Winter Is The Shortest Season Of The Year
A year is not an even number of days, and neither are the seasons. To try to achieve a value as close as possible to the exact length of the year, our Gregorian Calendar was constructed to give a close approximation to the tropical year, which is the actual length of time it takes for Earth to complete one orbit around the sun. It eliminates leap days in century years not evenly divisible by 400, such 1700, 1800 and 2100, and millennium years that are divisible by 4000, such as 8000 and 12000.
Another reason is that Earth’s elliptical orbit is changing its orientation relative to the sun (it skews), which causes Earth’s axis to constantly point in a different direction, called procession. Since the seasons are defined as beginning at strict 90-degree intervals, these positional changes affect the time Earth reaches each 90-degree location in its orbit around the sun.
The pull of gravity from the other planets also affects the location of Earth in its orbit.
The current seasonal start dates for 2017 in the Northern Hemisphere are:
Spring: Mar. 20, 6:29 A.M. EDT
Summer: June 21, 12:24 A.M. EDT
Autumn: Sept. 22, 4:02 P.M. EDT
Winter: Dec. 21, 11:28 A.M. EDT
The warm seasons of spring and summer combined are 7.573 days longer than the colder seasons, fall and winter (good news for warm weather admirers). However, spring is currently being reduced by approximately one minute per year and winter by about half a minute per year. Summer is gaining the minute lost from spring, and autumn is gaining the half-minute lost from winter. Winter is the shortest astronomical season, and with its seasonal duration continuing to decrease, it is expected to attain its minimum value — 88.71 days — by about the year 3500.