When does winter start?
How you define the first day of winter depends on whether you are referring to the astronomical or meteorological winter.
The day in our calendar that marks the first day of winter usually refers to the astronomical seasons which are a result of the Earth's axis and orbit around the sun.
This year, astronomical winter begins on 21 December 2018 and ends on 20 March 2019.
The astronomical calendar determines the seasons due to the 23.5 degree 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.
Astronomical seasons are relative to the position of the Earth's orbit around the sun taking into account equinoxes and solstices.
However, at the Met Office we often use a meteorological definition of the seasons.
The meteorological winter begins on 1 December 2018 and will end on 28 February 2019.
The meteorological seasons consists of splitting the seasons into four periods made up of three months each. These seasons are split to coincide with our Gregorian calendar making it easier for meteorological observing and forecasting to compare seasonal and monthly statistics. By the meteorological calendar, winter always starts on 1 December.