It is the first official day of autumn on Monday 23 September. It's known as the autumn equinox.
Equinoxes and solstices are key dates in the calendar.
They are used to define the transitional periods between the seasons, so when winter changes to spring, summer to autumn, and so on.
Equinoxes and solstices mark key moments in the Earth's journey around the Sun.
That's because the position of the Sun in relation to the Earth affects the length of days and nights.
This year, the autumn equinox takes place on 23 September.
An equinox happens when the amount of daytime is pretty much equal to the amount of night-time.
It marks a transition between seasons - so, the spring equinox marks the start of spring, while the autumn equinox marks the start of autumn.
There are two equinox each year, which take place when the Sun is positioned exactly above the equator - once around 20/21 March (spring) and again around 22/23 September (autumn).
The word equinox comes from two Latin words - equi which means equal and nox meaning night.
From the day of the spring equinox, the day is longer than the night.
From the day of the autumn equinox, the night becomes longer than the day.
However, an equinox occurs when day and night are almost completely equal in length. The moment when they are completely equal actually occurs a few days before the spring equinox, and a few days after the autumn equinox. This is called the equilux.
The solstice also occurs twice a year.
In the northern hemisphere, there is a summer solstice around 21 June and a winter solstice around the 21 December.
The summer solstice is the day when the northern hemisphere experiences its longest period of daylight all year. The winter solstice is the day when the northern hemisphere experiences its longest period of night all year.
During the summer solstice, in the middle of the day, the Sun is at its highest point in the sky that it will be all year.
During the winter solstice, this is reversed and the noon sun is at the lowest point it will be all year.
Because the Northern Hemisphere is tilted towards the Sun, some far northern countries such as Iceland and Norway experience continuous daylight for months on end!