Seasons divide the year into different periods, which are delimited by astronomical data - including calendar - or by characteristic climatic properties. In everyday parlance, this mainly refers to annual periods that can be clearly distinguished from one another in meteorological terms. In the temperate latitudes these are the four seasons of spring , summer , autumn and winter ; in the tropics there are rainy seasons and dry seasons .
The description of the seasons in this article refers to the northern hemisphere of the earth , in the southern hemisphere they are offset by six months according to the calendar. Summer and winter can each be understood as halves of a year, for example as the north summer half year and the south winter half year .
Different peoples distinguish other times of the year. The Sami in Scandinavia know eight seasons, Australian Aborigines in Arnhem Land know six seasons. In Russia the Rasputitsa is known as the mud season, during the snowmelt in spring and during the autumn rains.
Origin of seasons
Over the course of a year, the intensity, duration and angle of the incidence of sunlight change in a specific geographic location . These changes are repeated as seasonal fluctuations after the earth has orbited the sun . The position of the earth's axis of rotation relative to the plane of its orbit is decisive for the seasons that can be experienced on earth .
As with a top, the earth's axis maintains its orientation in space and is (almost) fixed in space at a certain angle to the ecliptic plane because of the conservation of angular momentum . This inclination of the earth's axis is not right-angled, but is (currently) about 66.6 °, so that the equatorial plane is inclined by about 23.4 ° (23 ° 26 ') compared to the orbit plane. That is why the angle of incidence of sunlight ( sun height at noon) changes during the course of the year during an earth orbit around the sun . In addition, the rotation of the earth around itself also changes the duration of daylight ( light day ) as the length of the day , in the regions remote from the poles outside the polar circles , from one revolution to the next. The longer and the steeper the sunlight hits the surface, the more this region can be warmed.
Change of angle of incidence and duration of irradiation
For the development of the seasons, the decisive factor is how much the respective share of the sun's radiant power that a specific geographic region receives fluctuates over the course of the year. The irradiance related to the relief of the surface depends on the angle of incidence and the duration of the irradiation. The angle of incidence reaches its daily maximum at noon and this solar height at noon fluctuates by ± 23.4 ° for all locations outside the polar regions during the year, with the average elevation angle becoming flatter towards the poles. The possible daily exposure time, the clear day, on the other hand, is on average the same length, but the annual fluctuation range of the day length increases with increasing geographical latitude . Since both factors, angle and duration of solar radiation, are related over the day arc - the highest position of the sun and the longest day coincide - and their fluctuations add up, the formation of the seasons depends primarily on the geographical latitude of a region.
After alone latitude can therefore be related to the incident angle of sunlight solar climates differ. The tropics close to the equator are demarcated as tropical zones between the tropics (23.4 ° latitude) and the ectropic zones - which then include the subtropics or mid-latitudes and (from around 66.6 ° latitude) the polar zones - with increasing distance from the equator pronounced seasonal differences.
During the time between the equinoxes of day and night in March and September, the northern hemisphere is more inclined towards the sun, so that the sun goes through a high arc for an observer located there. When the sun is high, the solar radiation hits the earth's surface steeply and thus provides a relatively high energy input per area. Furthermore, the greater part of the daily apparent path of the sun around the earth is an arc of the day above the horizon, so that the days are long and there is a lot of time for the input of energy. The increased energy input causes the northern hemisphere (northern hemisphere) to warm up during this period.
If the earth is at the opposite point of its orbit six months later, the northern hemisphere is inclined towards the sun - due to the relatively fixed position of the earth's axis, apart from precession and nutation . For an observer in the northern hemisphere, the result is a low daily solar path. If the sun is low, the solar radiation hits the earth's surface more flatly, so that it is distributed over a larger area and brings in less energy. In addition, only the smaller part of the daily solar path lies above the horizon, so that the energy input can only take place for a short period of time. The result is a cooling of the northern hemisphere.
Warming up and cooling down are first shown in the air temperatures (see figure); Because of the thermal inertia, the ground temperatures follow the high and low levels of the sun with a certain delay. The differences in the daily arc of the course of the sun increase with greater geographical latitude and have increasingly stronger effects (up to the polar night ); towards the equator, the seasonal fluctuations are lower.
Conditions in the southern hemisphere
In the tropical and subtropical areas on both sides of the Earth's equator , seasonal changes are less pronounced; instead, rainy and dry seasons occur. During the course of the year, two rainy seasons can be distinguished in the equatorial tropics. With increasing geographical latitude, they merge and thus become two peaks of a single rainy season, which differ depending on the hemisphere.
Change in distance to the sun
Although the earth moves on an orbit around the sun that is elliptical and not circular, so that the distance to the sun varies, the differences that result only affect the light intensity and are not great because of the low orbital eccentricity of the earth's orbit . The earth passes the point of orbit furthest from the sun in the first week of July, in summer in the northern hemisphere. The distance of the earth from the sun, which changes slightly over the course of the year due to the eccentric orbit, is therefore not the cause of the seasons. The change between perihelion and apple passage only makes the southern winters somewhat more severe and the northern winters somewhat milder (shorter and closer to the sun) than they would be with a circular earth orbit. Under the current circumstances, the earth is at its closest point to the sun ( perihelion ) in the northern winter - around January 3 at a distance of around 147.1 million km; in the southern winter it is furthest from the sun ( aphelion ) - around 5 July at a distance of around 152.1 million km. The reason for the seasons on earth is - as already explained above - in the angle and the duration of solar radiation. For Central Europe, the extremes of the angle in summer are 60 ° to 65 ° and the possible sunshine duration in Central Germany is 16-17 hours, in winter it is 7-8 hours or angles of only 13 ° to 18 °.
Shift of the seasons
Due to the gravitational effect mainly of the moon and sun on the rotating earth body, the earth's axis performs a precession movement , so that the position of the reference points for solstices and equinoxes gradually shifts and moves backwards ( retrograde ) around the earth's orbit in about 26,000 years ( cycle of precession ). The tropical year related to the vernal equinox is therefore around 20 minutes shorter than a complete orbit of the earth around the sun related to the fixed star background , a sidereal year . The so-called civil year of the calendar calendar is based on the length of the tropical year . The tropical year length is approximated to the calendar year in the Gregorian calendar by inserting leap days , which results in typical shifts for calendar information of the beginning of the seasons, for example for the beginning of autumn .
In addition, as a result of orbital disturbances from other planets, the apsidal line (straight through aphelion and perihelion) turns once every 111,000 years ( prograd ). Because of these opposing movements, the perihelion runs through all the seasons once every 21,000 years. In about ten thousand years the orbit point closest to the sun will coincide with the northern summer solstice. The winter seasons of the northern hemisphere will then take place longer and further away from the sun than today. In return, the southern hemisphere will have shorter winters closer to the sun.
Astronomically, the seasons are determined according to the apparent geocentric ecliptical longitude of the sun. Taking into account aberration and nutation , the apparent annual orbit of the sun is viewed from a hypothetical observation point in the center of the earth and divided into four sections. Each of the orbit sections is bounded by an equinox (from equinox, at 0 ° or at 180 °) and a solstitial point (from solstice, at 90 ° or at 270 °).
The astronomical seasons are defined as the time spans that pass while passing through a certain of the four sections and do not last the same length because of the different angular speeds. Due to the geocentric definition based on the center of the earth, an astronomical season begins or ends at the same point in time, regardless of location, worldwide (but with different times in different time zones).
- Astronomical spring begins when the sun's apparent geocentric longitude is 0 °. This is the time of the equinox of spring (primary equinox). Except for a few seconds, it coincides with the point in time at which the sun crosses the celestial equator from south to north.
- The astronomical summer begins when the sun's apparent geocentric longitude is 90 °. This is the time of the summer solstice . With the exception of a few minutes, it coincides with the point in time at which the sun reaches its greatest northern declination and thus its northernmost position on the celestial sphere.
- Astronomical autumn begins when the sun's apparent geocentric longitude is 180 °. This is the time of the equinox of autumn (secondary equinox). Except for a few seconds, it coincides with the point in time at which the sun crosses the celestial equator from north to south.
- The astronomical winter begins when the sun's apparent geocentric longitude is 270 °. This is the time of the winter solstice. With the exception of a few minutes, it coincides with the point in time at which the sun reaches its greatest southern declination and thus its southernmost position on the celestial sphere.
The beginning of the season is not exactly identical with crossing the celestial equator or reaching the greatest declination, because it is actually the center of gravity of the earth-moon system that moves evenly in the "earth orbit plane" around the sun, while the earth itself has this center of gravity circled and is usually slightly above or below this level. From the geocentric observer's point of view, the sun is therefore not exactly on the ecliptic (it has an ecliptical latitude not equal to zero). On the one hand, it does not happen exactly through the point of spring and autumn, on the other hand, its variable ecliptical latitude means that the maximum declination is usually not assumed exactly at the solstice points.
Beginning of the seasons
|2015||March, 20th||11:45 p.m. CET||June 21st||18:38 CEST||September 23rd||10:21 CEST||December 22||05:48 CET|
|2016||March, 20th||05:30 a.m. CET||June 21st||00:34 CEST||September 22||16:21 CEST||21st December||11:44 am CET|
|2017||March, 20th||11:29 am CET||June 21st||06:24 CEST||September 22||10:02 p.m. CEST||21st December||17:28 CET|
|2018||March, 20th||5:15 p.m. CET||June 21st||12:07 p.m. CEST||September 23rd||03:54 CEST||21st December||23:23 CET|
|2019||March, 20th||10:58 p.m. CET||June 21st||17:54 CEST||September 23rd||09:50 CEST||December 22||05:19 CET|
|2020||March, 20th||4:50 a.m. CET||20th June||23:44 CEST||September 22||15:31 CEST||21st December||11:02 am CET|
|2021||March, 20th||10:37 a.m. CET||June 21st||05:32 CEST||September 22||21:21 CEST||21st December||16:59 CET|
|2022||March, 20th||4:33 p.m. CET||June 21st||11:14 am CEST||September 23rd||03:04 CEST||21st December||10:48 p.m. CET|
|2023||March, 20th||10:24 p.m. CET||June 21st||16:58 CEST||September 23rd||08:50 CEST||December 22||04:27 CET|
|2024||March, 20th||4:06 a.m. CET||20th June||10:51 p.m. CEST||September 22||14:44 CEST||21st December||10:21 CET|
|2025||March, 20th||10:01 a.m. CET||June 21st||04:42 CEST||September 22||20:19 CEST||21st December||16:03 CET|
|2026||March, 20th||3:46 p.m. CET||June 21st||10:24 CEST||September 23rd||02:05 CEST||21st December||21:50 CET|
|2027||March, 20th||9:24 p.m. CET||June 21st||16:11 CEST||September 23rd||08:01 CEST||December 22||03:42 CET|
There is an average of 365 days, 5 hours and 49 minutes between the two beginnings of spring (see tropical year ). Every beginning of spring therefore falls almost 6 hours later than the previous one. This systematic drift can also be seen in the table when comparing the times given for successive years. Deviations of the individual time intervals from the mean are caused by the orbital disturbances caused by other planets as well as the already mentioned difference between the center of the earth and the earth-moon center of gravity (these influences are explained in more detail in the article Earth orbit ).
After four years, the beginning of spring has been postponed by almost 24 hours to later times. The Julian calendar now introduced a leap day (in the table: 2012, 2016, 2020, 2024) to move the beginning of spring back by 24 hours to earlier times. Since the correction by the leap day is 24 hours, but the beginning of spring is only shifted by almost 24 hours (namely by an average of 4 5 h 49 m = 23 h 16 m), the leap day results in overcompensation, so that the beginning of spring after a leap year cycle of four years is shifted on average by around 44 minutes to earlier times. This can also be seen in the table when comparing two beginnings of spring that are four years apart. This overcompensation is corrected in the Gregorian calendar by the fact that the leap day is canceled in three out of four hundred years (secular years). This has been the case in the past in the years 1700, 1800 and 1900.
Since the switching rule only allows a certain shift in the beginning of spring to appear before it corrects it again by inserting a leap day, the time of the beginning of spring (and accordingly that of all other beginning of the season) fluctuates in a range of around 18 hours. Usually midnight is in this area, so that the beginning of the season in question can take place on two different calendar days over the years. The beginning of autumn in the Central European time zone currently falls on September 22nd or 23rd. If the fluctuation range does not extend beyond midnight, the beginning of the season in question always takes place on the same calendar date. So currently the beginning of summer in the Central European time zone (but not in other time zones) always falls on June 21.
However, these ratios do not remain constant, since every leap day causes overcompensation and, as mentioned above, the beginning of the season slowly shifts over the longer term to earlier calendar times until this shift is corrected again by the switching rule for hundreds of years:
- For much of the 20th century and the first few years of the 21st century, the beginning of spring in the Central European Time Zone fell on March 20th or 21st. In 2011 it took place for the last time in this century on March 21st and has always been on March 20th ever since. In 2048 it will fall on March 19 for the first time and then more and more often. By the end of the century, March 19th and 20th will be about the same frequency. Due to the omitted leap day in 2100, the beginning of spring at the beginning of the 22nd century will again commute between March 20 and 21.
- At present, the beginning of summer in the Central European time zone (summer time) still takes place on June 21. In 2020 it will fall for the first time and then more and more often on June 20th. Towards the end of the century, the 20th will occur more frequently than the 21. The leap day, which fails in 2100, shifts the beginning of summer back to June 21st for some time.
- At the moment, the beginning of autumn in the Central European Time Zone (summer time) is about as often as 22 or 23 September. In the future, the 22nd will appear increasingly frequently; in 2067 the 23rd will occur for the last time in this century (provided that there is still summer time in those years, otherwise in 2063). The switching rule of the century then pushes the beginning of autumn back to September 22nd and 23rd.
- At present, winter begins roughly equally on December 21st and 22nd. The 21st will be more frequent in the future; in 2047 the 22nd will occur for the last time in this century. In 2084, for the first time since 1696, December 20 will be the beginning of winter. After the turn of the century, winter begins again on December 21st and 22nd.
Duration of the seasons
The astronomical seasons correspond to certain sections of the earth's orbit. Since the earth's orbit is slightly elliptical, the earth traverses these sections at variable speeds, so that the seasons are not all the same length.
At present the earth is near the perihelion at the beginning of winter and therefore passes through autumn and winter faster than spring and summer. Since the perihelion moves slowly through the seasons due to precession and gravitational orbit disturbances, the speeds at which the respective seasons are passed through also change.
The table shows the mean duration of the individual seasons in days:
In 1246, perihelion and winter solstice coincided, so winter was the same length as autumn and summer was the same length as spring. Since then, winter has been the shortest season. It will reach its shortest length (88.71 days) around the year 3500 and then get longer again. It remains the shortest season until around the year 6430 the perihelion coincides with the spring equinox.
Because of the tidal forces of the sun and moon acting on the earth's equatorial bulge, the earth's axis performs a precession motion: its inclination remains (essentially) constant; however, the direction in which it is inclined rotates once around 360 ° in the course of about 26,000 years. This does not change the sequence of the seasons, only the section of the orbit in which the respective season occurs is shifted: the summer solstice, for example, always occurs when the northern end of the earth's axis is exactly inclined towards the sun. The influence of this shift on the duration and severity of the seasons has already been explained.
For a point in time in 13,000 years, the earth's axis would have to be drawn inclined to the left instead of to the right in all the positions shown above. The globe in the right position would have turned the northern hemisphere towards the sun, it would be the time of the summer solstice instead of the winter solstice. The Gregorian calendar is set up in such a way that it takes part in this shift: the mean length of its calendar year (365.2425 days) roughly corresponds to the length of the tropical year (365.2422 days), so that the calendar date March 21 is always close to astronomical beginning of spring remains fixed and the other beginnings of the season wander along accordingly. In that orbit position it would be June instead of December, as is to be expected for the beginning of summer.
The constellation Orion and other characteristic winter constellations lie in the direction in which the night side of this globe points . In 13,000 years, summer will prevail in this section of the orbit, and Orion will be a summer constellation, but will then be significantly lower in the northern hemisphere.
The so-called meteorological seasons are simply divided into calendar months and each consist of three complete months. This means that they are set around three weeks earlier than the astronomical seasons. With the meteorological definition, the warmest months of June, July and August on average in the northern hemisphere, as in Germany , fall in the (meteorological) summer, and the average coldest months in December, January and February fall in the (meteorological) winter (see figure above, seasonal temperature curve (NH)). These quarterly periods of time, determined by the beginning and end of calendar months, often allow easier statistical recording of meteorological data; they are not geared to current weather changes.
- Spring : 1. March - 31 May
- Summer : 1. June - 31 August
- Autumn : September 1st - November 30th
- Winter : December 1st - February 28th / 29th
Seasonally different aspect of a beech group in the Scheldt Forest
In general, the division into four seasons is too rough to describe the seasonal development of nature that can be observed over the course of the year. Therefore, ten phenological seasons are distinguished in the phenology for Central Europe , each of which is described by the first occurrence of natural events. They refer to the developmental stages of pointer plants, for example the flowering of hazel bushes or apple trees , and the behavior of various animals, such as the flight of bees or the call of a cuckoo .
The beginning of such phenologically determined seasons is regionally different and also in the same place not the same from year to year. The vegetation periods of plants not only differ from species to species, but can also vary within a species for the same individual depending on the current climatic conditions at the respective location. The yearly different seasonal conditions are also reflected in the wood growth as an annual ring . Annual ring tables form the basis for dendrochronological dating; the Hohenheim annual ring calendar goes back almost 12,500 years without any gaps and thus covers the entire Holocene .
Seasons in lyric
The formative influence that the course of the seasons has on people's rhythm of life is also reflected in the language (youth corresponds to spring, flowering of life corresponds to summer, age corresponds to autumn).
While Homer and Hesiod only differentiate between three seasons, namely spring, summer and winter, Hippocrates of Kos also knows and defines autumn. For him, the spring lasts 48 days, from the equinox to the early sunrise of the Pleiades on May 12th, the summer 134 to 135 days, to the early sunrise of Arctur in the constellation Bear Guardian , autumn 48 days to the early sunset of the Pleiades on May 11th. November, and winter (which he divides into four parts) 135 days until the next equinox. With this, however, he deviates from the dates of Eudoxus of Knidos (May 14, July 25, September 14 and November 14), for Cicero the "prince of astronomers".
In Eastern European countries with a continental climate , the two seasons, spring and autumn, are extremely short and summer and winter are much hotter and colder.
- Erik Oppold: Learning module "Earth, Earth Orbit , Astronomical Seasons". Geometric-astronomical basics. In: WEBGEO basics / climatology. Institute for Physical Geography (IPG) at the University of Freiburg, accessed on December 14, 2010.
- Time-lapse films of the seasons - tree over four seasons, thawing snow, spring, discoloration in autumn
The times of the beginning of the season, rounded to the nearest minute, can be calculated to the nearest second. However, the calculation must be based on a strictly uniform time scale (e.g. terrestrial time TT), while the reader expects an indication of the civil time displayed by his clock (here CET or CEST ). The conversion from astronomical time to civil time is done by subtracting the time correction " ΔT ", which essentially depends on the number of leap seconds inserted up to the relevant point in time . While the times of the future beginning of the season in terrestrial time are known to the second, their specification in civil time requires an estimate of how many leap seconds will have been inserted by then.
Because of the irregularity of the earth's rotation , future leap seconds can only be estimated with a certain uncertainty, and this uncertainty (not an uncertainty in the astronomical calculations) creates an uncertainty of a few seconds in the future table entries shown here. If the number of leap seconds that has accumulated when the relevant point in time is reached differs from the number estimated when the table was created, a corresponding correction of the tabulated point in time results. These corrections are usually not visible in the times that are rounded to the nearest minute, but if a point in time falls close to half a minute and the rounding is therefore "on the brink", a correction of one minute can result. So happened z. E.g. with the winter solstice on December 21, 2012 at 12:12 CET, which was announced in older yearbooks and also in earlier versions of this article for 12:11 CET.
- The Eight Seasons of the Sami. In: Visit Sweden. Retrieved March 20, 2019 .
- Hans-Jürgen Martin: The six seasons. In: zebrafink.de. November 28, 2014, accessed March 20, 2019 .
- PD Jones et al .: Surface Air Temperature And Its Changes Over The Past 150 Years, Figure 7 ( Memento from July 16, 2010 in the Internet Archive ) (page 24 of 28 of the PDF file; 7.8 MB)
- added on May 15, 2006, September 10, 2006, October 28, 2005, March 10, 2006
- Season table of the USNO ( memento of October 8, 2015 in the Internet Archive ), accessed on March 18, 2015
- J. Meeus: Astronomical Tables of the Sun, Moon and Planets . 2nd ed., Willmann-Bell, Richmond 1995, ISBN 0-943396-45-X , p. 151 ff.
- J. Meeus: Astronomical Tables… . P. 99
- The poetry of the seasons - poems , selected by Evelyne Polt-Heinzl and Christine Schmidjell, Philipp Reclam jun., Stuttgart 2001 and 2003, ISBN 978-3-15-010535-1
- Georg Sticker : Hippokrates, Der Volkskrankheiten first and third book (around the year 434-430 BC). Translated, introduced and explained from the Greek. Johann Ambrosius Barth, Leipzig 1923 (= Classics of Medicine. Volume 29); Unchanged reprint: Central antiquariat of the German Democratic Republic, Leipzig 1968, p. 37 ( Der Volkskrankheiten first book. First volume. ) and 88 f., note 1.