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A season is a division of the year based on changes in weather, ecology, and the number of daylight hours in a given region. On Earth, seasons are the result of the axial parallelism of Earth's axial tilt around the Sun. (2025). 9780787693329, Thomson-Gale. ISBN 9780787693329 In temperate and polar regions, the seasons are marked by changes in the intensity of sunlight that reaches the Earth's surface, variations of which may cause animals to undergo hibernation or to migrate, and plants to be dormant. Various cultures define the number and nature of seasons based on regional variations, and as such there are a number of both modern and historical definitions of the seasons.
The Northern Hemisphere experiences most direct sunlight during May, June, and July (thus the traditional celebration of Midsummer in June), as the hemisphere faces the Sun. For the Southern Hemisphere it is instead in November, December, and January. It is Earth's axial tilt that causes the Sun to be higher in the sky during the summer , which increases the solar flux. Because of seasonal lag, June, July, and August are the warmest months in the Northern Hemisphere while December, January, and February are the warmest months in the Southern Hemisphere.
In temperate and Subpolar climate regions, four seasons based on the Gregorian calendar are generally recognized: spring, summer, autumn ( fall), and winter. Ecologists often use a six-season model for temperate climate regions which are not tied to any fixed calendar dates: prevernal, vernal, estival, serotinal, autumnal, and hibernal. Many tropical regions have two seasons: the wet season/ wet season/ monsoon season and the dry season. Some have a third cool, mild, or harmattan season. "Seasons" can also be dictated by the timing of important ecological events such as hurricane season, tornado season, and wildfire season. Some examples of historical importance are the ancient Egyptian seasons— flood, growth, and Shemu—which were previously defined by the former annual flooding of the Nile in Egypt.
Seasons often hold special significance for agrarian societies, whose lives revolve around planting and harvest times, and the change of seasons is often attended by ritual. The definition of seasons is also cultural. In India, from ancient times to the present day, six seasons or Ritu based on south Asian religious or cultural calendars are recognised and identified for purposes such as agriculture and trade.
Regardless of the time of year, the northern and southern hemispheres always experience opposite seasons. This is because during summer or winter, one part of the planet is more directly exposed to the rays of the Sun than the other, and this exposure alternates as the Earth revolves in its orbit.
For approximately half of the year (from around March20 to around September22), the Northern Hemisphere tips toward the Sun, with the maximum amount occurring on about June21. For the other half of the year, the same happens, but in the Southern Hemisphere instead of the Northern, with the maximum around December21. The two instants when the Sun is directly overhead at the Equator are the . Also at that moment, both the North Pole and the South Pole of the Earth are just on the terminator, and hence day and night are equally divided between the two hemispheres. Around the March equinox, the Northern Hemisphere will be experiencing spring as the hours of daylight increase, and the Southern Hemisphere is experiencing autumn as daylight hours shorten.
The effect of axial tilt is observable as the change in day length and the altitude of the Sun at solar noon (the Sun's culmination) during the year. The low angle of the Sun during the winter months means that incoming rays of solar radiation are spread over a larger area of the Earth's surface, so the light received is more indirect and of lower intensity. Between this effect and the shorter daylight hours, the axial tilt of the Earth accounts for most of the seasonal variation in climate in both hemispheres.
Orbital eccentricity can influence temperatures, but on Earth, this effect is small and is more than counteracted by other factors; research shows that the Earth as a whole is actually slightly warmer when farther from the Sun. This is because the Northern Hemisphere has more land than the Southern, and land warms more readily than sea. Any noticeable intensification of southern winters and summers due to Earth's elliptical orbit is mitigated by the abundance of water in the Southern Hemisphere.
In the temperate and polar regions, seasons are marked by changes in the amount of sunlight, which in turn often causes cycles of dormancy in plants and hibernation in animals. These effects vary with latitude and with proximity to bodies of water. For example, the South Pole is in the middle of the continent of Antarctica and therefore a considerable distance from the moderating influence of the southern oceans. The North Pole is in the Arctic Ocean, and thus its temperature extremes are buffered by the water. The result is that the South Pole is consistently colder during the southern winter than the North Pole during the northern winter.
The seasonal cycle in the polar and temperate zones of one hemisphere is opposite to that of the other. When it is summer in the Northern Hemisphere, it is winter in the Southern, and vice versa.
In Astronomy reckoning by hours of daylight alone, the solstices and are in the middle of the respective seasons. Because of seasonal lag due to thermal absorption and release by the oceans, regions with a continental climate, which predominate in the Northern Hemisphere, often consider these four dates to be the start of the seasons as in the diagram, with the cross-quarter days considered seasonal midpoints. The length of these seasons is not uniform because of Earth's elliptical orbit and its different speeds along that orbit." Astronomy Answers AstronomyAnswerBook: Seasons ," Astronomical Institute, Utrecht University, downloaded 1 August 2008
The four seasons have been in use since at least Roman times, as in Rerum rusticarum of Varro De Re Rustica 28. Latin original . Varro says that spring, summer, autumn, and winter start on the 23rd day of the sun's passage through Aquarius, Taurus, Leo, and Scorpio, respectively. Nine years before he wrote, Julius Caesar had reformed the calendar, so Varro was able to assign the dates of February 7, May 9, August 11, and November 10 to the start of spring, summer, autumn, and winter.
| + Meteorological temperate seasons |
| 31 May |
| 31 August |
| 30 November |
| 28 February (29th if leap year)[5], UK Met Office |
In Sweden and Finland, meteorologists and news outlets use the concept of thermal seasons, which are defined based on mean daily temperatures.
target="_blank" rel="nofollow"> The onsets of the thermal seasons, Finnish Meteorological Institute The beginning of spring is defined as when the mean daily temperature permanently rises above 0 °C. The beginning of summer is defined as when the temperature permanently rises above +10 °C, autumn as when the temperature permanently falls below +10 °C, and winter as when the temperature permanently falls below 0 °C. In Finland, "permanently" is defined as when the mean daily averaged temperature remains above or below the defined limit for seven consecutive days. (In Sweden the number of days ranges from 5 to 7 depending on the season.) This implies two things:
The India Meteorological Department (IMD) designates four climatological seasons:
In China, a common temperature-based reckoning holds that it is winter for the period when temperatures are below 10°C on average and summer for the period when temperatures are above 22°C on average. This means that areas with relatively extreme climates (such as the Paracel Islands and parts of the Tibetan plateau) may be said to have summer all year round or winter all year round.
Pliny the Elder, in his Natural History, mentions the two equinoxes and the two solstices and gives the lengths of the intervals (values which were fairly correct in his day but are no longer very correct because the perihelion has moved from December into January). He then defines the seasons of autumn, winter, spring, and summer as starting half-way through these intervals. 18:59 (paragraphs 220-2 in this Latin edition ) He gives "the eighth day to the Kalends of January" (December 25) as the date of the winter solstice, though actually it occurred on the 22nd or 23rd at that time.In a space of four years, the solstice occurs latest in the Julian Calendar in the year before a leap year. In 2019, it occurred on the 22nd in the Gregorian Calendar, or December 9 in the Julian, at 4:19 AM, according to Earth's Seasons Equinoxes, Solstices, Perihelion, and Aphelion . The number of days between successive winter solstices varied from 365.242883 to 365.242740 between the year 1 BC and AD 2000, according to . Therefore, the average value over the last 2000 years has been 365.24281 days, 0.00719 days less than an average Julian year. This means the solstice was 2000×0.00719=14.38 days later, that is, on December 23 in the middle of the day. A hundred years earlier it would have been on the 24th.
At the present time, the astronomical timing has winter starting at the winter solstice, spring at the spring equinox, and so on. This is used worldwide, although some countries like Australia, New Zealand, Pakistan and Russia prefer to use meteorological reckoning. The precise timing of the seasons is determined by the exact times of the Sun reaching the tropics of Cancer and Capricorn for the and the times of the Sun's transit over the equator for the , or a traditional date close to these times.
The following diagram shows the relation between the line of solstice and the line of apsides of Earth's elliptical orbit. The orbital ellipse (with eccentricity exaggerated for effect) goes through each of the six Earth images, which are sequentially the perihelion (periapsis—nearest point to the Sun) on anywhere from 2 January to 5 January, the point of March equinox on 19, 20 or 21 March, the point of June solstice on 20 or 21 June, the aphelion (apoapsis—farthest point from the Sun) on anywhere from 3 July to 6 July, the September equinox on 22 or 23 September, and the December solstice on 21 or 22 December.
These "astronomical" seasons are not of equal length, because of the ellipse of the orbit of the Earth, as discovered by Johannes Kepler. From the March equinox it currently takes 92.75 days until the June solstice, then 93.65 days until the September equinox, 89.85 days until the December solstice and finally 88.99 days until the March equinox. Thus the time from the March equinox to the September equinox is 7.56 days longer than from the September equinox to the March equinox.
The calendar equinox (used in the calculation of Easter) is 21 March, the same date as in the Easter tables current at the time of the Council of Nicaea in AD 325. The calendar is therefore framed to prevent the astronomical equinox wandering onto 22 March. From Nicaea to the date of the reform, the years 500, 600, 700, 900, 1000, 1100, 1300, 1400, and 1500, which would not have been leap years in the Gregorian calendar, amount to nine extra days, but astronomers directed that ten days be removed. Because of this, the (proleptic) Gregorian calendar agrees with the Julian calendar in the third century of the Christian era, rather than in the fourth.
Currently, the most common equinox and solstice dates are March 20, June 21, September 22 or 23, and December 21; the four-year average slowly shifts to earlier times as a century progresses. This shift is a full day in about 128 years (compensated mainly by the century "leap year" rules of the Gregorian calendar); as 2000 was a leap year, the current shift has been progressing since the beginning of the last century, when equinoxes and solstices were relatively late. This also means that in many years of the twentieth century, the dates March 21, June 22, September 23, and December 22 were much more common, so older books teach (and older people may still remember) these dates.
All the times are given in UTC (roughly speaking, the time at Greenwich, ignoring British Summer Time). People living farther to the east (Asia and Australia), whose local times are in advance, see the astronomical seasons apparently start later; for example, in Tonga (UTC+13), an equinox occurred on September 24, 1999, a date on which the equinox will not fall again until 2103. On the other hand, people living far to the west (America), whose clocks run behind UTC, may experience an equinox as early as March 19.
Smaller irregularities in the times are caused by perturbations of the Moon and the other planets.
The solar seasons change at the cross-quarter days, which are about 3–4 weeks earlier than the meteorological seasons and 6–7 weeks earlier than seasons starting at equinoxes and solstices. Thus, the day of greatest insolation is designated "midsummer" as noted in William Shakespeare's play A Midsummer Night's Dream, which is set on the summer solstice. On the Celtic calendar, the start of the seasons corresponds to four Pagan agricultural festivals – the traditional first day of winter is 1 November (Samhain, the Celtic origin of Halloween); spring starts 1 February (Celtic Imbolc); summer begins 1 May (Beltane, the Celtic origin of May Day); the first day of autumn is 1 August (Celtic Lughnasadh).
| +Irish seasons |
| 31 January |
| 30 April |
| 31 July |
| 31 October (Hallowe'en) |
In the Hindu calendar of tropical and subtropical India, there are six seasons or Ritu that are calendar-based in the sense of having fixed dates: Vasanta (spring), Grishma (summer), Varsha (monsoon), Sharada (autumn), Hemanta (early winter), and Shishira (prevernal or late winter). The six seasons are ascribed to two months each of the twelve months in the Hindu calendar. The rough correspondences are:
| Vasanta | Mid-March | Mid-May | Chaitra, Vaishakha | spring |
| Grishma | Mid-May | Mid-July | Jyeshtha, Ashadha | summer |
| Varshā | Mid-July | Mid-September | Shravana, Bhadrapada | monsoon |
| Sharada | Mid-September | Mid-November | Ashvin, Kartika | autumn |
| Hemanta | Mid-November | Mid-January | Agrahayana, Pausha | early winter or late autumn |
| Shishira | Mid-January | Mid-March | Magha, Phalguna | prevernal or late winter |
The Bengali Calendar is similar but differs in start and end times. It has the following seasons or ritu:
| Grīshmo (গ্রীষ্ম)(summer) | Mid-April | Mid-June | Boishakh, Joishtho | Summer |
| Bôrsha (বর্ষা) (monsoon) | Mid-June | Mid-August | Asharh, Srabon | Monsoon |
| Shôrôt (শরৎ) (autumn/ fall) | Mid-August | Mid-October | Bhadro, Ashwin | Autumn |
| Hemônto (হেমন্ত) (frost/ late autumn) | Mid-October | Mid-December | Kartik, Ogrohayon | Dry season |
| Shīto (শীত) (winter) | Mid-December | Mid-February | Poush, Magh | Winter |
| Bôsônto বসন্ত(spring) | Mid-February | Mid-April | Falgun, Choitro | Spring |
| April–June |
| June–August |
| August–October |
| October–December |
| December–February |
| February–April |
The Tamil calendar follows a similar pattern of six seasons
| MuthuVenil (summer) | April 15 to June 14 | Chithirai and Vaikasi |
| Kaar (monsoon) | June 15 to August 14 | Aani and Aadi |
| Kulir (autumn) | August 15 to October 14 | Avani and Purattasi |
| MunPani (winter) | October 15 to December 14 | Aipasi and Karthikai |
| PinPani (prevernal) | December 15 to February 14 | Margazhi and Thai |
| IlaVenil (spring) | February 15 to April 14 | Maasi and Panguni |
The North American Cree and possibly other Algonquian speaking peoples used or still use a 6-season system. The extra two seasons denoting the freezing and breaking up of the ice on rivers and lakes.
| Pipon | Jan/Feb | Winter |
| Sekwun | Mar/Apr | Break-up |
| Mithoskumin | May/Jun | Spring |
| Nepin | Jul/Aug | Summer |
| Tukwakin | Sep/Oct | Autumn |
| Mikiskaw | Nov/Dec | Freeze-up |
The Noongar people of South-West Western Australia recognise maar-keyen bonar, or six seasons. Each season's arrival is heralded not by a calendar date, but by environmental factors such as changing winds, flowering plants, temperature and migration patterns and lasts approximately two standard calendar months. The seasons also correlate to aspects of the human condition, intrinsically linking the lives of the people to the world that surrounds them and also dictating their movements, as with each season, various parts of country would be visited which were particularly abundant or safe from the elements.
| Birak (first summer) | December to January | Season of the young |
| Bunuru (second summer) | February to March | Season of adolescence |
| Djeran (autumn) | April to May | Season of adulthood |
| Makuru (the first rains) | June to July | Fertility season |
| Djilba (the second rains) | August to September | Season of conception |
| Kambarang (wildflower season) | October to November | Season of birth |
Floral and animal activity variation near the equator depends more on wet/dry cycles than seasonal temperature variations, with different species flowering (or emerging from cocoons) at specific times before, during, or after the monsoon season. Thus, the tropics are characterized by numerous "mini-seasons" within the larger seasonal blocks of time.
In the tropical parts of Australia in the northern parts of Queensland, Western Australia, and the Northern Territory, wet and dry seasons are observed in addition to or in place of temperate season names.
| +Meteorological Tropical seasons |
| 30 April |
| 31 October |
| Ruedu nao (cold season) | mid October to mid February |
| Ruedu ron (hot season) | mid February to mid May |
| Ruedu fon (rainy season) | mid May to mid October |
For example, at the military and weather station Alert located at 82°30′05″N and 62°20′20″W, on the northern tip of Ellesmere Island, Canada (about 450 or 830 km from the North Pole), the Sun begins to peek above the horizon for minutes per day at the end of February and each day it climbs higher and stays up longer; by 21 March, the Sun is up for over 12 hours. On 6 April the Sun is perceived as rising at 0522 UTC and remains above the horizon until it sets below the horizon again on 6 September at 0335 UTC. By October 13 the Sun is above the horizon for only 1 hour 30 minutes, and on October 14 it does not rise above the horizon at all and remains below the horizon until it rises again on 27 February.
First light comes in late January because the sky has twilight, being a glow on the horizon, for increasing hours each day, for more than a month before the Sun first appears with its disc above the horizon. From mid-November to mid-January, there is no twilight.
In the weeks surrounding 21 June, in the northern polar region, the Sun is at its highest elevation, appearing to circle the sky there without going below the horizon. Eventually, it does go below the horizon, for progressively longer periods each day until around the middle of October, when it disappears for the last time until the following February. For a few more weeks, "day" is marked by decreasing periods of twilight. Eventually, from mid-November to mid-January, there is no twilight and it is continuously dark. In mid-January the first faint wash of twilight briefly touches the horizon (for just minutes per day), and then twilight increases in duration with increasing brightness each day until sunrise at end of February, then on 6 April the Sun remains above the horizon until mid-October.
For navies, the presence of accessible ports and bases can allow naval operations during certain (variable) seasons of the year. The availability of ice-free harbor or can make navies much more effective. Thus Russia, historically navally constrained when confined to using Arkhangelsk (before the 18th century) and even Kronstadt, has particular interests in maintaining and in preserving access to Baltiysk, Vladivostok, and Sevastopol. Storm seasons or polar winter-weather conditions can inhibit surface warships at sea.
Pre-modern armies, especially in Europe, tended to campaign in the summer months - peasant conscription tended to melt away at harvest time, nor did it make economic sense in an agricultural society to neglect the sowing season. Any modern war of manoeuvre profits from firm ground – summer can provide dry conditions suitable for marching and transport, frozen snow in winter can also offer a reliable surface for a period, but rasputitsa can inhibit campaigning. Rainy-season floods may make rivers temporarily impassable, and winter snow tends to block mountain passes. Taliban offensives are usually confined to the Afghanistan fighting season.
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