doodle
Google is marking the start of the "fall season" with a special doodle, inspired by John Keats' poem "To Autumn", in which he described it as the "season of mists and mellow fruitfulness".
The autumn brings an end to the long days of summer and the beginning of the harvest season.
"To bend with apples the moss’d cottage trees / And fill all fruit with ripeness to the core," Keats wrote.
The doodle appears on Google's homepage throughout most of the Northern hemisphere:
When do the clocks change?
The autumnal equinox should not be confused with the day the clocks change, which also happens around this time of year.
In the UK, the clocks will "fall back" one hour on October 27.