The 2017 December solstice will come on the December 21 at 16:28 UTC. That’s 10:28 a.m. on December 21, for those in the central time zone in North America. It’s when the sun reaches its southernmost point for the year. This solstice marks the beginning of the winter season in the Northern Hemisphere, and the start of the summer season in the Southern Hemisphere. And, no matter where you are on Earth, it marks the beginning of your shortest season.
By season, we mean the time between a solstice and an equinox, or vice versa. The upcoming season – between the December solstice and March equinox – is a touch shy of 89 days.
Contrast the number of days of the upcoming season with that of the longest season, a Northern Hemisphere summer or Southern Hemisphere winter. The longest season as measured from the June solstice to the September equinox lasts 93.65 days.
Why is the upcoming season nearly 5 days shorter? Every year in early January, the Earth swings closest to the sun for the year. Because Earth is nearest the sun at this time, Earth moves most swiftly in its orbit. That’s why a Northern Hemisphere winter or Southern Hemisphere summer is the shortest of the four seasons.
On the other hand, in early July, Earth is farthest from the sun and moving most slowly in its orbit.
Lengths of the astronomical seasons:
December solstice to March equinox: 88.99 days
March equinox to June solstice: 92.76 days
June solstice to September equinox: 93.65 days
September equinox to December solstice: 89.84 days
According to the computational wizard Jean Meeus, a Northern Hemisphere winter or Southern Hemisphere summer became the shortest season after the year 1246. The astronomical season between the December solstice and the March equinox will reach a minimum value of 88.71 days around the year 3500, and will continue to reign as the shortest season until about 6430.
Bottom line: By season, we mean the time between a solstice and an equinox, or vice versa. Shortest season begins on December 21, 2017. It’s a touch shy of 89 days in length.