Winter
Astronomical and other calendar-based reckoning

In the Northern Hemisphere, some authorities define the period of winter based on astronomical fixed points (i.e., based solely on the position of the Earth in its orbit around the Sun), regardless of weather conditions. In one version of this definition, winter begins at the winter solstice and ends at the March equinox. These dates are somewhat later than those used to define the beginning and end of the meteorological winter — usually considered to span the entirety of December, January, and February in the Northern Hemisphere and June, July, and August in the Southern.

Astronomically, the winter solstice — being the day of the year that has fewest hours of daylight — ought to be in the middle of the season, but seasonal lag means that the coldest period normally follows the solstice by a few weeks. In some cultures, the season is regarded as beginning at the solstice and ending on the following equinox. In the Northern Hemisphere, depending on the year, this corresponds to the period between 20, 21 or 22 December and 19, 20 or 21 March.

In an old Norwegian tradition, winter begins on 14 October and ends on the last day of February.

In many countries in the Southern Hemisphere, including Australia, New Zealand, and South Africa, winter begins on 1 June and ends on 31 August.

In Celtic nations such as Ireland (using the Irish calendar) and in Scandinavia, the winter solstice is traditionally considered as midwinter, with the winter season beginning 1 November, on All Hallows, or Samhain. Winter ends and spring begins on Imbolc, or Candlemas, which is 1 or 2 February. In Chinese astronomy and other East Asian calendars, winter is taken to commence on or around 7 November, on Lìdōng, and end with the arrival of spring on 3 or 4 February, on Lìchūn. Late Roman Republic scholar Marcus Terentius Varro defined winter as lasting from the fourth day before the Ides of November (10 November) to the eighth day before the Ides of Februarius (6 February).

This system of seasons is based on the length of days exclusively. The three-month period of the shortest days and weakest solar radiation occurs during November, December and January in the Northern Hemisphere and May, June and July in the Southern Hemisphere.

Many mainland European countries tended to recognize Martinmas or St. Martin's Day (11 November) as the first calendar day of winter. The day falls at the midpoint between the old Julian equinox and solstice dates. Also, Valentine's Day (14 February) is recognized by some countries as heralding the first rites of spring, such as flowers blooming.

The three-month period associated with the coldest average temperatures typically begins somewhere in late November or early December in the Northern Hemisphere and lasts through late February or early March. This "thermological winter" is earlier than the solstice delimited definition, but later than the daylight (Celtic or Chinese) definition. Depending on seasonal lag, this period will vary between climatic regions.

Since by almost all definitions valid for the Northern Hemisphere, winter spans 31 December and 1 January, the season is split across years, just like summer in the Southern Hemisphere. Each calendar year includes parts of two winters. This causes ambiguity in associating a winter with a particular year, e.g. "Winter 2018". Solutions for this problem include naming both years, e.g. "Winter 18/19", or settling on the year the season starts in or on the year most of its days belong to, which is the later year for most definitions.
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