1.
Antarctic
–
The Antarctic is a polar region, specifically the region around the Earths South Pole, opposite the Arctic region around the North Pole. The Antarctic comprises in the sense the continent of Antarctica. The region covers some 20% of the Southern Hemisphere, of which 5. 5% is the area of the Antarctic continent itself. All of the land and ice shelves south of 60°S latitude are administrated under the Antarctic Treaty System, in a biogeographic sense, the Antarctic ecozone is one of eight ecozones of the Earths land surface. Most of the Antarctic region is situated south of 60°S latitude parallel, there are only two species of flowering plant, Antarctic hair grass and Antarctic pearlwort, but a range of mosses, liverworts, lichens and macrofungi. The first Antarctic land discovered was the island of South Georgia, the first human born in the Antarctic was Solveig Gunbjørg Jacobsen born on 8 October 1913 in Grytviken, South Georgia. However, the region is visited by more than 40,000 tourists annually, the definitive results of the conference was presented at the Antarctic Treaty states meeting in Uruguay in May 2010. The Antarctic hosts the worlds largest protected area comprising 1.07 million km2, the South Georgia, because Antarctica surrounds the South Pole, it is theoretically located in all time zones. For practical purposes, time zones are based on territorial claims or the time zone of a stations owner country or supply base. Antarctic Circle History of Antarctica Krupnik, Igor, Michael A. Lang, smithsonian at the Poles, Contributions to International Polar Year Science
Antarctic
Antarctic
–
The Antarctic
Antarctic
–
The
Amundsen–Scott South Pole Station, the
geographic South Pole is signposted in the background
Antarctic
–
Moubray Bay and
Mount Herschel, Eastern Antarctica
2.
Circle of latitude
–
A circle of latitude on the Earth is an abstract east–west circle connecting all locations around the earth at a given latitude. Circles of latitude are called parallels because they are parallel to each other – that is. A locations position along a circle of latitude is given by its longitude and their length can be calculated by a common sine or cosine function. The 60th circle of latitude is half as long as the equator, a circle of latitude is perpendicular to all meridians. The latitude of the circle is approximately the angle between the equator and the circle, with the vertex at the Earths centre. The equator is at 0°, and the North and South poles are at 90° north, the Equator is the longest circle of latitude and is the only circle of latitude which also is a great circle. There is no limit to how precisely latitude can be measured, on a map, the circles of latitude may or may not be parallel, and their spacing may vary, depending on which projection is used to map the surface of the Earth onto a plane. On an equirectangular projection, centered on the equator, the circles of latitude are horizontal, parallel, on other cylindrical and pseudocylindrical projections, the circles of latitude are horizontal and parallel, but may be spaced unevenly to give the map useful characteristics. On most non-cylindrical and non-pseudocylindrical projections, the circles of latitude are neither straight nor parallel, North American nations and states have also mostly been created by straight lines, which are often parts of circles of latitudes. For instance, the border of Colorado is at 41°N while the southern border is at 37°N. Roughly half the length of border between the United States and Canada follows 49°N, there are five major circles of latitude, listed below from north to south. The position of the Equator is fixed but the latitudes of the other circles depend on the tilt of this axis relative to the plane of the Earths orbit, the equator is the circle that is equidistant from the North Pole and South Pole. It divides the Earth into the Northern Hemisphere and the Southern Hemisphere, of the parallels or circles of latitude, it is the longest, and the only great circle. All the other parallels are smaller and centered only on the earths axis, the Arctic Circle is the southernmost latitude in the Northern Hemisphere at which the sun can remain continuously above or below the horizon for 24 hours. Similarly, the Antarctic Circle marks the northernmost latitude in the Southern Hemisphere at which the sun can remain continuously above or below the horizon for 24 hours. The Tropic of Cancer and Tropic of Capricorn mark the northernmost and southernmost latitudes at which the sun may be directly overhead. The latitude of the circles is equal to the Earths axial tilt. In 2000 the mean value of the tilt was about 23° 26′ 21″, the main long-term cycle causes the axial tilt to fluctuate between about 22. 1° and 24. 5° with a period of 41,000 years
Circle of latitude
3.
Earth
–
Earth, otherwise known as the World, or the Globe, is the third planet from the Sun and the only object in the Universe known to harbor life. It is the densest planet in the Solar System and the largest of the four terrestrial planets, according to radiometric dating and other sources of evidence, Earth formed about 4.54 billion years ago. Earths gravity interacts with objects in space, especially the Sun. During one orbit around the Sun, Earth rotates about its axis over 365 times, thus, Earths axis of rotation is tilted, producing seasonal variations on the planets surface. The gravitational interaction between the Earth and Moon causes ocean tides, stabilizes the Earths orientation on its axis, Earths lithosphere is divided into several rigid tectonic plates that migrate across the surface over periods of many millions of years. About 71% of Earths surface is covered with water, mostly by its oceans, the remaining 29% is land consisting of continents and islands that together have many lakes, rivers and other sources of water that contribute to the hydrosphere. The majority of Earths polar regions are covered in ice, including the Antarctic ice sheet, Earths interior remains active with a solid iron inner core, a liquid outer core that generates the Earths magnetic field, and a convecting mantle that drives plate tectonics. Within the first billion years of Earths history, life appeared in the oceans and began to affect the Earths atmosphere and surface, some geological evidence indicates that life may have arisen as much as 4.1 billion years ago. Since then, the combination of Earths distance from the Sun, physical properties, in the history of the Earth, biodiversity has gone through long periods of expansion, occasionally punctuated by mass extinction events. Over 99% of all species that lived on Earth are extinct. Estimates of the number of species on Earth today vary widely, over 7.4 billion humans live on Earth and depend on its biosphere and minerals for their survival. Humans have developed diverse societies and cultures, politically, the world has about 200 sovereign states, the modern English word Earth developed from a wide variety of Middle English forms, which derived from an Old English noun most often spelled eorðe. It has cognates in every Germanic language, and their proto-Germanic root has been reconstructed as *erþō, originally, earth was written in lowercase, and from early Middle English, its definite sense as the globe was expressed as the earth. By early Modern English, many nouns were capitalized, and the became the Earth. More recently, the name is simply given as Earth. House styles now vary, Oxford spelling recognizes the lowercase form as the most common, another convention capitalizes Earth when appearing as a name but writes it in lowercase when preceded by the. It almost always appears in lowercase in colloquial expressions such as what on earth are you doing, the oldest material found in the Solar System is dated to 4. 5672±0.0006 billion years ago. By 4. 54±0.04 Gya the primordial Earth had formed, the formation and evolution of Solar System bodies occurred along with the Sun
Earth
–
"
The Blue Marble " photograph of Earth, taken during the
Apollo 17 lunar mission in 1972
Earth
–
Artist's impression of the early Solar System's planetary disk
Earth
–
World map color-coded by relative height
Earth
–
The summit of
Chimborazo, in Ecuador, is the point on Earth's surface farthest from its center.
4.
Geographical zone
–
The five main latitude regions of the Earths surface comprise geographical zones, divided by the major circles of latitude. The differences between them relate to climate and they are as follows, The North Frigid Zone, between the Arctic Circle 66.5 degrees north and the North Pole 90 degrees north. Covers 4. 12% of Earths surface The North Temperate Zone, covers 25. 99% of Earths surface The Torrid Zone, between the Tropic of Cancer 23.5 degrees north and the Tropic of Capricorn 23.5 degrees south. Covers 39. 78% of Earths surface The South Temperate Zone, covers 25. 99% of Earths surface The South Frigid Zone, from Antarctic Circle 66.5 degrees south and the South Pole 90 degrees south. On the basis of latitudinal extent, the globe is divided into three broad heat zones, the Torrid or Tropical Zone is also known as the Tropics. This happens annually, but in the region between the sun passes overhead twice a year, after the September equinox the sun passes into the Southern Hemisphere. It then passes similarly over the tropical regions until it reaches the Tropic of Capricorn at the December solstice. In the two Temperate Zones, consisting of the latitudes, the Sun is never directly overhead. The four annual seasons, spring, summer, autumn and winter, the North Temperate Zone includes Europe, Northern Asia, and North and Central America. The South Temperate Zone includes Southern Australasia, southern South America, in the center of the zone the day is one year long with six months of daylight and six months of night. The Frigid Zones are the coldest regions of Earth and are covered in ice. The concept of a zone was first hypothesized by the ancient Greek scholar Parmenides. Both philosophers theorized the Earth divided into three types of climatic zones based on their distance from the equator, like Parmeneides, thinking that the area near the equator was too hot for habitation, Aristotle dubbed the region around the equator the Torrid Zone. Both philosophers reasoned the region from the Arctic Circle to the pole to be permanently frozen and this region, thought uninhabitable, was called the Frigid Zone. The only area believed to be habitable was the northern Temperate Zone, however, humans have inhabited almost all climates on Earth, including inside the Arctic Circle. As knowledge of the Earths geography improved, a second Temperate Zone was discovered south of the equator, although Aristotles map was oversimplified, the general idea was correct. Circles of latitude Climate classification Tropics Subtropics Temperate Polar region Hardiness zone
Geographical zone
–
Map of annual average temperatures as a function of location.
Geographical zone
–
Köppen's map
5.
Sun
–
The Sun is the star at the center of the Solar System. It is a perfect sphere of hot plasma, with internal convective motion that generates a magnetic field via a dynamo process. It is by far the most important source of energy for life on Earth. Its diameter is about 109 times that of Earth, and its mass is about 330,000 times that of Earth, accounting for about 99. 86% of the total mass of the Solar System. About three quarters of the Suns mass consists of hydrogen, the rest is mostly helium, with smaller quantities of heavier elements, including oxygen, carbon, neon. The Sun is a G-type main-sequence star based on its spectral class and it formed approximately 4.6 billion years ago from the gravitational collapse of matter within a region of a large molecular cloud. Most of this matter gathered in the center, whereas the rest flattened into a disk that became the Solar System. The central mass became so hot and dense that it eventually initiated nuclear fusion in its core and it is thought that almost all stars form by this process. The Sun is roughly middle-aged, it has not changed dramatically for more than four billion years and it is calculated that the Sun will become sufficiently large enough to engulf the current orbits of Mercury, Venus, and probably Earth. The enormous effect of the Sun on Earth has been recognized since prehistoric times, the synodic rotation of Earth and its orbit around the Sun are the basis of the solar calendar, which is the predominant calendar in use today. The English proper name Sun developed from Old English sunne and may be related to south, all Germanic terms for the Sun stem from Proto-Germanic *sunnōn. The English weekday name Sunday stems from Old English and is ultimately a result of a Germanic interpretation of Latin dies solis, the Latin name for the Sun, Sol, is not common in general English language use, the adjectival form is the related word solar. The term sol is used by planetary astronomers to refer to the duration of a solar day on another planet. A mean Earth solar day is approximately 24 hours, whereas a mean Martian sol is 24 hours,39 minutes, and 35.244 seconds. From at least the 4th Dynasty of Ancient Egypt, the Sun was worshipped as the god Ra, portrayed as a falcon-headed divinity surmounted by the solar disk, and surrounded by a serpent. In the New Empire period, the Sun became identified with the dung beetle, in the form of the Sun disc Aten, the Sun had a brief resurgence during the Amarna Period when it again became the preeminent, if not only, divinity for the Pharaoh Akhenaton. The Sun is viewed as a goddess in Germanic paganism, Sól/Sunna, in ancient Roman culture, Sunday was the day of the Sun god. It was adopted as the Sabbath day by Christians who did not have a Jewish background, the symbol of light was a pagan device adopted by Christians, and perhaps the most important one that did not come from Jewish traditions
Sun
–
The Sun in visible wavelength with filtered white light on 8 July 2014. Characteristic
limb darkening and numerous
sunspots are visible.
Sun
–
During a total
solar eclipse, the solar
corona can be seen with the naked eye, during the brief period of totality.
Sun
–
Taken by
Hinode 's Solar Optical Telescope on 12 January 2007, this image of the Sun reveals the filamentary nature of the plasma connecting regions of different magnetic polarity.
Sun
–
Visible light photograph of sunspot, 13 December 2006
6.
Horizon
–
At many locations, the true horizon is obscured by trees, buildings, mountains, etc. and the resulting intersection of earth and sky is called the visible horizon. When looking at a sea from a shore, the part of the sea closest to the horizon is called the offing. The word horizon derives from the Greek ὁρίζων κύκλος horizōn kyklos, separating circle, from the verb ὁρίζω horizō, to divide, to separate, a pilot can also retain his or her spatial orientation by referring to the horizon. For observers near sea level the difference between this geometrical horizon and the horizon is imperceptible to the naked eye. In astronomy the horizon is the plane through the eyes of the observer. It is the plane of the horizontal coordinate system, the locus of points that have an altitude of zero degrees. While similar in ways to the horizon, in this context a horizon may be considered to be a plane in space. One typically sees further along the Earths curved surface than a simple geometric calculation allows for because of refraction error, the reverse happens if the ground is hotter than the air above it, as often happens in deserts, producing mirages. Examples, For an observer standing on the ground with h =1.70 metres, for an observer standing on the ground with h =2 metres, the horizon is at a distance of 5 kilometres. For an observer standing on a hill or tower of 100 metres in height, for an observer standing at the top of the Burj Khalifa, the horizon is at a distance of 103 kilometres. For an observer atop Mount Everest, the horizon is at a distance of 336 kilometres, with d in miles and h in feet, d ≈1.5 h ≈1.22 h. Examples, assuming no refraction, For an observer on the ground with eye level at h =5 ft 7 in, for an observer standing on a hill or tower 100 feet in height, the horizon is at a distance of 12.2 miles. For an observer on the summit of Aconcagua, the horizon to the west is at a distance of 184 miles. The secant-tangent theorem states that O C2 = O A × O B, the equation can also be derived using the Pythagorean theorem. Since the line of sight is a tangent to the Earth and this sets up a right triangle, with the sum of the radius and the height as the hypotenuse. Another relationship involves the distance s along the surface of the Earth to the horizon, with γ in radians, s = R γ. Solving for s gives s = R cos −1 R R + h, the distances d and s are nearly the same when the height of the object is negligible compared to the radius. If the observer is close to the surface of the earth, then it is valid to disregard h in the term, and the formula becomes d =2 R h
Horizon
–
A water horizon, in northern
Wisconsin, U.S.
Horizon
–
View of Earth's horizon as seen from
Space Shuttle Endeavour, 2002
Horizon
–
A view across a 20-km-wide bay in the coast of
Spain. Note the
curvature of the Earth hiding the base of the buildings on the far shore.
Horizon
–
The curvature of the horizon is easily seen in this photograph, taken from a space shuttle at an altitude of 226 km in 2008.
7.
Hour
–
An hour is a unit of time conventionally reckoned as 1⁄24 of a day and scientifically reckoned as 3, 599–3,601 seconds, depending on conditions. The seasonal, temporal, or unequal hour was established in the ancient Near East as 1⁄12 of the night or daytime, such hours varied by season, latitude, and weather. It was subsequently divided into 60 minutes, each of 60 seconds, the modern English word hour is a development of the Anglo-Norman houre and Middle English ure, first attested in the 13th century. It displaced the Old English tide and stound, the Anglo-Norman term was a borrowing of Old French ure, a variant of ore, which derived from Latin hōra and Greek hṓrā. Like Old English tīd and stund, hṓrā was originally a word for any span of time, including seasons. Its Proto-Indo-European root has been reconstructed as *yeh₁-, making hour distantly cognate with year, the time of day is typically expressed in English in terms of hours. Whole hours on a 12-hour clock are expressed using the contracted phrase oclock, Hours on a 24-hour clock are expressed as hundred or hundred hours. Fifteen and thirty minutes past the hour is expressed as a quarter past or after and half past, respectively, fifteen minutes before the hour may be expressed as a quarter to, of, till, or before the hour. Sumerian and Babylonian hours divided the day and night into 24 equal hours, the ancient Egyptians began dividing the night into wnwt at some time before the compilation of the Dynasty V Pyramid Texts in the 24th century BC. By 2150 BC, diagrams of stars inside Egyptian coffin lids—variously known as diagonal calendars or star clocks—attest that there were exactly 12 of these. The coffin diagrams show that the Egyptians took note of the risings of 36 stars or constellations. Each night, the rising of eleven of these decans were noted, the original decans used by the Egyptians would have fallen noticeably out of their proper places over a span of several centuries. By the time of Amenhotep III, the priests at Karnak were using water clocks to determine the hours and these were filled to the brim at sunset and the hour determined by comparing the water level against one of its twelve gauges, one for each month of the year. During the New Kingdom, another system of decans was used, the later division of the day into 12 hours was accomplished by sundials marked with ten equal divisions. The morning and evening periods when the failed to note time were observed as the first and last hours. The Egyptian hours were closely connected both with the priesthood of the gods and with their divine services, by the New Kingdom, each hour was conceived as a specific region of the sky or underworld through which Ras solar bark travelled. Protective deities were assigned to each and were used as the names of the hours, as the protectors and resurrectors of the sun, the goddesses of the night hours were considered to hold power over all lifespans and thus became part of Egyptian funerary rituals. The Egyptian for astronomer, used as a synonym for priest, was wnwty, the earliest forms of wnwt include one or three stars, with the later solar hours including the determinative hieroglyph for sun
Hour
–
Key concepts
Hour
–
Midnight on a 24-hour digital clock
8.
Midnight
–
Midnight is the transition time period from one day to the next, the moment when the date changes. In ancient Roman timekeeping, midnight was halfway between sunset and sunrise, varying according to the seasons, by clock time, midnight is the opposite of noon, differing from it by 12 hours. Solar midnight is the time opposite to solar noon, when the sun is closest to the nadir, due to the advent of time zones, which make time identical across a range of meridians, and daylight saving time, it rarely coincides with midnight on a clock. Solar midnight is dependent on longitude and time of the rather than on a time zone. In the northern hemisphere, midnight had an ancient geographic association with north, modern Polish and Belarusian preserve this association with its word for midnight, which also means north. Midnight marks the beginning and ending of each day in time throughout the world. It is the point between one day and another. In the United States and Canada, digital clocks and computers commonly display 12 a. m. right at midnight, while that phrase may be used practically, it helps to understand that any particular time is actually an instant. In other words,11,59 p. m. shows until midnight, at the instant of midnight, it changes to 12,00. Simultaneously, the p. m. changes to a. m. though, strictly speaking, a. m. does not apply to the instant of midnight which separates p. m. and a. m. In 24-hour time notation,0,00 and 0,00,00 refer to midnight at the start of a given date, some styles, such as ISO8601, allow 24,00 to refer to the end of a day. While computers and digital clocks display 12,00 a. m. and 12,00 p. m. those notations provide no clear, strictly speaking, it is actually incorrect to use a. m. and p. m. when referring to noon or midnight. The abbreviation a. m. stands for ante meridiem or before noon, since noon is neither after noon nor before noon, and midnight can equally be twelve hours before and after noon, neither abbreviation is correct. m. Some religious calendars continue to begin the day at another time — for example, at sunset in the Hebrew calendar, 12-hour clock Noon New Years Eve National Institute of Standards and Technology
Midnight
–
A photo taken at midnight
Midnight
–
One way for a digital clock to show midnight
9.
Noon
–
Noon is usually defined as 12 oclock in the daytime, as opposed to midnight. The term midday is also used colloquially to refer to a period of time in the middle of the day. Solar noon is when the Sun transits the celestial meridian and is at its highest altitude in the sky. The local or clock time of solar noon depends on the longitude, the opposite of noon is midnight. In many cultures in the Northern Hemisphere, noon had ancient geographic associations with the direction south, remnants of the noon = south association are preserved in the words for noon in French and Italian, both of which also refer to the southern parts of the respective countries. Modern Polish, Belarusian, Ukrainian, and Serbian go a step farther, with the words for noon also meaning south, the word noon is derived from Latin nona hora, the ninth hour of the day, and is related to the liturgical term none. The Roman and Western European medieval monastic day began at 6,00 a. m. at the equinox by modern timekeeping, in English, the meaning of the word shifted to midday and the time gradually moved back to 12,00 local time. The change began in the 12th century and was fixed by the 14th century, solar noon is the moment when the Sun transits the celestial meridian – roughly the time when it is highest above the horizon on that day. This is also the origin of the ante meridiem and post meridiem as noted below. The sun is overhead at solar noon at the Equator on the equinoxes, at the Tropic of Cancer on the June solstice. The elapsed time from local solar noon of one day to the day is exactly 24 hours only four instances in any particular year. This occurs when the effects of Earth’s obliquity of ecliptic and its orbital speed around the Sun offset each other and these four days for the current epoch are centered about Feb 11, May 13, July 25 and Nov 3. It occurs at one particular line of Longitude each event. This line varies year to year since Earth’s true year is not a number of days. This event time and location also varies due to Earth’s orbit being gravitationally perturbed by the planets and these four 24-hour days occur in both N and S hemispheres simultaneously. The precise UTC times for four days also mark when the opposite line of longitude 180 degrees away experiences precisely 24 hours from local midnight to local midnight the next day. Thus four varying great circles of Longitude define from year to year when a 24-hour day occurs, the two longest time spans from noon to noon occur twice each year around Jun 20 and around Dec 21. The shortest time spans occur twice each year around March 25, noon is commonly indicated by 12 p. m. and midnight by 12 a. m
Noon
–
A clock tower showing 12 o'clock noon
10.
Polar circle
–
A polar circle is either the Arctic Circle or the Antarctic Circle. On Earth, the Arctic Circle is located at a latitude of 66°33′46. 6″ N, the latitude of the polar circles is 90 degrees minus the axial tilt of the Earths axis of daily rotation relative to the ecliptic, the plane of the Earths orbit. This tilt varies slightly, a described as nutation. Therefore, the latitudes noted above are calculated by averaging values of tilt observed over many years, the axial tilt also exhibits long-term variations as described in the reference article. Arctic Circle Antarctic Circle Polar region Arctic Antarctica Frigid Zones Polar climate Polar day & Polar night
Polar circle
–
The polar circle in Finland, 1975.
Polar circle
–
The polar circle in Norway at
Saltfjellet mountain plateau.
11.
Northern Hemisphere
–
The Northern Hemisphere is the half of Earth that is north of the equator. For other planets in the Solar System, north is defined as being in the celestial hemisphere relative to the invariable plane of the solar system as Earths North pole. Due to the Earths axial tilt, winter in the Northern Hemisphere lasts from the December solstice to the March Equinox, the dates vary each year due to the difference between the calendar year and the astronomical year. Its surface is 60. 7% water, compared with 80. 9% water in the case of the Southern Hemisphere, the Arctic is the region north of the Arctic Circle. Its climate is characterized by cold winters and cool summers, precipitation mostly comes in the form of snow. The Arctic experiences some days in summer when the Sun never sets, the duration of these phases varies from one day for locations right on the Arctic Circle to several months near the North Pole, which is the middle of the Northern Hemisphere. Between the Arctic Circle and the Tropic of Cancer lies 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 are generally hot all year round and tend to experience a rainy season during the summer months, and a dry season during 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 circulation patterns in the North Atlantic. For the same reason, flows of air down toward the 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 tends to draw air toward it in a counterclockwise pattern. Hurricanes and tropical storms spin counter-clockwise in the Northern Hemisphere, the shadow of a sundial moves clockwise in the Northern Hemisphere. 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. The Northern Hemisphere is home to approximately 6.57 billion people which is around 90% of the total human population of 7.3 billion people
Northern Hemisphere
–
Northern Hemisphere highlighted in blue
12.
Arctic Circle
–
The Arctic Circle is the most northerly of the abstract five major circles of latitude as shown on maps of the Earth. The region north of this circle is known as the Arctic, the position of the Arctic Circle is not fixed, as of 7 April 2017, it runs 66°33′46. 6″ north of the Equator. Its latitude depends on the Earths axial tilt, which fluctuates within a margin of 2° over a 40, 000-year period, consequently, the Arctic Circle is currently drifting northwards at a speed of about 15 m per year. The word arctic comes from the Greek word ἀρκτικός and that from the word ἄρκτος, directly on the Arctic Circle these events occur, in principle, exactly once per year, at the June and December solstices, respectively. That is true at sea level, those limits increase with elevation above sea level, tens of thousands of years ago, waves of people migrated from eastern Siberia across the Bering Strait into North America to settle. Much later, in the period, there has been migration into some Arctic areas by Europeans. The largest communities north of the Arctic Circle are situated in Russia and Norway, Murmansk, Norilsk, Tromsø, rovaniemi in Finland is the largest settlement in the immediate vicinity of the Arctic Circle, lying slightly south of the line. In contrast, the largest North American community north of the Arctic Circle, of the Canadian and United States Arctic communities, Barrow, Alaska is the largest settlement with about 4,000 inhabitants. The Arctic Circle is roughly 16,000 kilometres, the area north of the Circle is about 20,000,000 km2 and covers roughly 4% of Earths surface. The Arctic Circle passes through the Arctic Ocean, the Scandinavian Peninsula, North Asia, Northern America, the land within the Arctic Circle is divided among eight countries, Norway, Sweden, Finland, Russia, the United States, Canada, Denmark, and Iceland. In the interior, summers can be warm, while winters are extremely cold
Arctic Circle
–
A sign along the
Dalton Highway marking the location of the Arctic Circle in Alaska.
Arctic Circle
–
Map of the
Arctic, with the Arctic Circle in blue and the July 10 °C mean
isotherm in red
Arctic Circle
–
Arctic Circle line in
Rovaniemi,
Finland
Arctic Circle
–
Aurora Borealis above Arctic Circle sign along the
Dempster Highway in
Yukon,
Canada
13.
Equator
–
The Equator usually refers to an imaginary line on the Earths surface equidistant from the North Pole and South Pole, dividing the Earth into the Northern Hemisphere and Southern Hemisphere. The Equator is about 40,075 kilometres long, some 78. 7% lies across water and 21. 3% over land, other planets and astronomical bodies have equators similarly defined. Generally, an equator is the intersection of the surface of a sphere with the plane that is perpendicular to the spheres axis of rotation. The latitude of the Earths equator is by definition 0° of arc, the equator is the only line of latitude which is also a great circle — that is, one whose plane passes through the center of the globe. The plane of Earths equator when projected outwards to the celestial sphere defines the celestial equator, in the cycle of Earths seasons, the plane of the equator passes through the Sun twice per year, at the March and September equinoxes. To an observer on the Earth, the Sun appears to travel North or South over the equator at these times, light rays from the center of the Sun are perpendicular to the surface of the Earth at the point of solar noon on the Equator. Locations on the Equator experience the quickest sunrises and sunsets because the sun moves nearly perpendicular to the horizon for most of the year. The Earth bulges slightly at the Equator, the diameter of the Earth is 12,750 kilometres. Because the Earth spins to the east, spacecraft must also launch to the east to take advantage of this Earth-boost of speed, seasons result from the yearly revolution of the Earth around the Sun and the tilt of the Earths axis relative to the plane of revolution. During the year the northern and southern hemispheres are inclined toward or away from the sun according to Earths position in its orbit, the hemisphere inclined toward the sun receives more sunlight and is in summer, while the other hemisphere receives less sun and is in winter. At the equinoxes, the Earths axis is not tilted toward the sun, instead it is perpendicular to the sun meaning that the day is about 12 hours long, as is the night, across the whole of the Earth. Near the Equator there is distinction between summer, winter, autumn, or spring. The temperatures are usually high year-round—with the exception of high mountains in South America, the temperature at the Equator can plummet during rainstorms. In many tropical regions people identify two seasons, the wet season and the dry season, but many places close to the Equator are on the oceans or rainy throughout the year, the seasons can vary depending on elevation and proximity to an ocean. The Equator lies mostly on the three largest oceans, the Pacific Ocean, the Atlantic Ocean, and the Indian Ocean. The highest point on the Equator is at the elevation of 4,690 metres, at 0°0′0″N 77°59′31″W and this is slightly above the snow line, and is the only place on the Equator where snow lies on the ground. At the Equator the snow line is around 1,000 metres lower than on Mount Everest, the Equator traverses the land of 11 countries, it also passes through two island nations, though without making a landfall in either. Starting at the Prime Meridian and heading eastwards, the Equator passes through, Despite its name, however, its island of Annobón is 155 km south of the Equator, and the rest of the country lies to the north
Equator
–
Left: A monument marking the Equator near the town of
Pontianak, Indonesia Right: Road sign marking the Equator near
Nanyuki, Kenya
Equator
Equator
–
The Equator marked as it crosses
Ilhéu das Rolas, in São Tomé and Príncipe
14.
Axial tilt
–
In astronomy, axial tilt, also known as obliquity, is the angle between an objects rotational axis and its orbital axis, or, equivalently, the angle between its equatorial plane and orbital plane. At an obliquity of zero, the two axes point in the direction, i. e. the rotational axis is perpendicular to the orbital plane. Over the course of an orbit, the obliquity usually does not change considerably, and this causes one pole to be directed more toward the Sun on one side of the orbit, and the other pole on the other side — the cause of the seasons on the Earth. Earths obliquity oscillates between 22.1 and 24.5 degrees on a 41, 000-year cycle, the mean obliquity is currently 23°26′13. 4″. There are two methods of specifying tilt. The IAU also uses the rule to define a positive pole for the purpose of determining orientation. Using this convention, Venus is tilted 177° and it is denoted by the Greek letter ε. Earth currently has a tilt of about 23. 4°. This value remains about the relative to a stationary orbital plane throughout the cycles of axial precession. But the ecliptic due to planetary perturbations, and the obliquity of the ecliptic is not a fixed quantity. At present, it is decreasing at a rate of about 47″ per century, Earths obliquity may have been reasonably accurately measured as early as 1100 BC in India and China. The ancient Greeks had good measurements of the obliquity since about 350 BC, about 830 AD, the Caliph Al-Mamun of Baghdad directed his astronomers to measure the obliquity, and the result was used in the Arab world for many years. It was widely believed, during the Middle Ages, that both precession and Earths obliquity oscillated around a value, with a period of 672 years. Earths axis remains tilted in the direction with reference to the background stars throughout a year. This means that one pole will be directed away from the Sun at one side of the orbit and this is the cause of Earths seasons. Summer occurs in the Northern hemisphere when the pole is directed toward the Sun. Variations in Earths axial tilt can influence the seasons and is likely a factor in climate change. The exact angular value of the obliquity is found by observation of the motions of Earth, from 1984, the Jet Propulsion Laboratorys DE series of computer-generated ephemerides took over as the fundamental ephemeris of the Astronomical Almanac
Axial tilt
Axial tilt
15.
Tidal force
–
The tidal force is a force that is the secondary effect of the force of gravity, it is responsible for the phenomenon of tides. It arises because the force exerted by one body on another is not constant across it. Thus, the force is differential. Consider the gravitational attraction of the Moon on the oceans nearest to the Moon, the solid Earth, there is a mutual attraction between the Moon and the solid Earth, which can be considered to act on its centre of mass. However, the oceans are more strongly attracted and, especially since they are fluid, they approach the Moon slightly. The far oceans are attracted less, viewing the Earth as a whole, we see that all its mass experiences a mutual attraction with that of the Moon, but the near oceans more so than the far oceans, leading to a separation of the two. When a body is acted on by the gravity of another body, Figure 2 shows the differential force of gravity on a spherical body exerted by another body. These so-called tidal forces cause strains on both bodies and may distort them or even, in cases, break one or the other apart. These strains would not occur if the field were uniform, because a uniform field only causes the entire body to accelerate together in the same direction. In the case of a small elastic sphere, the effect of a tidal force is to distort the shape of the body without any change in volume. The sphere becomes an ellipsoid with two bulges, pointing towards and away from the other body, larger objects distort into an ovoid, and are slightly compressed, which is what happens to the Earths oceans under the action of the Moon. The Earth and Moon rotate about their center of mass or barycenter. To an observer on the Earth, very close to this barycenter, all parts of the Earth are subject to the Moons gravitational forces, causing the water in the oceans to redistribute, forming bulges on the sides near the Moon and far from the Moon. When a body rotates while subject to forces, internal friction results in the gradual dissipation of its rotational kinetic energy as heat. In the case for the Earth, and Earths Moon, the loss of kinetic energy results in a gain of about 2 milliseconds per century. If the body is enough to its primary, this can result in a rotation which is tidally locked to the orbital motion. Tidal heating produces dramatic volcanic effects on Jupiters moon Io, stresses caused by tidal forces also cause a regular monthly pattern of moonquakes on Earths Moon. Tidal forces contribute to ocean currents, which moderate global temperatures by transporting heat energy toward the poles and it has been suggested that in addition to other factors, harmonic beat variations in tidal forcing may contribute to climate changes
Tidal force
–
Figure 1:
Comet Shoemaker-Levy 9 in 1994 after breaking up under the influence of
Jupiter 's tidal forces during a previous pass in 1992.
Tidal force
Tidal force
–
Figure 3:
Saturn 's rings are inside the orbits of its principal moons. Tidal forces oppose gravitational coalescence of the material in the rings to form moons.
Tidal force
–
Tidal force is responsible for the merge of galactic pair MRK 1034.
16.
Orbit of the Moon
–
Not to be confused with Lunar orbit. The Moon orbits Earth in the direction and completes one revolution relative to the stars in approximately 27.323 days. Earth and the Moon orbit about their barycentre, which lies about 4,600 km from Earths center, on average, the Moon is at a distance of about 385,000 km from Earths centre, which corresponds to about 60 Earth radii. With a mean velocity of 1.022 km/s, the Moon appears to move relative to the stars each hour by an amount roughly equal to its angular diameter. The Moon differs from most satellites of planets in that its orbit is close to the plane of the ecliptic. The plane of the orbit is inclined to the ecliptic by about 5°. The properties of the orbit described in this section are approximations, the Moons orbit around Earth has many irregularities, whose study has a long history. The orbit of the Moon is distinctly elliptical, with an eccentricity of 0.0549. The non-circular form of the lunar orbit causes variations in the Moons angular speed and apparent size as it moves towards, the mean angular movement relative to an imaginary observer at the barycentre is 13. 176° per day to the east. The Moons elongation is its angular distance east of the Sun at any time, at new moon, it is zero and the Moon is said to be in conjunction. At full moon, the elongation is 180° and it is said to be in opposition, in both cases, the Moon is in syzygy, that is, the Sun, Moon and Earth are nearly aligned. When elongation is either 90° or 270°, the Moon is said to be in quadrature, the orientation of the orbit is not fixed in space, but rotates over time. This orbital precession is also called apsidal precession and is the rotation of the Moons orbit within the orbital plane, the Moons apsidal precession is distinct from, and should not be confused with its axial precession. The mean inclination of the orbit to the ecliptic plane is 5. 145°. The rotational axis of the Moon is also not perpendicular to its plane, so the lunar equator is not in the plane of its orbit. Therefore, the angle between the ecliptic and the equator is always 1. 543°, even though the rotational axis of the Moon is not fixed with respect to the stars. The period from moonrise to moonrise at the poles is quite close to the sidereal period, when the sun is the furthest below the horizon, the moon will be full when it is at its highest point. The nodes are points at which the Moons orbit crosses the ecliptic, the Moon crosses the same node every 27.2122 days, an interval called the draconic or draconitic month
Orbit of the Moon
–
Earth's lunar orbit perturbations
Orbit of the Moon
–
Orbit of the Moon
Orbit of the Moon
–
Physical lunar features
17.
Midnight sun
–
The midnight sun is a natural phenomenon that occurs in the summer months in places north of the Arctic Circle or south of the Antarctic Circle, when the sun remains visible at the local midnight. Around the summer solstice the sun is visible for the full 24 hours, the number of days per year with potential midnight sun increases the farther towards either pole one goes. A quarter of Finlands territory lies north of the Arctic Circle, in Svalbard, Norway, the northernmost inhabited region of Europe, there is no sunset from approximately 19 April to 23 August. The extreme sites are the poles, where the sun can be visible for half the year. The opposite phenomenon, polar night, occurs in winter, when the sun stays below the horizon throughout the day, since the axial tilt of the Earth is considerable, the sun does not set at high latitudes in local summer. The duration of sunlight increases from one day during the solstice at the polar circle, to several weeks only 100 km closer to the pole. At extreme latitudes, the sun is usually referred to as polar day. At the poles themselves, the sun rises and sets only once each year, for example, Iceland is known for its midnight sun, even though most of it is slightly south of the Arctic Circle. For the same reasons, the period of sunlight at the poles is slightly longer than six months, even the northern extremities of Scotland experience twilight in the northern sky at around the summer solstice. Observers at heights appreciably above sea level can experience extended periods of midnight sun as a result of the dip of the horizon viewed from altitude, the term midnight sun refers to the consecutive 24-hour periods of sunlight experienced north of the Arctic Circle and south of the Antarctic Circle. Other phenomena are sometimes referred to as midnight sun, but they are caused by time zones, for instance, in Fairbanks, Alaska, which is south of the Arctic Circle, the sun sets at 12,47 am at the summer solstice. This is because Fairbanks is 51 minutes ahead of its time zone. This means that solar culmination occurs at about 1,51 pm instead of at 12 noon, if a precise moment for the genuine midnight sun is required, the observers longitude, the local civil time and the equation of time must be taken into account. The moment of the suns closest approach to the horizon coincides with its due north at the observers position. These two effects must be added, furthermore, the equation of time must be added, a positive value on a given date means that the sun is running slightly ahead of its average position, so the value must be subtracted. The equation of time at that date is -2.0 minutes, therefore, the suns lowest elevation occurs 120 -103.2 +2.0 minutes after midnight, at 00.19 Central European Summer time. On other nearby dates the only thing different is the equation of time, the suns altitude remains within half a degree of the minimum of about 5 degrees for about 45 minutes either side of this time. The midnight sun is visible at the Arctic Circle from 12 June until 1 July and this period extends as one travels north, At Cape Nordkinn, Norway, the northernmost point of Continental Europe, the midnight sun lasts approximately from 14 May to 29 July
Midnight sun
–
The midnight sun at
Nordkapp,
Norway
Midnight sun
–
The
Altafjord in
Alta,
Norway bathed in the Midnight Sun
Midnight sun
–
Midnight sun in
Kiruna,
Sweden
18.
Polar night
–
The polar night occurs when the night lasts for more than 24 hours. This occurs only inside the polar circles, the opposite phenomenon, the polar day, or midnight sun, occurs when the Sun stays above the horizon for more than 24 hours. Night is understood as the center of the Sun being below a free horizon. Since the atmosphere bends the rays of the Sun, the day is longer than the polar night. The polar circle is located at a latitude between two areas, at the latitude of approximately 66.5 degrees. In the most northern city of Sweden, Kiruna, at 67°51N, while it is day in the Arctic Circle it is night in the Antarctic Circle. The polar shortest day is not totally dark everywhere inside the circle, but only in places within about 5° of the poles. Regions located at the border of the polar circles experience polar twilight instead of polar night. In fact, polar regions typically get more twilight throughout the year than equatorial regions, however, not all this time is classified as polar night since sunlight may be visible because of refraction. Also, the time when the Sun is above the horizon at the poles is 186 days, the numerical asymmetry occurs because the time when the Sun is partially above the horizon is counted towards the daytime. Also, the numbers are average numbers, the ellipticity of the Earths orbit makes the South Pole receive a week more of Sun-below-horizon than the North Pole. As there are kinds of twilight, there also exist various kinds of polar night. Each kind of night is defined as when its darker than the corresponding kind of twilight. The descriptions below are based on relatively clear skies, so the sky will be darker in the presence of dense clouds. Polar twilight occurs in areas that are located at the border of the polar circles. There is then no true daylight at the culmination, only civil twilight. This means that the Sun is below the horizon, but by less than 6°, during civil twilight, there may still be enough light for normal outdoor activities because of light scattering by the upper atmosphere and refraction. Thus, polar twilight is limited to more than 66.5 degrees from the equator, in other words
Polar night
–
Characteristic polar night blue twilight,
Longyearbyen,
Svalbard,
Norway located at 78° north.
Polar night
–
Polar night at the
South Pole,
Antarctica.
Polar night
–
Polar night in
Naryan-Mar,
Russia. December 23, 2014, 11:27 (noon)
Polar night
–
Early afternoon during the polar night in
Tromsø,
Norway.
19.
Southern Hemisphere
–
The Southern Hemisphere is the half sphere of Earth which is south of the equator. It contains all or parts of five continents, four oceans and its surface is 80. 9% water, compared with 60. 7% water in the case of the Northern Hemisphere, and it contains 32. 7% of Earths land. Due to the tilt of Earths rotation relative to the Sun, September 22 or 23 is the vernal equinox and March 20 or 21 is the autumnal equinox. The South Pole is in the middle of the southern hemispherical region, Southern Hemisphere climates tend to be slightly milder than those at similar latitudes in the Northern Hemisphere, except in the Antarctic which is colder than the Arctic. This is because the Southern Hemisphere has significantly more ocean and much land, water heats up. In the Southern Hemisphere the sun passes from east to west through the north, sun-cast shadows turn anticlockwise throughout the day and sundials have the hours increasing in the anticlockwise direction. Cyclones and tropical storms spin clockwise in the Southern Hemisphere due to the Coriolis effect, the southern temperate zone, a subsection of the Southern Hemisphere, is nearly all oceanic. Forests in the Southern Hemisphere have special features which set apart from those in the Northern Hemisphere. Both Chile and Australia share, for example, unique species or Nothofagus. The eucalyptus is native to Australia but is now planted in Southern Africa and Latin America for pulp production and, increasingly. Approximately 800,000,000 humans live in the Southern Hemisphere and this is due to the fact that there is significantly less land in the Southern Hemisphere than in the Northern Hemisphere. Africa Antarctica Asia Australia South America Zealandia Media related to Southern Hemisphere at Wikimedia Commons
Southern Hemisphere
–
A famous photo of Earth from
Apollo 17 (
Blue Marble) originally had the south pole at the top; however, it was turned upside-down to fit the traditional perspective
Southern Hemisphere
–
Aurora australis appearing in the night sky of Swifts Creek, 100 km (62 mi) north of
Lakes Entrance,
Victoria,
Australia
Southern Hemisphere
–
Aurora australis appearing from
Stewart Island / Rakiura at the southern point of the
South Island of
New Zealand
20.
December solstice
–
The December solstice, also known as the southern solstice, is the solstice that occurs each December, typically between the 20th and the 22nd day of the month according to the Gregorian calendar. In the southern hemisphere, the December solstice is the summer solstice and it is thus the length of time between adjacent December solstices. The length of the December solstice year has been relatively stable between 6000 BC and 2000 at 49,30 to 50,00 in excess of 365 days and 5 hours, after 2000 it is getting shorter. In 4000 the excess time will be 48,52 and in 1000046,45, the following tables contain information on the length of the day on the winter solstice of the Northern Hemisphere and the summer solstice of the Southern Hemisphere. The data was collected from the website of the Finnish Meteorological Institute on 22 December 2015, the data is arranged geographically and within the tables from the shortest day to the longest one. The figures in the show the differences between the Gregorian calendar and Persian Jalāli calendar in reference to the actual yearly time of the Southern solstice. The date of the solstice is not the same as the date of the latest sunrise, because the Earth is moving along its solar orbital path, for each solar day the Earth has to do more than one full rotation. Because the Earths orbit is elliptical, the speed at which the Earth moves along its orbit varies, consequently, solar days are not the same length throughout the year. Mean time is our way of modifying this, for our convenience, making each day the same length, the maximum correction is ±15 minutes to the mean but its value changes quite rapidly around the solstices. Christmas Dies Natalis Solis Invicti Brumalia Dongzhi Festival
December solstice
–
Gregorian calendar
21.
Solstice
–
The Solstice occurs twice each year as the Sun reaches its most northerly or southerly excursion relative to the celestial equator on the celestial sphere. The seasons of the year are directly connected to both the solstices and the equinoxes, the term solstice can also be used in a broader sense, as the day when this occurs. The day of the solstice has either the most sunlight of the year or the least sunlight of the year for any other than the equator. Alternative terms, with no ambiguity as to which hemisphere is the context, are June solstice and December solstice, at latitudes outside the tropics, the summer solstice marks the day when the sun appears highest in the sky. Within the tropics, the sun directly overhead from days to months before the solstice and again after the solstice. For an observer on the North Pole, the sun reaches the highest position in the sky once a year in June, the day this occurs is called the June solstice day. Similarly, for an observer on the South Pole, the sun reaches the highest position on December solstice day, when it is the summer solstice at one Pole, it is the winter solstice on the other. The suns westerly motion never ceases as the Earth is continually in rotation, however, the suns motion in declination comes to a stop at the moment of solstice. In that sense, solstice means sun-standing and this modern scientific word descends from a Latin scientific word in use in the late Roman republic of the 1st century BC, solstitium. Pliny uses it a number of times in his Natural History with a meaning that it has today. It contains two Latin-language morphemes, sol, sun, and -stitium, stoppage, the Romans used standing to refer to a component of the relative velocity of the Sun as it is observed in the sky. Relative velocity is the motion of an object from the point of view of an observer in a frame of reference, from a fixed position on the ground, the sun appears to orbit around the Earth. To an observer in a frame of reference, the planet Earth is seen to rotate about an axis. The Earths axis is tilted with respect to the plane of the Earths orbit, an observer on Earth therefore sees a solar path that is the result of both rotation and revolution. At maximum or minimum elevation, the relative motion of the Sun perpendicular to the horizon stops. Outside of the tropics, the maximum occurs at the summer solstice. The path of the Sun, or ecliptic, sweeps north and south between the northern and southern hemispheres, the days are longer around the summer solstice and shorter around the winter solstice. When the Suns path crosses the equator, the length of the nights at latitudes +L° and -L° are of equal length and this is known as an equinox
Solstice
Solstice
–
Diagram of the Earth's seasons as seen from the north. Far right:
southern solstice
Solstice
–
Diagram of the Earth's seasons as seen from the south. Far left:
northern solstice
Solstice
–
Two images showing the amount of reflected sunlight at southern and northern summer solstices respectively (watts / m²).
22.
Atmospheric refraction
–
Atmospheric refraction is the deviation of light or other electromagnetic wave from a straight line as it passes through the atmosphere due to the variation in air density as a function of height. This refraction is due to the velocity of light through air decreasing with increased density, Atmospheric refraction near the ground produces mirages and can make distant objects appear to shimmer or ripple, elevated or lowered, stretched or shortened with no mirage involved. The term also applies to the refraction of sound, Atmospheric refraction is considered in measuring the position of both astronomical and terrestrial objects. Astronomical or celestial refraction causes astronomical objects to higher in the sky than they are in reality. Refraction not only affects lightrays but all electromagnetic radiation, although in varying degrees, for example, in visible light, blue is more affected than red. This may cause objects to be spread out into a spectrum in high-resolution images. Whenever possible, astronomers will schedule their observations around the time of culmination of an object when it is highest in the sky, likewise sailors will never shoot a star which is not at least 20° or more above the horizon. If observations close to the horizon cannot be avoided, it is possible to equip a telescope with control systems to compensate for the shift caused by the refraction, if the dispersion is a problem too, atmospheric refraction correctors can be employed as well. Surveyors, on the hand, will often schedule their observations in the afternoon when the magnitude of refraction is minimum. Atmospheric refraction becomes more severe when there are strong temperature gradients and this is the cause of twinkling of the stars and various deformations of the shape of the sun at sunset and sunrise. On the horizon refraction is slightly greater than the apparent diameter of the Sun, so when the bottom of the disc appears to touch the horizon. If the atmosphere suddenly vanished, the sun would too, the altitude of a celestial body is normally given for the center of the body’s disc. In the case of the Moon, additional corrections are needed for the Moon’s horizontal parallax and its apparent semi-diameter, at and below the horizon, values of refraction significantly higher than the nominal value of 35. 4′ have been observed in a wide range of climates. A glow on the northern horizon resolved itself into the sun at 11 am that day. A quarter of an hour later the unreasonable visitor disappeared again, only to rise again at 11,40 am, set at 1 pm, rise at 1,10 pm and these curious phenomena were due to refraction which amounted to 2°37 at 1,20 pm. The temperature was 15° below 0° Fahr. and we calculated that the refraction was 2° above normal, because atmospheric refraction is nominally 34′ on the horizon, but only 29′ at 0. 5° above it, the setting or rising sun seems to be flattened by about 5′. Young distinguished several regions where different methods for calculating astronomical refraction were applicable, in the upper portion of the sky, with a zenith distance of less than 70°, various simple refraction formulas based on the index of refraction at the observer are adequate. Closer to the horizon, actual measurements of the changes with height of the temperature gradient need to be employed in the numerical integration
Atmospheric refraction
–
Plot of refraction vs. altitude using Bennett’s 1982 formula
23.
Mirage
–
A mirage is a naturally occurring optical phenomenon in which light rays are bent to produce a displaced image of distant objects or the sky. The word comes to English via the French mirage, from the Latin mirari, meaning to look at and this is the same root as for mirror and to admire. In contrast to a hallucination, a mirage is an optical phenomenon that can be captured on camera. What the image appears to represent, however, is determined by the faculties of the human mind. For example, inferior images on land are very easily mistaken for the reflections from a body of water. For exhausted travelers in the desert, an inferior mirage may appear to be a lake of water in the distance, an inferior mirage is called inferior because the mirage is located under the real object. The real object in a mirage is the sky or any distant object in that same direction. The mirage causes the observer to see a bright and bluish patch on the ground in the distance, light rays coming from a particular distant object all travel through nearly the same air layers and all are bent over about the same amount. Therefore, rays coming from the top of the object will arrive lower than those from the bottom, the image usually is upside down, enhancing the illusion that the sky image seen in the distance is really a water or oil puddle acting as a mirror. Hot air rises, and cooler air descends, so the layers will mix, the image will be distorted accordingly. It may be vibrating, it may be extended or horizontally extended. If there are several layers, several mirages may mix. In any case, mirages are usually not larger than half a degree high. When appearing on roads due to the hot asphalt, it is referred to as a highway mirage. This produces a blurred shimmering effect, which affects the ability to resolve objects, light from the sky at a shallow angle to the road is refracted by the index gradient, making it appear as if the sky is reflected by the roads surface. The mind interprets this as a pool of water on the road, the illusion fades as one gets closer. On tarmac roads it may look as if water, or even oil, has been spilled and these kinds of inferior mirages are often called desert mirages or highway mirages. Heat haze is not related to the phenomenon of haze
Mirage
Mirage
–
An inferior mirage on the
Mojave Desert in spring
Mirage
–
Heat haze caused by jet engine exhaust
Mirage
–
A hot-road mirage, "fake water" on the road, the most common example of an inferior mirage
24.
Summer solstice
–
The summer solstice, also known as midsummer, occurs when a planets rotational axis, in either northern or southern hemispheres, is most inclined toward the star that it orbits. Earths maximum axial tilt toward the Sun is 23° 26′ and this happens twice each year, at which times the Sun reaches its highest position in the sky as seen from the north or the south pole. The summer solstice occurs during the hemispheres summer and this is the northern solstice in the northern hemisphere and the southern solstice in the southern hemisphere. The same dates in the opposite hemisphere are referred to as the winter solstice, when on a geographic pole, the Sun reaches its greatest height at the moment of solstice. It can be noon only along that longitude which at that moment lies in the direction of the Sun from the pole, for other longitudes, it is not noon. Noon has either passed or has yet to come, hence the notion of a solstice day is useful. The term is used like midsummer to refer to the day on which solstice occurs. The summer solstice day has the longest period of daylight – except in the polar regions,2016 was the first time in nearly 70 years that a full moon and the Northern Hemispheres summer solstice concur on the same day. The 2016 summer solstices full moon rose just as the Sun set, worldwide, interpretation of the event has varied among cultures, but most recognize the event in some way with holidays, festivals, and rituals around that time with themes of religion or fertility. In some regions, the solstice is seen as the beginning of summer. In other cultural conventions, the solstice is closer to the middle of summer, Solstice is derived from the Latin words sol and sistere. Day of Private Reflection Fremont Solstice Parade Guru Purnima Christmas typically marks the end of the solstice in the southern hemisphere. The data was collected from the website of the Finnish Meteorological Institute on 20 June 2016 as well as certain other websites. The data is arranged geographically and within the tables from the longest day to the shortest one, day length Equinox Midsummers day Stonehenge Winter solstice Xiazhi Table of dates/times from 1600-2400
Summer solstice
–
Diagram of the
Earth 's seasons as seen from the north. Far left: summer
solstice for the Northern Hemisphere. Front right: summer
solstice for the Southern Hemisphere.
25.
Minute and second of arc
–
A minute of arc, arcminute, arc minute, or minute arc is a unit of angular measurement equal to 1/60 of one degree. Since one degree is 1/360 of a turn, one minute of arc is 1/21600 of a turn, a second of arc, arcsecond, or arc second is 1/60 of an arcminute, 1/3600 of a degree, 1/1296000 of a turn, and π/648000 of a radian. To express even smaller angles, standard SI prefixes can be employed, the number of square arcminutes in a complete sphere is 4 π2 =466560000 π ≈148510660 square arcminutes. The standard symbol for marking the arcminute is the prime, though a single quote is used where only ASCII characters are permitted. One arcminute is thus written 1′ and it is also abbreviated as arcmin or amin or, less commonly, the prime with a circumflex over it. The standard symbol for the arcsecond is the prime, though a double quote is commonly used where only ASCII characters are permitted. One arcsecond is thus written 1″ and it is also abbreviated as arcsec or asec. In celestial navigation, seconds of arc are used in calculations. This notation has been carried over into marine GPS receivers, which normally display latitude and longitude in the format by default. An arcsecond is approximately the angle subtended by a U. S. dime coin at a distance of 4 kilometres, a milliarcsecond is about the size of a dime atop the Eiffel Tower as seen from New York City. A microarcsecond is about the size of a period at the end of a sentence in the Apollo mission manuals left on the Moon as seen from Earth, since antiquity the arcminute and arcsecond have been used in astronomy. The principal exception is Right ascension in equatorial coordinates, which is measured in units of hours, minutes. These small angles may also be written in milliarcseconds, or thousandths of an arcsecond, the unit of distance, the parsec, named from the parallax of one arcsecond, was developed for such parallax measurements. It is the distance at which the radius of the Earths orbit would subtend an angle of one arcsecond. The ESA astrometric space probe Gaia is hoped to measure star positions to 20 microarcseconds when it begins producing catalog positions sometime after 2016, there are about 1.3 trillion µas in a turn. Currently the best catalog positions of stars actually measured are in terms of milliarcseconds, apart from the Sun, the star with the largest angular diameter from Earth is R Doradus, a red supergiant with a diameter of 0.05 arcsecond. The dwarf planet Pluto has proven difficult to resolve because its angular diameter is about 0.1 arcsecond, space telescopes are not affected by the Earths atmosphere but are diffraction limited. For example, the Hubble space telescope can reach a size of stars down to about 0. 1″
Minute and second of arc
–
Comparison of angular diameter of the Sun, Moon, planets and the International Space Station. To get a true representation of the sizes, view the image at a distance of 103 times the width of the "Moon: max." circle. For example, if this circle is 10 cm wide on your monitor, view it from 10.3 m away.
26.
Winter solstice
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The winter solstice, also known as midwinter, is an astronomical phenomenon marking the day with the shortest period of daylight and the longest night of the year. In the Northern Hemisphere this is the December solstice and in the Southern Hemisphere this is the June solstice. The axial tilt of Earth and gyroscopic effects of its daily rotation mean that the two points in the sky to which the Earths axis of rotation points change very slowly. As the Earth follows its orbit around the Sun, the hemisphere that faced away from the Sun, experiencing winter, will, in half a year, face towards the Sun. This is because the two hemispheres face opposite directions along Earths axis, and so as one polar hemisphere experiences winter, although the winter solstice itself lasts only a moment in time, the term sometimes refers to the day on which it occurs. Other names are midwinter, the extreme of winter, or the shortest day, in some cultures it is seen as the middle of winter, while in others it is seen as the beginning of winter. In meteorology, winter in the Northern Hemisphere spans the period of December through February. The seasonal significance of the solstice is in the reversal of the gradual lengthening of nights. The solstice may have been a moment of the annual cycle for some cultures even during neolithic times. Astronomical events were used to guide activities such as the mating of animals, the sowing of crops. Many cultural mythologies and traditions are derived from this and this is attested by physical remains in the layouts of late Neolithic and Bronze Age archaeological sites, such as Stonehenge in England and Newgrange in Ireland. The primary axes of both of these seem to have been carefully aligned on a sight-line pointing to the winter solstice sunrise. It is significant that at Stonehenge the Great Trilithon was erected outwards from the middle of the monument, the winter solstice was immensely important because the people were economically dependent on monitoring the progress of the seasons. Starvation was common during the first months of the winter, January to April or July to October, in temperate climates, the midwinter festival was the last feast celebration, before deep winter began. Most cattle were slaughtered so they would not have to be fed during the winter, the majority of wine and beer made during the year was finally fermented and ready for drinking at this time. The concentration of the observances were not always on the day commencing at midnight or at dawn, but at the beginning of the pagan day, also reversal is yet another frequent theme, as in Saturnalias slave and master reversals. Iranian people celebrate the night of the Northern Hemispheres winter solstice as, Yalda night, in this night all the family gather together, usually at the house of the oldest, and celebrate it by eating, drinking and reading poems. Nuts, pomegranates and watermelons are particularly served during this festival, the pagan Scandinavian and Germanic people of northern Europe celebrated a twelve-day midwinter holiday called Yule
Winter solstice
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Lawrence Hall of Science visitors observe sunset on the day of the winter solstice using the Sunstones II
Winter solstice
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Japanese Sun goddess
Amaterasu emerging from a cave.
Winter solstice
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Neolithic site of
Goseck circle. The yellow lines are the direction the Sun rises and sets at winter solstice.
Winter solstice
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Sunrise at Stonehenge on the Winter Solstice
27.
Iceberg
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An iceberg or ice mountain is a large piece of freshwater ice that has broken off a glacier or an ice shelf and is floating freely in open water. It may subsequently become frozen into pack ice, as it drifts into shallower waters, it may come into contact with the seabed, a process referred to as seabed gouging by ice. Almost 91% of an iceberg is below the surface of the water, the word iceberg is a partial loan translation from Dutch ijsberg, literally meaning ice mountain, cognate to Danish isbjerg, German Eisberg, Low Saxon Iesbarg and Swedish isberg. Because the density of ice is about 920 kg/m³. The shape of the underwater portion can be difficult to judge by looking at the portion above the surface and this has led to the expression tip of the iceberg, for a problem or difficulty that is only a small manifestation of a larger problem. Icebergs generally range from 1 to 75 metres above sea level, the largest known iceberg in the North Atlantic was 168 metres above sea level, reported by the USCG icebreaker East Wind in 1958, making it the height of a 55-story building. These icebergs originate from the glaciers of western Greenland and may have a temperature of −15 to −20 °C. Icebergs are usually confined by winds and currents to move close to the coast, the largest icebergs recorded have been calved, or broken off, from the Ross Ice Shelf of Antarctica. Iceberg B-15, photographed by satellite in 2000, measured 295 by 37 kilometres and this iceberg was larger than Belgium. When a piece of ice melts, it makes a fizzing sound called Bergie Seltzer. This sound is made when the water-ice interface reaches compressed air trapped in the ice. As this happens, each bubble bursts, making a popping sound, the bubbles contain air trapped in snow layers very early in the history of the ice, that eventually got buried to a given depth and pressurized as it transformed into firn then to glacial ice. About a month later, this split into two pieces upon crashing into Joe Island in the Nares Strait next to Greenland. In June 2011, large fragments of the Petermann Ice Islands were observed off the Labrador coast, iceberg B-17B140 km2,1999, shipping alert issued December 2009. In addition to classification, icebergs can be classified on the basis of their shape. The two basic types of forms are tabular and non-tabular. Tabular icebergs have steep sides and a top, much like a plateau. This type of iceberg, also known as an ice island, can be quite large, Antarctic icebergs formed by breaking off from an ice shelf, such as the Ross Ice Shelf or Filchner-Ronne Ice Shelf, are typically tabular
Iceberg
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An artist's impression of an iceberg viewed from the side, with the majority underwater
Iceberg
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Grotto in an iceberg, photographed during the
British Antarctic Expedition of 1911–1913, 5 Jan 1911. Photographer: Herbert Ponting, Alexander Turnbull Library
Iceberg
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The
calving of Iceberg A-38 off
Ronne Ice Shelf
Iceberg
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Iceberg A22A in the South
Atlantic Ocean
28.
Detaille Island
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Detaille Island is a small island off the northern end of the Arrowsmith Peninsula in Graham Land, Antarctica. From 1956 to 1959 it was home to Base W of the British Antarctic Survey and it is now often visited by Antarctic cruise ships but is otherwise unoccupied. Thanks to the hasty departure and the necessity that they take little with them. The base had been intended to host dog-sledging survey parties which would cross the sea ice to the nearby Antarctic Peninsula, but the ice was dangerously unstable. When Base W was vacated, heavy sea ice prevented resupply ship Biscoe from approaching closer than 50 kilometres, so the men were forced to close up the base, load sledges with only their most valuable gear and use dog teams to reach the ship. The base was established in 1956 as a British research station primarily for survey, geology and meteorology and it consists of a hut and associated structures and outbuildings, including a small emergency storage building, bitch and pup pens, anemometer tower, and two tubular steel radio masts. As a relatively unaltered research station of the late 1950s, it provides a reminder of the science, in 2009, the base was designated a Historic Site and Monument, following a proposal by the United Kingdom to the Antarctic Treaty Consultative Meeting. Detaille Island is maintained by the United Kingdom Antarctic Heritage Trust, the organization works to conserve Antarctic buildings and artefacts, and to promote and encourage the publics interest in its Antarctic heritage. The normal occupancy of Base W was eight to 10 people, in addition to the main building, there are two nearby huts. One was used as a store and the other to keep dogs. List of Antarctic and subantarctic islands Antarctic Treaty Visitor Site Guide Antarctic Heritage Trust Information Sheet Station W in 2008 Antarctic Heritage Trust - Detaille Blog
Detaille Island
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Detaille Island, Antarctica
29.
Demographics of Antarctica
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Antarctica has no permanent residents, although it contains research stations and field camps that are staffed seasonally or year-round, and former whaling settlements. The largest station, McMurdo Station, has a population of about 1,000 people. Approximately 29 nations, all signatory to the Antarctic Treaty, send personnel to perform seasonal or year-round research on the continent, at least ten children have been born in West Antarctica. The first was Emilio Marcos Palma, born on 7 January 1978 to Argentine parents at Esperanza, Hope Bay, in 1984, Juan Pablo Camacho was born at the Presidente Eduardo Frei Montalva Base, becoming the first Chilean born in Antarctica. Soon after, a girl, Gisella, was born at the same station, in 2001, National Geographic reported that eight children had been born at Esperanza alone. Antarctica at the CIA World Factbook
Demographics of Antarctica
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Children, adolescents and teachers of the school of
Esperanza Base.
30.
Research stations in Antarctica
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A number of governments maintain permanent research stations in Antarctica and these bases are widely distributed. Unlike the bases set up in the Arctic, the stations of the Antarctic are constructed either on rock or on ice that is fixed in place. Many of the stations are staffed around the year, a total of 30 countries, all signatories to the Antarctic Treaty, operate seasonal and year-round research stations on the continent. The population of people performing and supporting research on the continent. In addition to permanent stations, approximately 30 field camps are established each summer to support specific projects. During the Heroic Age of Antarctic Exploration in the early 20th century, in 1903, Dr William S. Bruces Scottish National Antarctic Expedition set off to Antarctica, with one of its aims to establish a meteorological station in the area. After the expedition failed to land, Bruce decided to head back to the South Orkneys. The islands were well-situated as a site for a meteorological station, Bruce instituted a comprehensive programme of work, involving meteorological readings, trawling for marine samples, botanical excursions, and the collection of biological and geological specimens. The major task completed during this time was the construction of a stone building and this was to act as living accommodation for the parties that would remain on Laurie Island to operate the proposed meteorological laboratory. The building was constructed from local materials using the dry method, with a roof improvised from wood. The completed house was 20 feet by 20 feet square, with two windows, fitted as quarters for six people, rudmose Brown wrote, Considering that we had no mortar and no masons tools it is a wonderfully fine house and very lasting. I should think it will be standing a century hence, Bruce later offered to Argentina the transfer of the station and instruments on the condition that the government committed itself to the continuation of the scientific mission. Bruce informed the British officer William Haggard of his intentions in December 1903, in 1906, Argentina communicated to the international community the establishment of a permanent base on South Orkney Islands. Little happened for the forty years until the Second World War. The chief reason was to establish solid British claims to various uninhabited islands and parts of Antarctica, prior to the start of the war, German aircraft had dropped markers with swastikas across Queen Maud Land in an attempt to create a territorial claim, see New Swabia. Led by Lieutenant James Marr, the 14-strong team left the Falkland Islands in two ships, HMS William Scoresby and Fitzroy, on Saturday January 29,1944, Marr had accompanied the British explorer Sir Ernest Shackleton on his final Antarctic expedition in 1921 -1922. A further base was founded at Hope Bay on February 13,1945 and these bases were the first ever to be constructed on the mainland Antarctica. The Operation provoked a massive expansion in international activity after the war, chile organized its First Chilean Antarctic Expedition in 1947–48
Research stations in Antarctica
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"Omond House", the first permanent base, built in 1903 by the
Scottish National Antarctic Expedition
Research stations in Antarctica
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Map shows the location of permanent Antarctic research stations
Research stations in Antarctica
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View of Chile's
Captain Arturo Prat Base, established in 1947
