Season

The seasons divide the year into different periods, which are characterized by typical astronomical or climatic characteristics. In everyday parlance, this refers mainly meteorological distinguishable from each other significantly year periods; in temperate latitudes are the spring, summer, autumn and winter, in the tropics it is dry season and rainy season. The description of the seasons in this article refers to the northern hemisphere, in the southern hemisphere they are offset by half a year according to the calendar. Summer and winter can be ever understood as halves of a year, for example, as the boreal summer half of the year or the Südwinterhalbjahr.

Various peoples divide or divided the year into different seasons. So the seeds know in Scandinavia 8 seasons Australian Aborigines in Arnhem Land who speak Gagudju, 6 seasons. In Russia, you know the Rasputiza, two mud times each during snowmelt in the spring and during the autumn rains.

• 3.1 precession

Emergence of seasons

In the course of a year to change to a specific geographical location intensity, duration and angle of sunlight incidence. Although the Earth is running on a path around the sun, which is elliptical and not circular, so that the distance from the Sun varies, but affect the resulting differences alone, the light intensity and are going through because of the small orbital eccentricity is not large ( the sun next path point ( perihelion ) the earth in the first week of January, in the winter in the northern hemisphere ). On the other hand determines the tangible on earth seasons the position of the axis of rotation relative to the orbit plane. Like a gyroscope keeps the Earth's axis due to conservation of angular momentum in their orientation in space and is thus ( almost ) fixed in space at a certain angle to the ecliptic plane. But this position is not at right angles (90 °), but is (at present ) is about 66.6 ° so that the equatorial plane is inclined at about 23.4 ° (23 ° 26 ' ) relative to the orbital plane. Therefore ( solar elevation at noon ) changes during a Erdumlaufs around the sun the angle of incidence of sunlight during the year, as well as the Earth's rotation around itself and the length of daylight ( lights day) and day length in the polfernen regions outside the polar circles each of a rotation of the earth to the next.

For the generation of seasons first crucial how strong the respective shares of the radiation output of the sun, a specific geographic region - based depending on their surface relief of both angle of incidence as well as day length - receives as irradiance, varies during the year. During the height of the sun each year varies lunch for all locations outside of the polar regions to ± 23.4 °, and the average elevation angle to the poles is shallower, the clear day takes an average of the same length, but increases the range of variation in day length with increasing latitude. Since both factors together hanging over the diurnal arc - the highest position of the sun and longest day fall together - and their fluctuations add up, training of seasons thus depends primarily on the geographical latitude of a region.

Therefore, based on the angle of incidence of sunlight solar climates alone are distinguished according to the latitude and so the equatorial tropics between the tropics ( 23.4 ° latitude ) delimited from the ectropic zones - the subtropics and mid-latitudes and the polar regions (from 66.6 ° width) - with remote equatorial increasingly more marked seasonal differences.

During the time between the day - and -night match in March and September, the northern hemisphere is more inclined towards the sun, so the sun for an observer located there is undergoing a high arch. When the sun is high solar radiation hits steeply to the soil surface, thus providing a relatively high energy input per unit area. Furthermore, is the greater part of a day passed through Sun's apparent path around the earth as a day arc above the horizon, so that the days are long and has plenty of time for the energy input is available. The thus increased energy introduced in this period, a warming of the northern hemisphere.

The earth is six months later at the opposite point of its orbit, the northern hemisphere the sun is averse - as a result of apart from precession and nutation relatively space-fixed position of Earth's axis. For an observer in the northern hemisphere then results in a low running daily path of the sun. When the sun is low, so the sun rays hit flat on the surface, so that they spread over a larger area and enters less energy. In addition, is only the smaller part of the daily path of the sun above the horizon, so that the energy input can be done only for a short period of time. The result is a cooling of the northern hemisphere.

Heating and cooling initially show up in the air temperatures ( see figure); because of the thermal inertia of the soil temperatures follow the high and the low of the sun with a certain delay. The differences in the daily arc of the solar cycle increase with higher latitude and increasingly have a greater impact ( up to the polar night ) to get to the equator, the seasonal variations lower.

In the southern hemisphere, respectively those of the northern hemisphere prevail opposite seasons: Is it summer in the south, as in northern winter, and vice versa. In tropical and subtropical areas, a distinction is instead between the rainy and dry seasons. In the tropics there are two rainy seasons per year, which, however, are changing with increasing latitude to a single, bimodal, and finally in the subtropics eingipfeligen to a rainy season.

The course of the year slightly different because of the eccentric round distance of the earth from the sun is not the cause of the seasons. It merely present the southern winter somewhat stricter and the northern winter milder (less sun and closer ) than they would be in a circular orbit of the earth. The Earth is in fact at the present perihelion and Apheldurchgang in the north winter sun at its closest point ( perihelion to the January 3, at 147.1 million km ), while it is further away in the southern winter something from the sun ( aphelion to the July 5, at 152.1 million km ). The reason for the seasons is against it - as stated above - the angle and duration of sunlight. For Central Europe the extremes of the angles in the summer of 60 ° to 65 ° and of possible sunshine duration in Central Germany 16-17 hours, in winter there are here, however, 7-8 hours or angle of only 13 ° to 18 °.

As a result of perturbations by the other planets, the apsides (the line between aphelion and perihelion ) rotates in good 111,000 years once prograde (ie in the direction of motion of the planets ). Due to the interference caused by the planets and the moon also the earth's axis executes a precessional motion, so that the position of the reference points of the solstices and equinoxes gradually shifts and in about 26,000 years, even declining (against the direction of movement) around the entire Earth's orbit travels. Because of these mutually opposing movements perihelion runs in about 21,000 years, even through all the seasons. In about 10,000 years the sun next track point will coincide with the beginning of summer. The winter seasons in the northern hemisphere are then longer and sun also take place today. In return, the southern hemisphere is getting shorter and more winter sun.

Astronomical seasons

Definitions

Astronomically, the seasons are determined by the apparent geocentric ecliptic longitude of the sun. Taking account of aberration and nutation, the apparent annual path of the sun is it considered from a hypothetical observation site in the center of the earth and divided into four sections. Each of the track sections is bounded by a respective equinoctial point (of equinox, at 0 ° or at 180 ° ) and a respective solstitial point (of solstice, at 90 ° or at 270 °).

The astronomical seasons are defined as those periods which elapse during the passage of a particular one of the four sections in each case and take account of the different angular velocity is not the same length. Due to the geocentric, related to the Earth's center definition begins or ends an astronomical season anywhere around the world at the same time (but the corresponding different times in different time zones).

• The astronomical spring begins when the apparent geocentric longitude of the Sun is 0 °. This is the time of the Spring Day and night are equally long ( Primaräquinoktium ). He falls to within a few seconds together with the time at which the sun passes the celestial equator from south to north.

• Astronomical summer begins when the apparent length of the geocentric sun is 90 °. This is the time of the summer solstice. He falls down to a few minutes along with the time at which the sun reaches its greatest northern declination and thus their northerly position on the celestial sphere.

• Astronomical fall begins when the apparent length of the geocentric solar is 180 °. This is the time of the Autumn Day and night are equally long ( Sekundaräquinoktium ). He falls to within a few seconds together with the time at which the sun passes through the celestial equator from north to south.

• Astronomical winter begins when the apparent length of the geocentric solar is 270 °. This is the time of the winter solstice. He falls down to a few minutes along with the time at which the sun reaches its greatest southern declination, and thus its southernmost position on the celestial sphere.

The season starts are not exactly identical with the crossing of the celestial equator, the attainment of the greatest declination because it is actually the focal point of the Earth - Moon system, the uniform in the " Earth's orbital plane " around the sun moves as the earth itself this focus orbits and is in generally slightly above or below this level. From the geocentric observer as seen from the sun, therefore, does not exactly on the ecliptic (she has a non-zero ecliptic latitude ). Therefore, do not happen to a precisely through the Spring and Autumn period, on the other hand performs its variable ecliptic latitude means that the maximum declination is generally not accepted exactly at the solstice points.

Beginning of the seasons

The table lists the astronomical season starts in 2018 for the Central European Time zone (highlighted in leap years ) in: [note 1]

Between two spring beginnings is on average a period of 365 days, 5 hours and 49 minutes (see tropical year ). Therefore, any early spring falls on a later time by almost six hours than the previous one. This systematic drift can also be seen in the table, when comparing the times for successive years. Deviations of the individual time intervals from the mean caused by the induced perturbations from other planets as well as the already mentioned difference between center of the Earth and Earth-Moon gravity.

After four years, the beginning of spring has shifted by nearly 24 hours later times. The Julian calendar now led a leap day (in the table: 2008, 2012, 2016 ) to around 24 hours to move the beginning of spring back to earlier time points. Since the correction by the leap day is 24 hours, the beginning of spring is postponed until just under 24 hours ( ie an average of 4 · 5 h 49 m = 23 h 16 m), has the leap overcompensation result, so that the beginning of spring is shifted to a leap year cycle of four years, on average, about 44 minutes at earlier time points. This also shows up in the table comparing two spring starts, the spaced apart by four years. This overcompensation is corrected in the long term in the Gregorian calendar by three of the four hundred years the leap fails ( the next time in 2100 ).

Since the switching rule can only accumulate a certain amount of displacement of the spring first, before they corrected by insertion of a leap day again, varies the time beginning of spring (and correspondingly that of any other season starts ) in a range of about 18 hours. Usually a midnight is in this range, so that the annual Commencement concerned may take place at two different calendar days over the years. So, now falls the beginning of autumn in the Central European time zone on 22 or 23 September. If the range of variation is not a midnight extends across, finding the season starts in question always held on the same calendar date. So currently (but not in other time zones ) falls the start of summer in the Central European Time Zone always on June 21.

However, these conditions do not remain constant because each leap causes an overcompensation and, as mentioned above, move the season starts long-term slow to earlier calendar time points out to this shift by the switching rule for hundreds of years is corrected:

• For much of the 20th century and the first years of the 21st century, the beginning of spring was in the Central European time zone on the 20th or 21st of March. In 2011, he was for the last time in this century on March 21 instead and it has since always on March 20. In the year 2048 he will increasingly fall on 19 March for the first time and then. Towards the end of the century will occur about equally often 19 and 20 March. Because of the emergent in the year 2100 leap day in the early spring will oscillate at the beginning of the 22 th century again between the 20th and 21st of March.

• Currently, the beginning of summer still takes place on June 21 in the Central European Time Zone ( DST ). In 2020, he will increasingly fall on 20 June for the first time and then. Towards the end of the century, the 20 more frequent than the 21 The precipitated in the year 2100 leap day shifts the beginning of summer for some time again on June 21.

• Currently, the beginning of autumn arrives in the Central European Time Zone ( DST ) with approximately equal frequency on the 22nd or 23rd of September. In the future, increasingly frequently occur after 22; in the year 2067, the 23 occur for the last time (provided that there is a summer time in those years, or in 2063 ). The century- switching rule then pushes the beginning of fall back on the 22 and 23 September.

• Currently, winter begins about the same frequency on 21 and 22 December. The 21 will in future be more frequent; in 2047 will occur for the last time in this century of 22. In the year 2084 December 20 will be the beginning of winter for the first time since 1696. After the turn of the century to the beginning of winter is back on the 21 and 22 December.

Duration of the seasons

The astronomical seasons respectively correspond to certain sections of the Earth's orbit. Since the Earth's orbit is slightly elliptical, the Earth passes through these sections with variable speed, so that the seasons are not all the same length.

Currently, the Earth is at the beginning of winter in the vicinity of the perihelion and therefore passes through autumn and winter faster than spring and summer. Since the perihelion slowly migrates through the seasons by other planets because of the perturbations to the velocities with which the respective seasons are run through change.

The table shows the average length of seasons in days:

In 1246 perihelion and winter solstice coincided, the winter so had the same length as the autumn and the summer of the same length as the spring. Since winter is the shortest season. It is about the year 3500 to reach its lowest length ( 88.71 days ) and then get longer. He remains the shortest season until around the year 6430 coincides with the perihelion of the spring day and night are equally long.

Precession

Due to the resulting gravitational disturbances by the planets and the moon, as mentioned, the Earth's axis precesses from: their tendency remains (essentially) constant; the direction in which it is inclined, but pivots through 360 ° in the course of approximately 26,000 years. This does not change the sequence of the seasons, only the web portion, in which the respective season occurs shifts: the summer solstice, for example, always occurs when the north end of Earth's axis the sun is exactly inclined. The impact of this shift on the duration and severity of the seasons has already been explained.

For a time in 13,000 years, the earth's axis on the figure above in all positions shown to the left instead of to draw would be inclined to right. The globe in the right position would then have to face the northern hemisphere the sun, it would be the date of the summer instead of the winter solstice. The Gregorian calendar is set up so that he 's experienced this shift: the average length of its calendar year ( 365.2425 days ) is approximately equal to the length of the tropical year ( 365.2422 days ), so that the calendar date of March 21 always near At first astronomical spring is fixed and the other co-migrate season starts accordingly. In that web position then so June would be held in December, as is expected for a summer start.

In the direction in which shows the dark side of this globe are the constellation Orion and other characteristic winter constellations. In 13,000 years, summer will reign in this web section, and Orion will be a summer constellation, however, are in the northern hemisphere then much lower.

Meteorological seasons

The meteorological seasons are divided by calendar month and each includes getting three complete months. They are thus a total of about three weeks earlier recognized as the astronomical seasons. With the meteorological definition, on the northern hemisphere as in Germany average warmest months of June, July and August in the summer and the coldest months of December, January and February in the winter (see figure above, seasonal temperature variation (NH) ) fall. This is also more representative of the human nature feeling. In addition, at least in the period before the invention of the computer statistical evaluations were so simple to carry out.

• Spring: March 1 to May 31
• Summer: June 1 to August 31
• Fall: September 1 to November 30
• Winter: December 1 - 28 / February 29

Spring (1 ) Summer ( 2) Autumn (3) and Winter Landscape (4) in the Eifel near Monschau:

Phenological seasons

In order to describe the observed seasonal trend in flow state of nature through the seasons, the subdivision in four seasons in general is too coarse. In the phenology is therefore knows up to ten seasons, the beginning is given locally differ by the occurrence of various natural events (for example, apple blossom ).

Others

The dramatic impact that the sequence of the seasons has with the lifestyle of the people is reflected linguistically. In German it is called a period in which the different pace of life in an area considerably from the normal, a fifth season.

In Eastern European countries, where there is the continental climate, the two seasons of spring and autumn are extremely short and for the summer and winter periods are considerably hotter or colder.