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79-664: 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 months , which increases the solar flux . Due to seasonal lag , June, July, and August are

158-492: A change in daily floral and animal events can be observed, the season is changing. In this sense, ecological seasons are defined in absolute terms, unlike calendar-based methods in which the seasons are relative. If specific conditions associated with a particular ecological season do not normally occur in a particular region, then that area cannot be said to experience that season on a regular basis. Six ecological seasons can be distinguished without fixed calendar-based dates like

237-583: A counterclockwise pattern. Hurricanes and tropical storms (massive low-pressure systems) spin counterclockwise in the Northern Hemisphere. The shadow of a sundial moves clockwise on latitudes north of the subsolar point and anticlockwise to the south. During the day at these latitudes, the Sun tends to rise to its maximum at a southerly position. Between the Tropic of Cancer and the Equator,

316-404: A four-season model to demarcate the warmest and coldest seasons, which are further separated by two intermediate seasons. Calendar-based reckoning defines the seasons in relative rather than absolute terms, so the coldest quarter-year is considered winter even if floral activity is regularly observed during it, despite the traditional association of flowers with spring and summer. The major exception

395-426: A little over one half of Earth is illuminated at any point in time (with exceptions during eclipses ), the terminator path varies by time of day due to Earth's rotation on its axis. The terminator path also varies by time of year due to Earth's orbital revolution around the Sun; thus, the plane of the terminator is nearly parallel to planes created by lines of longitude during the equinoxes , and its maximum angle

474-621: Is north of the Equator . For other planets in the Solar System , north is defined as being in the same celestial hemisphere relative to the invariable plane of the Solar System as Earth's North Pole . Due to Earth's axial tilt of 23.439281°, there is a seasonal variation in the lengths of the day and night. There is also a seasonal variation in temperatures, which lags the variation in day and night. Conventionally, winter in

553-784: 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 ). The traditional calendar in China has 4 seasons based on 24 periods, twelve of which are called zhōngqi and twelve of which are known as jiéqi . These periods are collectively known in English as "solar terms" or "solar breaths". The four seasons chūn ( 春 ), xià ( 夏 ), qiū ( 秋 ), and dōng ( 冬 )—translated as "spring", "summer", "autumn", and "winter"—each center on

632-585: Is a 7% variation in sunlight received. 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

711-419: Is a moving line that divides the daylit side and the dark night side of a planetary body . The terminator is defined as the locus of points on a planet or moon where the line through the center of its parent star is tangent . An observer on the terminator of such an orbiting body with an atmosphere would experience twilight due to light scattering by particles in the gaseous layer. On Earth ,

790-516: Is a type of skywave propagation . Under good conditions, radio waves can travel along the terminator to antipodal points. The lunar terminator is the division between the illuminated and dark hemispheres of the Moon . It is the lunar equivalent of the division between night and day on the Earth spheroid , although the Moon's much lower rate of rotation means it takes longer for it to pass across

869-455: Is also known as "obliquity of the ecliptic ".) 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 March   20 to around September   22),

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948-406: Is approximately 23.5° to the pole during the solstices . At the equator , under flat conditions (without obstructions like mountains or at a height above any such obstructions), the terminator moves at approximately 463 metres per second (1,040 mph). This speed can appear to increase when near obstructions, such as the height of a mountain, as the shadow of the obstruction will be cast over

1027-411: Is called "summer" and the rainy season (May to October) is called "winter", even though it is located in the northern hemisphere. There is no noticeable change in the amount of sunlight at different times of the year. Instead, many regions (such as the northern Indian Ocean ) have varying monsoon rain and wind cycles. Northern Hemisphere The Northern Hemisphere is the half of Earth that

1106-582: Is contradictory. These are mainly a matter of custom and not generally proclaimed by governments north or south of the equator for civil purposes. Meteorological seasons are reckoned by temperature, with summer being the hottest quarter of the year and winter the coldest quarter of the year. In 1780 the Societas Meteorologica Palatina (which became defunct in 1795), an early international organization for meteorology, defined seasons as groupings of three whole months as identified by

1185-418: 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

1264-460: Is in the tropics where, as already noted, the winter season is not observed. The four seasons have been in use since at least Roman times, as in Rerum rusticarum of Varro 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

1343-405: Is measured when the Sun still or already illuminates it while the base of the mountain remains in shadow.   Low Earth orbit satellites take advantage of the fact that certain polar orbits set near the terminator do not suffer from eclipse , therefore their solar cells are continuously lit by sunlight. Such orbits are called dawn-dusk orbits, a type of Sun-synchronous orbit . This prolongs

1422-633: Is mitigated by the abundance of water in the Southern Hemisphere. Seasonal weather fluctuations (changes) also depend on factors such as proximity to oceans or other large bodies of water, currents in those oceans, El Niño /ENSO and other oceanic cycles, and prevailing winds . 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,

1501-444: Is one of the primary reasons for the Earth's seasons, as illustrated by the diagram to the right. Minor variation in the direction of the axis, known as axial precession , takes place over the course of 26,000 years, and therefore is not noticeable to modern human civilization. The seasons result from the Earth's axis of rotation being tilted with respect to its orbital plane by an angle of approximately 23.4 degrees . (This tilt

1580-420: The rainy / wet / 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 low water —which were previously defined by

1659-633: The Midwestern United States and parts of eastern Europe . The actual dates for each season vary by climate region and can shift from one year to the next. Average dates listed here are for mild and cool temperate climate zones in the Northern Hemisphere: Indigenous people in polar, temperate and tropical climates of northern Eurasia, the Americas, Africa, Oceania, and Australia have traditionally defined

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1738-465: The Northern temperate zone . The changes in these regions between summer and winter are generally mild, rather than extreme hot or cold. However, a temperate climate can have very unpredictable weather. Tropical regions (between the Tropic of Cancer and the Equator, 0° latitude) are generally hot all year round and tend to experience a rainy season during the summer months, and a dry season during

1817-641: The Paracel Islands and parts of the Tibetan plateau ) may be said to have summer all year round or winter all year round. Astronomical timing as the basis for designating the temperate seasons dates back at least to the Julian Calendar used by the ancient Romans. As mentioned above, Varro wrote that spring, summer, autumn, and winter start on the 23rd day of the sun's passage through Aquarius, Taurus, Leo, and Scorpio, respectively, and that (in

1896-654: 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

1975-630: The elliptical nature 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

2054-720: 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. The Earth's orbit exhibits approximate axial parallelism , maintaining its direction toward Polaris (the "North Star") year-round. This

2133-583: The last glacial period ended about 10,000 years ago. Earth is currently in an interglacial period of the Quaternary , called the Holocene . The glaciations that occurred during the glacial period covered many areas of the Northern Hemisphere. The Arctic is a region around the North Pole (90° latitude ). Its climate is characterized by cold winters and cool summers. Precipitation mostly comes in

2212-580: 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 . Most calendar-based partitions use

2291-422: The westerlies , push the currents back to the east, producing a closed clockwise loop. Its surface is 60.7% water, compared with 80.9% water in the case of the Southern Hemisphere , and it contains 67.3% of Earth's land. The continents of North America and mainland Eurasia are located entirely in the Northern Hemisphere, together with about two-thirds of Africa and a small part of South America . During

2370-512: 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

2449-516: The 2.5 million years of the Pleistocene , numerous cold phases called glacials ( Quaternary ice age ), or significant advances of continental ice sheets, in Europe and North America , occurred at intervals of approximately 40,000 to 100,000 years. The long glacial periods were separated by more temperate and shorter interglacials which lasted about 10,000–15,000 years. The last cold episode of

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2528-455: 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

2607-459: 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. He gives "the eighth day to

2686-491: The Gregorian calendar. According to this definition, for temperate areas in the northern hemisphere, spring begins on 1 March, summer on 1 June, autumn on 1 September, and winter on 1 December. For the southern hemisphere temperate zone, spring begins on 1 September, summer on 1 December, autumn on 1 March, and winter on 1 June. In Australasia the meteorological terms for seasons apply to

2765-476: The ICZ migrates to a position south of the Equator. In meteorological terms, the solstices (the maximum and minimum insolation ) do not fall in the middles of summer and winter. The heights of these seasons occur up to 7 weeks later because of seasonal lag . Seasons, though, are not always defined in meteorological terms. In astronomical reckoning by hours of daylight alone, the solstices and equinoxes are in

2844-469: The Julian Calendar) these days were February 7, May 9, August 11, and November 10. He points out that the lengths are not equal, being 91 (in non-leap years), 94, 91, and 89 days for spring, summer, autumn, and winter, respectively. The midpoints of these seasons were March 24 or 25, June 25, September 25 or 26, and December 24 or 25, which are near to the equinoxes and solstices of his day. Pliny

2923-469: 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. Nowadays 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

3002-403: The March equinox. The times of the equinoxes and solstices are not fixed with respect to the modern Gregorian calendar, but fall about six hours later every year, amounting to one full day in four years. They are reset by the occurrence of a leap year. The Gregorian calendar is designed to keep the March equinox no later than 21 March as accurately as is practical. The calendar equinox (used in

3081-464: The Northern Hemisphere compared to the Southern Hemisphere, making the Northern Hemisphere more suitable for deep-space observation, as it is not "blinded" by the Milky Way. As of 2015, the Northern Hemisphere is home to approximately 6.4 billion people, which is around 87.0% of the Earth's total human population of 7.3 billion people. Terminator (solar) A terminator or twilight zone

3160-610: The Northern Hemisphere is taken as the period from the December solstice (typically December 21 UTC ) to the March equinox (typically March 20 UTC), while summer is taken as the period from the June solstice through to the September equinox (typically on 23 September UTC). The dates vary each year due to the difference between the calendar year and the astronomical year . Within the Northern Hemisphere, oceanic currents can change

3239-465: The Northern Hemisphere tips toward the Sun, with the maximum amount occurring on about June   21. For the other half of the year, the same happens, but in the Southern Hemisphere instead of the Northern, with the maximum around December   21. The two instants when the Sun is directly overhead at the Equator are the equinoxes . Also at that moment, both the North Pole and the South Pole of

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3318-539: The Sami people in Scandinavia. Many indigenous people who no longer live directly off the land in traditional often nomadic styles, now observe modern methods of seasonal reckoning according to what is customary in their particular country or region. 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

3397-570: 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. Compared to axial parallelism and axial tilt, other factors contribute little to seasonal temperature changes. The seasons are not

3476-556: The Sun can be seen to the north, directly overhead, or to the south at noon, depending on the time of year. In the Southern Hemisphere, the midday Sun is predominantly in the north. When viewed from the Northern Hemisphere, the Moon appears inverted compared to a view from the Southern Hemisphere. The North Pole faces away from the Galactic Center of the Milky Way . This results in the Milky Way being sparser and dimmer in

3555-417: The Sun is low in the sky. For this reason, much lunar photographic study centers on the illuminated area near the lunar terminator, and the resulting shadows provide accurate descriptions of the lunar terrain . The lunar terminator (or tilt) illusion is an optical illusion arising from the expectation of an observer on Earth that the direction of sunlight illuminating the Moon (i.e. a line perpendicular to

3634-401: The average temperature difference between summer and winter in location — will also change over time because the Earth's axial tilt fluctuates between 22.1 and 24.5 degrees. Smaller irregularities in the times are caused by perturbations of the Moon and the other planets. Solar timing is based on insolation in which the solstices and equinoxes are seen as the midpoints of the seasons. This was

3713-646: 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,

3792-609: The case with the seasons described by the Roman scholar Varro (see above). It was the method for reckoning seasons in medieval Europe, especially by the Celts , and is still ceremonially observed in Ireland and some East Asian countries. Summer is defined as the quarter of the year with the greatest insolation and winter as the quarter with the least. The solar seasons change at the cross-quarter days, which are about 3–4 weeks earlier than

3871-434: 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

3950-413: The dark side of the terminator, whereas the E and F layers above the D layer take longer to form. This time-difference puts the ionosphere into a unique intermediate state along the terminator, called the "grey line". Amateur radio operators take advantage of conditions along the terminator to perform long-distance communications. Called "gray-line" or "grey-line" propagation , this signal path

4029-499: 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

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4108-652: The form of snow. Areas inside the Arctic Circle (66°34′ latitude) experience some days in summer when the Sun never sets, and some days during the winter when it never rises. The duration of these phases varies from one day for locations right on the Arctic Circle to several months near the Pole, which is the middle of the Northern Hemisphere. Between the Arctic Circle and the Tropic of Cancer (23°26′ latitude) lies

4187-430: The ground in advance of the terminator along a flat landscape. The speed of the terminator decreases as it approaches the poles, where it can reach a speed of zero (full-day sunlight or darkness). Supersonic aircraft like jet fighters or Concorde and Tupolev Tu-144 supersonic transports are the only aircraft able to overtake the maximum speed of the terminator at the equator. However, slower vehicles can overtake

4266-429: The ice on rivers and lakes. 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

4345-481: 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. In the tropics, where seasonal dates also vary, it is more common to speak of the rainy (or wet, or monsoon ) season versus the dry season . For example, in Nicaragua the dry season (November to April)

4424-535: The meteorological and astronomical seasons. Oceanic regions tend to experience the beginning of the hibernal season up to a month later than continental climates . Conversely, prevernal and vernal seasons begin up to a month earlier near oceanic and coastal areas. For example, prevernal crocus blooms typically appear as early as February in coastal areas of British Columbia , the British Isles , but generally do not appear until March or April in locations like

4503-534: 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

4582-532: The number of seasons between summer and winter can number from one to three. The dates are fixed at even intervals of months. 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

4661-408: The particles within an atmosphere are at a higher elevation, the light source can remain visible even after it has set at ground level. These particles scatter the light, reflecting some of it to the ground. Hence, the sky can remain illuminated even after the sun has set. Images showing a planetary terminator can be used to map topography: the position of the tip of a mountain behind the terminator line

4740-449: 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. The tropical and (to a lesser degree) subtropical regions see little annual fluctuation of sunlight and temperature due to Earth's moderate 23.4-degree tilt being insufficient to appreciably affect the strength of the sun's rays annually. The slight differences between

4819-470: The respective solstice or equinox. Astronomically, the seasons are said to begin on Lichun ( 立春 , "the start of spring") on about 4 February, Lixia ( 立夏 ) on about 6 May, Liqiu ( 立秋 ) on about 8 August, and Lidong ( 立冬 ) on about 8 November. This system forms the basis of other such systems in East Asian lunisolar calendars. Some calendars in south Asia use a six-season partition where

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4898-602: The result of the variation in Earth's distance to the Sun because of its elliptical orbit . In fact, Earth reaches perihelion (the point in its orbit closest to the Sun) in January, and it reaches aphelion (the point farthest from the Sun) in July, so the slight contribution of orbital eccentricity opposes the temperature trends of the seasons in the Northern Hemisphere. In general, the effect of orbital eccentricity on Earth's seasons

4977-410: The seasons ecologically by observing the activity of the plants, animals and weather around them. Each separate tribal group traditionally observes different seasons determined according to local criteria that can vary from the hibernation of polar bears on the arctic tundras to the growing seasons of plants in the tropical rainforests. In Australia, some tribes have up to eight seasons in a year, as do

5056-449: The seasons is determined by the exact times of the sun reaching the tropics of Cancer and Capricorn for the solstices and the times of the sun's transit over the equator for the equinoxes , 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

5135-483: 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

5214-489: The solstices and the equinoxes cause seasonal shifts along a rainy low-pressure belt called the Intertropical Convergence Zone (ICZ). As a result, the amount of precipitation tends to vary more dramatically than the average temperature. When the ICZ is north of the Equator, the northern tropics experience their wet season while the southern tropics have their dry season. This pattern reverses when

5293-403: The surface. At the equator, it moves at 15.4 kilometres per hour (9.6 mph), as fast as an athletic human can run on earth. Due to the angle at which sunlight strikes this portion of the Moon, shadows cast by craters and other geological features are elongated, thereby making such features more apparent to the observer. This phenomenon is similar to the lengthening of shadows on Earth when

5372-412: The temperate zone that occupies all of New Zealand , New South Wales , Victoria , Tasmania , the south-eastern corner of South Australia and the south-west of Western Australia, and the south east Queensland areas south of Brisbane . In Sweden and Finland, meteorologists and news outlets use the concept of thermal seasons, which are defined based on mean daily temperatures. The beginning of spring

5451-406: The terminator at higher latitudes , and it is possible to walk faster than the terminator at the poles, near to the equinoxes . The visual effect is that of seeing the sun rise in the west, or set in the east. Strength of radio propagation changes between day- and night-side of the ionosphere . This is primarily because the D layer , which absorbs high frequency signals , disappears rapidly on

5530-411: The terminator is a circle with a diameter that is approximately that of Earth. The terminator passes through any point on Earth's surface twice a day, at sunrise and at sunset , apart from polar regions where this only occurs when the point is not experiencing midnight sun or polar night . The circle separates the portion of Earth experiencing daylight from that experiencing darkness (night). While

5609-400: The terminator) should correspond with the position of the Sun , but does not appear to do so. The illusion results from misinterpreting the arrangement of objects in the sky according to intuition based on planar geometry . Examination of a terminator can yield information about the surface of a planetary body; for example, the presence of an atmosphere can create a fuzzier terminator. As

5688-524: 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

5767-554: The twelve months in the Hindu calendar. The rough correspondences are: The Bengali Calendar is similar but differs in start and end times. It has the following seasons or ritu: The Odia Calendar is similar but differs in start and end times. The Tamil calendar follows a similar pattern of six seasons Ecologically speaking, a season is a period of the year in which only certain types of floral and animal events happen (e.g.: flowers bloom—spring; hedgehogs hibernate—winter). So, if

5846-601: The warmest months in the Northern Hemisphere while December, January, and February are the warmest months in the Southern Hemisphere. In temperate and sub-polar 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:

5925-448: The weather patterns that affect many factors within the north coast. For the same reason, flows of air down toward the northern surface of the Earth tend to spread across the surface in a clockwise pattern. Thus, clockwise air circulation is characteristic of high pressure weather cells in the Northern Hemisphere. Conversely, air rising from the northern surface of the Earth (creating a region of low pressure) tends to draw air toward it in

6004-462: The weather patterns that affect many factors within the north coast. Such events include El Niño–Southern Oscillation . Trade winds blow from east to west just above the equator. The winds pull surface water with them, creating currents, which flow westward due to the Coriolis effect . The currents then bend to the right, heading north. At about 30 degrees north latitude, a different set of winds,

6083-470: The west (America), whose clocks run behind UTC, may experience an equinox as early as March 19. Over thousands of years, the Earth's axial tilt and orbital eccentricity vary (see Milankovitch cycles ). The equinoxes and solstices move westward relative to the stars while the perihelion and aphelion move eastward. Thus, ten thousand years from now Earth's northern winter will occur at aphelion and northern summer at perihelion. The severity of seasonal change —

6162-489: The winter months. In the Northern Hemisphere, objects moving across or above the surface of the Earth tend to turn to the right because of the Coriolis effect . As a result, large-scale horizontal flows of air or water tend to form clockwise-turning gyres . These are best seen in ocean circulation patterns in the North Atlantic and North Pacific oceans. Within the Northern Hemisphere, oceanic currents can change

6241-408: 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. As noted, a variety of dates and even exact times are used in different countries or regions to mark changes of the calendar seasons. These observances are often declared "official" within their respective areas by the local or national media, even when the weather or climate

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