TODAY is the first day of winter according to the astronomical calendar.

This year, astronomical winter begins on December 22 2019 and ends on March 20 2020.

The Met Office states that the astronomical calendar determines the seasons due to the 23.5 degrees of tilt of the Earth's rotational axis in relation to its orbit around the Sun.

Both Equinoxes and Solstices are related to the Earth's orbit around the Sun. However, the dates of the Equinox and Solstice aren't fixed due to the Earth's elliptical orbit of the Sun.

The Met Office use a meteorological definition of the seasons, meaning, by the meteorological calendar, the first day of winter is always December 1; ending on 28 (or 29 during a Leap Year) February.

Using this method - we are already three weeks into winter.

The winter solstice also marks the shortest day of the year.

Learn more here.

On December 22, people gathered at Stonehenge in Wiltshire to mark the winter solstice, and to witness the sunrise after the longest night of the year.