If you live in the Northern Hemisphere, it's almost time to get your coats out of storage, as autumn will officially begin tonight.
The autumn equinox will take place at 20:21 BST today (September 22) – officially marking the arrival of autumn and colder temperatures for the Northern Hemisphere.
The event occurs in September every year and marks the point when the Sun appears to shine directly over the equator, and daytime and nighttime are nearly equal lengths all around the world.
Along with the two solstices, the two equinoxes of the year mark the changing of seasons as Earth travels around the Sun.
After the autumn equinox, the Northern Hemisphere starts seeing fewer hours of daylight than darkness each day.
There are two equinoxes every year - in September and March - when the Sun shines directly on the equator and the length of day and night is nearly equal. During equinoxes, we experience pretty average temperatures and equal lengths of day and night
'In the Northern hemisphere, the September equinox marks the start of a period bringing us later sunrises and earlier sunsets,' NASA explained.
'We will also feel cooler days with chillier winds, and dry, falling leaves.'
The Earth is tilted on its axis, meaning that as our planet orbits the Sun, the Sun illuminates the northern or southern hemisphere more depending on where the Earth is along its orbit.
However, at two points in the year the Sun will illuminate the northern and southern hemispheres equally. These are known as the equinoxes.
Anna Ross, an astronomer at the Royal Observatory Greenwich, explains: 'The Earth takes one year (or 365-and-a-quarter days) to orbit once around the Sun and it is tilted over slightly on its axis.
'So for half of the year, the Northern Hemisphere is tilted slightly towards the sun, meaning we have longer daylight hours and hotter temperatures.
NASA graphic explains the difference between the solstice and the equinox. The solstices take place in June and December; the equinoxes take place in March and September
At an equinox, the Earth's terminator - the dividing line between day and night - becomes vertical and connects the north and south poles. Pictured, September equinox with the terminator line being vertical
'For the other half of the year, we are tilted slightly further away from the Sun so we have longer nights and colder weather. It's this tilt that gives us our seasons.
'There are two points in the year where the Earth's tilt is at its most extreme relative to the Sun.'
These two extreme tilts happen each year – one in March called the vernal equinox, and one in September (heralding autumn