Seasons are caused by the tilt in the Earth's axis.
Winter Solstice is the time of the longest night and shortest day.
Many ancient monuments are lined up with the solstice, which suggests it was an important moment of the year for many cultures.
One is Maeshowe on Orkney.
Others include Stonehenge and Newgrange.
Winter solstice sunset at Stonehenge in the mid-1980s. Image via Wikimedia Commons.