The First Day of Winter and longest night of the year, the 2019 Winter Solstice, occurs on Saturday, December 21 at 11:19 p.m. Eastern Standard Time (4:19 p.m. Universal Time), according to NOAA.
The Winter Solstice can occur on December 20, 21, 22, or 23, depending on calendar events such as leap year and when the Solstice begins relative to Coordinated Universal Time.
The Winter Solstice, as pictured in the above NASA image, is caused by a tilt of the earth’s rotating axis and marks the first day of winter. It is the shortest day and longest night of the year in the northern hemisphere for locations like Melbourne, Florida, but the reverse happens in the southern hemisphere for locations such as Melbourne, Australia.
According to NASA, it is not the Sun that is moving north or south through the seasons, but a change in the orientation and angles between the Earth and its nearest star.
The axis of the Earth has tilted 23.5 degrees relative to the Sun and the ecliptic plane. The axis is tilted away from the Sun at the December solstice and toward the Sun at the June solstice, spreading more and less light on each hemisphere. At the equinoxes, the tilt is at a right angle to the Sun and the light is spread evenly.
Although the December Solstice marks the beginning of Northern Winter, it is often called Mid-Winter instead of the First Day of Winter.
The difference between the First Day of Winter and Mid-Winter lies in the definitions created by culture, agriculture, and astronomy.
According to astronomers, December 21st marks the beginning of Winter in the Northern Hemisphere and the beginning of Summer in the Southern Hemisphere.
The official start and end of Winter can vary by country – not because Winter starts a week or so earlier in one country than another in the same hemisphere, but because the recognition of the start of Winter is often influenced by historical or cultural reasons particular to that country.
Most countries recognize Winter as starting on dates ranging in November and ending sometime in March.
Blame the oceans, which heat up and cool down slowly. Although the Winter Solstice marks the lowest exposure of the northern Atlantic and Pacific Oceans to the Sun’s heating radiation, the oceans are still warm in the Northern Hemisphere from the summertime, and that delays the peak heat by about a month and a half.