1.
Compass
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A compass is an instrument used for navigation and orientation that shows direction relative to the geographic cardinal directions, or points. Usually, a called a compass rose shows the directions north, south, east. When the compass is used, the rose can be aligned with the geographic directions, so, for example. Frequently, in addition to the rose or sometimes instead of it, North corresponds to zero degrees, and the angles increase clockwise, so east is 90 degrees, south is 180, and west is 270. These numbers allow the compass to show azimuths or bearings, which are stated in this notation. The magnetic compass was first invented as a device for divination as early as the Chinese Han Dynasty, the first usage of a compass recorded in Western Europe and the Islamic world occurred around the early 13th century. The magnetic compass is the most familiar compass type and it functions as a pointer to magnetic north, the local magnetic meridian, because the magnetized needle at its heart aligns itself with the horizontal component of the Earths magnetic field. The needle is mounted on a pivot point, in better compasses a jewel bearing. When the compass is level, the needle turns until, after a few seconds to allow oscillations to die out. In navigation, directions on maps are usually expressed with reference to geographical or true north, the direction toward the Geographical North Pole, the rotation axis of the Earth. Depending on where the compass is located on the surface of the Earth the angle between north and magnetic north, called magnetic declination can vary widely with geographic location. The local magnetic declination is given on most maps, to allow the map to be oriented with a parallel to true north. The location of the Earths magnetic poles slowly change with time, the effect of this means a map with the latest declination information should be used. Some magnetic compasses include means to compensate for the magnetic declination. The first compasses in ancient Han dynasty China were made of lodestone, the compass was later used for navigation during the Song Dynasty of the 11th century. Later compasses were made of iron needles, magnetized by striking them with a lodestone, dry compasses began to appear around 1300 in Medieval Europe and the Islamic world. This was supplanted in the early 20th century by the magnetic compass. Modern compasses usually use a needle or dial inside a capsule completely filled with a liquid
2.
Bowditch's American Practical Navigator
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The American Practical Navigator, originally written by Nathaniel Bowditch, is an encyclopedia of navigation. It serves as a handbook on oceanography and meteorology, and contains useful tables. In 1867 the copyright and plates were bought by the Hydrographic Office of the United States Navy and it is considered one of Americas nautical institutions. The most popular text of the late 18th century was The New Practical Navigator by John Hamilton Moore of the Royal Navy. To have exact tables to work from, Bowditch recomputed all of Moores tables and he contacted the US publisher of the work, Edmund March Blunt, who asked him to correct and revise the third edition on his fifth voyage. The task was so extensive that Bowditch decided to write his own book, and to put down in the book nothing I cant teach the crew. On that trip, it is said that man of the crew of 12, including the ships cook, became competent to take and calculate lunar observations. The New Practical Navigator was published in 1799, followed by an edition in 1800. The current edition of the American Practical Navigator traces its pedigree to that 1802 edition, Edmund M. Blunt continued to publish the book until 1833, upon his retirement, his sons, Edmund and George, assumed publication. The elder Blunt died in 1862, his son Edmund followed in 1866, the next year,1867, George Blunt sold the copyright to the government for $25,000. The government has published Bowditch ever since, Nathaniel Bowditch continued to correct and revise the book until his death in 1838. Upon his death, the responsibility for The New American Practical Navigator passed to his son. Very few significant changes were made under him, editions from 1837 through 1880 are nearly identical in content. Ingersoll Bowditch continued editing the Navigator until George Blunt sold the copyright to the government and he outlived all of the principals involved in publishing and editing the Navigator, dying in 1889. The U. S. government has published some 52 editions since acquiring the copyright to the book that has come to be simply by its original author’s name. Since the government began production, the book has been known by its year of publishing, instead of by the edition number. After the first major revision, an overhaul of the books content completed in 1880 under the direction of Commander Philip H. Cooper, USN. Much of Bowditch’s original content, including his methods for clearing lunar distance observations, were dropped in 1880, after numerous incremental revisions and printings in the period from 1914 to 1944, Bowditch was extensively revised between 1946 and 1958
3.
Iron
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Iron is a chemical element with symbol Fe and atomic number 26. It is a metal in the first transition series and it is by mass the most common element on Earth, forming much of Earths outer and inner core. It is the fourth most common element in the Earths crust, like the other group 8 elements, ruthenium and osmium, iron exists in a wide range of oxidation states, −2 to +6, although +2 and +3 are the most common. Elemental iron occurs in meteoroids and other low oxygen environments, but is reactive to oxygen, fresh iron surfaces appear lustrous silvery-gray, but oxidize in normal air to give hydrated iron oxides, commonly known as rust. Unlike the metals that form passivating oxide layers, iron oxides occupy more volume than the metal and thus flake off, Iron metal has been used since ancient times, although copper alloys, which have lower melting temperatures, were used even earlier in human history. Pure iron is soft, but is unobtainable by smelting because it is significantly hardened and strengthened by impurities, in particular carbon. A certain proportion of carbon steel, which may be up to 1000 times harder than pure iron. Crude iron metal is produced in blast furnaces, where ore is reduced by coke to pig iron, further refinement with oxygen reduces the carbon content to the correct proportion to make steel. Steels and iron alloys formed with metals are by far the most common industrial metals because they have a great range of desirable properties. Iron chemical compounds have many uses, Iron oxide mixed with aluminium powder can be ignited to create a thermite reaction, used in welding and purifying ores. Iron forms binary compounds with the halogens and the chalcogens, among its organometallic compounds is ferrocene, the first sandwich compound discovered. Iron plays an important role in biology, forming complexes with oxygen in hemoglobin and myoglobin. Iron is also the metal at the site of many important redox enzymes dealing with cellular respiration and oxidation and reduction in plants. A human male of average height has about 4 grams of iron in his body and this iron is distributed throughout the body in hemoglobin, tissues, muscles, bone marrow, blood proteins, enzymes, ferritin, hemosiderin, and transport in plasma. The mechanical properties of iron and its alloys can be evaluated using a variety of tests, including the Brinell test, Rockwell test, the data on iron is so consistent that it is often used to calibrate measurements or to compare tests. An increase in the content will cause a significant increase in the hardness. Maximum hardness of 65 Rc is achieved with a 0. 6% carbon content, because of the softness of iron, it is much easier to work with than its heavier congeners ruthenium and osmium. Because of its significance for planetary cores, the properties of iron at high pressures and temperatures have also been studied extensively
4.
Magnetite
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Magnetite is a mineral and one of the main iron ores. With the chemical formula Fe3O4, it is one of the oxides of iron, Magnetite is ferrimagnetic, it is attracted to a magnet and can be magnetized to become a permanent magnet itself. It is the most magnetic of all the minerals on Earth. Naturally-magnetized pieces of magnetite, called lodestone, will attract small pieces of iron, today it is mined as iron ore. Small grains of magnetite occur in almost all igneous and metamorphic rocks, Magnetite is black or brownish-black with a metallic luster, has a Mohs hardness of 5–6 and leaves a black streak. The chemical IUPAC name is iron oxide and the chemical name is ferrous-ferric oxide. In addition to rocks, magnetite also occurs in sedimentary rocks, including banded iron formations and in lake. Magnetite nanoparticles are also thought to form in soils, where they probably oxidize rapidly to maghemite, Magnetite has an inverse spinel crystal structure. As a member of the group, it can form solid solutions with similarly structured minerals, including ulvospinel. Titanomagnetite, also known as titaniferous magnetite, is a solution between magnetite and ulvospinel that crystallizes in many mafic igneous rocks. Titanomagnetite may undergo oxyexsolution during cooling, resulting in ingrowths of magnetite and ilmenite, Magnetite has been important in understanding the conditions under which rocks form. Magnetite reacts with oxygen to produce hematite, and the pair forms a buffer that can control oxygen fugacity. Commonly, igneous rocks contain solid solutions of both titanomagnetite and hemoilmenite or titanohematite, Magnetite also is produced from peridotites and dunites by serpentinization. Lodestones were used as a form of magnetic compass. At low temperatures, magnetite undergoes a crystal structure phase transition from a structure to a cubic structure known as the Verwey transition. The Verwey transition occurs around 121 K and is dependent on size, domain state. An isotropic point also occurs near the Verwey transition around 130 K, the Curie temperature of magnetite is 858 K. Magnetite is sometimes found in large quantities in beach sand. Such black sands are found in places, such as Lung Kwu Tan of Hong Kong, California of the United States
5.
Long Island Sound
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From east to west, the sound stretches 110 miles from the East River in New York City, along the North Shore of Long Island, to Block Island Sound. A mix of freshwater from tributaries and saltwater from the ocean, Long Island Sound is 21 miles at its widest point, several major cities are situated along Long Island Sound and more than 8 million people live within its watershed. Major Connecticut cities on the Sound include Bridgeport, New London, Stamford, Norwalk, property values in Westchester County, Long Island, and southwestern Connecticut are among the highest in the nation, due to the proximity to New York City and their location on The Sound. About 18,000 years ago, Connecticut, Long Island Sound, sea level at that time was about 330 feet lower than today. When the ice sheet stopped advancing 18,000 years ago, an amount of drift was deposited, known as the Ronkonkoma Moraine. Later, another period of equilibrium resulted in the Harbor Hill Moraine along most of northern Long Island, the next moraines to the north were created just on and off the Connecticut coast. These moraines, created by much smaller deposits are discontinuous and much smaller than those to the south, the Connecticut coast moraines are in two groups, the Norwalk area and the Madison-Old Saybrook area. The Captain Islands off Greenwich, Connecticut, along with the Norwalk Islands and Falkner Island off Guilford, other islands, including the Thimble Islands, are for the most part exposed bedrock with a thin amount of drift, often not continuous. Other shoals and islands off the Connecticut coast are a mixture of two extremes. The glacier also created several sandy outwash deltas off the coast, including one off Bridgeport, Connecticut and another off New Haven, fishers Island, New York appears to be related to the Harbor Hill Moraine. To the east of the Thimble Islands, inland moraines along the Connecticut coast include the broken Madison Moraine, the Long Island Sound basin existed before the glaciers came. It probably had been formed by stream flows, a relatively thick cover of sand and gravel was left in the basin from glacial meltwater streams. On the west, a rising to about 65 feet below the present sea level is called the Mattatuck Sill. Its lowest point is about 80 feet below sea level, glacial meltwater formed Lake Connecticut, a freshwater lake in the basin, until about 8,000 years ago, when the sea level rose to about 80 feet below todays level. Seawater then overflowed into the basin, transforming it from a nontidal, freshwater lake to a tidal, keene West Lebanon Seaweeds in the Sound occur in greatest abundance in rocky areas between high tide and low tide as well as on rocks on the sea floor. Green seaweed populations fluctuate with the seasons, monostroma, reproduces in the early spring and dies out by late summer. Grinnellia appears in August and disappears four to six weeks later, also present are Ectocarpus and red algas Polysiphonia, Neosiphonia, Porphyra and Chondrus. In the marshy areas of the zone can be found Cladophora, Ulva
6.
Measuring instrument
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A measuring instrument is a device for measuring a physical quantity. In the physical sciences, quality assurance, and engineering, measurement is the activity of obtaining and comparing physical quantities of objects and events. Established standard objects and events are used as units, and the process of measurement gives a number relating the item under study, measuring instruments, and formal test methods which define the instruments use, are the means by which these relations of numbers are obtained. All measuring instruments are subject to varying degrees of instrument error, scientists, engineers and other humans use a vast range of instruments to perform their measurements. These instruments may range from simple objects such as rulers and stopwatches to electron microscopes, virtual instrumentation is widely used in the development of modern measuring instruments. In the past, a common time measuring instrument was the sundial, today, the usual measuring instruments for time are clocks and watches. For highly accurate measurement of time a clock is used. Stop watches are used to measure time in some sports. Energy is measured by an energy meter, examples of energy meters include, An electricity meter measures energy directly in kilowatt hours. A gas meter measures energy indirectly by recording the volume of gas used and this figure can then be converted to a measure of energy by multiplying it by the calorific value of the gas. A physical system that exchanges energy may be described by the amount of energy exchanged per time-interval, for the ranges of power-values see, Orders of magnitude. Action describes energy summed up over the time a process lasts and its dimension is the same as that of an angular momentum. A phototube provides a measurement which permits the calculation of the quantized action of light. This includes basic quantities found in classical- and continuum mechanics, dumpy level Laser line level Spirit level Gyroscope Ballistic pendulum, indirectly by calculation and or gauging Considerations related to electric charge dominate electricity and electronics. Electrical charges interact via a field and that field is called electric if the charge doesnt move. If the charge moves, thus realizing an electric current, especially in a neutral conductor. Electricity can be given a quality — a potential, and electricity has a substance-like property, the electric charge. Energy in elementary electrodynamics is calculated by multiplying the potential by the amount of charge found at that potential, potential times charge, electrometer is often used to reconfirm the phenomenon of contact electricity leading to triboelectric sequences
7.
Polaris
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Polaris, designated Alpha Ursae Minoris, commonly the North Star or Pole Star, is the brightest star in the constellation of Ursa Minor. It is very close to the celestial pole, making it the current northern pole star. The revised Hipparcos parallax gives a distance to Polaris of about 433 light-years, Polaris is a multiple star, comprising the main star in orbit with a smaller companion, the pair in orbit with Polaris B. There were once thought to be two more distant components—Polaris C and Polaris D—but these have been not to be physically associated with the Polaris system. Polaris Aa is a 4.5 solar mass F7 yellow supergiant of spectral type Ib and this is the first classical Cepheid to have a mass determined from its orbit. William Herschel discovered the star in 1780 while using a hand-built reflecting telescope, in 1929 it was discovered, by examining the spectrum of Polaris A, that it was a very close binary, with the secondary being a dwarf, which had been theorized in earlier observations. In January 2006, NASA released images, from the Hubble telescope, that showed the three members of the Polaris ternary system. The nearest dwarf star is in an orbit of only 18.5 au from Polaris A, about the distance between the Sun and Uranus), which explains why its light is swamped by its close and much brighter companion. Polaris A, the supergiant primary component, is a low-amplitude Population I classical Cepheid variable, cepheids constitute an important standard candle for determining distance, so Polaris, as the closest such star, is heavily studied. The variability of Polaris had been suspected since 1852, this variation was confirmed by Ejnar Hertzsprung in 1911, the range of brightness of Polaris during its pulsations is given as 1. 86–2.13, but the amplitude has changed since discovery. Prior to 1963, the amplitude was over 0.1 magnitude and was gradually decreasing. After 1966 it very rapidly decreased until it was less than 0.05 magnitude, since then and it has been reported that the amplitude is now increasing again, a reversal not seen in any other Cepheid. The period, roughly 4 days, has changed over time. It has steadily increased by around 4.5 seconds per year except for a hiatus in 1963–1965 and this was originally thought to be due to secular redward evolution across the Cepheid instability strip, but it may be due to interference between the primary and the first-overtone pulsation modes. Authors disagree on whether Polaris is a fundamental or first-overtone pulsator, the temperature of Polaris varies by only a small amount during its pulsations, but the amount of this variation is variable and unpredictable. The erratic changes of temperature and the amplitude of changes during each cycle, from less than 50 K to at least 170 K. Research reported in Science suggests that Polaris is 2.5 times brighter today than when Ptolemy observed it, because of its importance in celestial navigation, Polaris is known by numerous names. It became known as Polaris during the Renaissance, its derived from the Latin polaris of/near the pole
8.
Milliradian
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A milliradian, often called a mil or mrad, is an SI derived unit for angular measurement which is defined as a thousandth of a radian. Mils are used in adjustment of firearm sights by adjusting the angle of the sight compared to the barrel, Mils are also used for comparing shot groupings, or to compare the difficulty of hitting different sized targets at different distances. Using optics with mil markings in the one can make a range estimation of a known size target, or vice versa to determine a target size if the distance is known. In such applications it is useful to use a unit for target size that is a thousandth of the unit for range, for instance by using the metric units millimeters for target size and meters for range. This coincides with the definition of the milliradian where the arc length is defined as 1/1000 of the radius, a common adjustment value in firearm sights is 1 cm at 100 meters which equals 10 mm/100 m = 1/10 mil. The true definition of a milliradian is based on a circle with a radius of one. There are other definitions used for mapping and artillery which are rounded to more easily be divided into smaller parts. The milliradian was first used in the mid nineteenth century by Charles-Marc Dapples, degrees and minutes were the usual units of angular measurement but others were being proposed, with grads under various names having considerable popularity in much of northern Europe. However, Imperial Russia used a different approach, dividing a circle into equilateral triangles, around the time of the start of World War I, France was experimenting with the use of milliemes for use with artillery sights instead of decigrades. The United Kingdom was also trialing them to replace degrees and minutes and they were adopted by France although decigrades also remained in use throughout World War I. The United States, which copied many French artillery practices, adopted mils, before 2007 the Swedish defence forces used streck which is closer to the milliradian but then changed to NATO mils. After the Bolshevik Revolution and the adoption of the system of measurement the Red Army expanded the 600 unit circle into a 6000 mil one. Hence the Russian mil has a different origin than those derived from French artillery practices. In the 1950s, NATO adopted metric units of measurement for land, Mils, meters, and kilograms became standard, although degrees remained in use for naval and air purposes, reflecting civil practices. The approximation error by using the linear formula will increase as the angle increases. New shooters are often explained the principle of subtensions in order to understand that a milliradian is an angular measurement, subtension is the physical amount of space covered by an angle and varies with distance. Thus, the corresponding to a mil varies with range. Subtensions always change with distance, but a mil is always a mil regardless of distance, therefore ballistic tables and shot corrections are given in mils thereby avoiding the need of mathematical calculations
9.
National Geospatial-Intelligence Agency
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NGA was known as the National Imagery and Mapping Agency until 2003. NGA headquarters is located at Fort Belvoir in Springfield, Virginia, the NGA campus, at 2.3 million square feet, is the third-largest government building in the Washington metropolitan area after The Pentagon and the Ronald Reagan Building. U. S. mapping and charting efforts remained relatively unchanged until World War I, using stereo viewers, photo-interpreters reviewed thousands of images. Many of these were of the target at different angles and times, giving rise to what became modern imagery analysis. The Engineer Reproduction Plant was the Army Corps of Engineerss first attempt to centralize mapping production, printing and it was located on the grounds of the Army War College in Washington, D. C. Previously, topographic mapping had largely been a function of individual field engineer units using field surveying techniques or copying existing or captured products, in addition, ERP assumed the supervision and maintenance of the War Department Map Collection, effective April 1,1939. With the advent of the Second World War aviation, field surveys began giving way to photogrammetry, photo interpretation, during wartime, it became increasingly possible to compile maps with minimal field work. Out of this emerged AMS, which absorbed the existing ERP in May 1942 and it was located at the Dalecarlia Site on MacArthur Blvd. just outside Washington, D. C. in Montgomery County, Maryland, and adjacent to the Dalecarlia Reservoir. AMS was designated as an Engineer field activity, effective July 1,1942, by General Order 22, OCE, the Army Map Service also combined many of the Armys remaining geographic intelligence organizations and the Engineer Technical Intelligence Division. The agencys credit union, Constellation Federal Credit Union, was chartered during the Army Map Service era and it has continued to serve all successive legacy agencies employees and their families. After the war, as capacity and range improved, the need for charts grew. The Army Air Corps established its map unit, which was renamed ACP in 1943 and was located in St. Louis, ACP was known as the U. S. Air Force Aeronautical Chart and Information Center from 1952 to 1972. A credit union was chartered for the ACP in 1948, called Aero Chart Credit Union and it was renamed Arsenal Credit Union in 1952, a nod to the St. Louis sites Civil War-era use as an arsenal. Shortly before leaving office in January 1961, President Dwight D. Eisenhower authorized the creation of the National Photographic Interpretation Center, lundahl, combining Central Intelligence Agency, Army, Navy, and Air Force assets to solve national intelligence problems. NPIC was a component of the CIAs Directorate of Science and Technology, NPIC first identified the Soviet Unions basing of missiles in Cuba in 1962. The Defense Mapping Agency was created on January 1,1972, dMAs birth certificate, DoD Directive 5105.40, resulted from a formerly classified Presidential directive, Organization and Management of the U. S. Foreign Intelligence Community, which directed the consolidation of mapping functions previously dispersed among the military services, DMA became operational on July 1,1972, pursuant to General Order 3, DMA. On Oct.1,1996, DMA was folded into the National Imagery, DMA was first headquartered at the United States Naval Observatory in Washington, D. C, then at Falls Church, Virginia
10.
VHF omnidirectional range
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It uses frequencies in the very high frequency band from 108 to 117.95 MHz. Developed in the United States beginning in 1937 and deployed by 1946, VOR is the standard air navigational system in the world, used by both commercial and general aviation. By 2000 there were about 3,000 VOR stations around the world including 1,033 in the US, reduced to 967 by 2013 with more stations being decommissioned with the widespread adoption of GPS. A VOR ground station sends out a master signal. This signal is timed so that its phase varies as the secondary signal rotates, by comparing the phase of the secondary signal with the master, the angle to the aircraft from the station can be determined. This line of position is called the radial from the VOR, the intersection of radials from two different VOR stations can be used to fix the position of the aircraft, as in earlier radio direction finding systems. VOR stations are short range, the signals are useful for up to 200 miles. Each station broadcasts a VHF radio composite signal including the signal, stations identifier and voice. The navigation signal allows the receiving equipment to determine a bearing from the station to the aircraft. The stations identifier is typically a string in Morse code. The voice signal, if used, is usually the name, in-flight recorded advisories. At some locations, this signal is a continuous recorded broadcast of Hazardous Inflight Weather Advisory Service or HIWAS. Developed from earlier Visual Aural Radio Range systems, the VOR was designed to provide 360 courses to and from the station, early vacuum tube transmitters with mechanically-rotated antennas were widely installed in the 1950s, and began to be replaced with fully solid-state units in the early 1960s. They became the radio navigation system in the 1960s, when they took over from the older radio beacon. Some of the older range stations survived, with the four-course directional features removed, a worldwide land-based network of air highways, known in the US as Victor airways and jetways, was set up linking VORs. VOR signals provide considerably greater accuracy and reliability than NDBs due to a combination of factors, most significant is that VOR provides a bearing from the station to the aircraft which does not vary with wind or orientation of the aircraft. VHF radio is less vulnerable to diffraction around terrain features and coastlines, Phase encoding suffers less interference from thunderstorms. VOR signals offer a predictable accuracy of 90 m,2 sigma at 2 NM from a pair of VOR beacons, as compared to the accuracy of unaugmented Global Positioning System which is less than 13 meters and this limits VOR range to the horizon—or closer if mountains intervene
11.
Runway
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According to the International Civil Aviation Organization, a runway is a defined rectangular area on a land aerodrome prepared for the landing and takeoff of aircraft. Runways may be a surface or a natural surface. Runways are named by a number between 01 and 36, which is generally the magnetic azimuth of the heading in decadegrees. This heading differs from true north by the magnetic declination. A runway numbered 09 points east, runway 18 is south, runway 27 points west, when taking off from or landing on runway 09, a plane would be heading 90°. A runway can normally be used in both directions, and is named for each separately, e. g. runway 33 in one direction is runway 15 when used in the other. The two numbers usually differ by 18, Runway Zero Three Left becomes Runway Two One Right when used in the opposite direction. In some countries, if parallel runways are too close to each other, at large airports with four or more parallel runways some runway identifiers are shifted by 10 degrees to avoid the ambiguity that would result with more than three parallel runways. For example, in Los Angeles, this results in runways 6L, 6R, 7L. At Dallas/Fort Worth International Airport, there are five parallel runways, named 17L, 17C, 17R, 18L, for clarity in radio communications, each digit in the runway name is pronounced individually, runway three six, runway one four, etc. A leading zero, for example in runway zero six or runway zero one left, is included for all ICAO, however, most U. S. civil aviation airports drop the leading zero as required by FAA regulation. This also includes some military airfields such as Cairns Army Airfield and this American anomaly may lead to inconsistencies in conversations between American pilots and controllers in other countries. It is very common in a such as Canada for a controller to clear an incoming American aircraft to, for example, runway 04. In flight simulation programs those of American origin might apply U. S. usage to airports around the world, for example, runway 05 at Halifax will appear on the program as the single digit 5 rather than 05. Runway designations change over time because the magnetic poles slowly drift on the Earths surface, depending on the airport location and how much drift takes place, it may be necessary over time to change the runway designation. As runways are designated with headings rounded to the nearest 10 degrees, for example, if the magnetic heading of a runway is 233 degrees, it would be designated Runway 23. If the magnetic heading changed downwards by 5 degrees to 228, if on the other hand the original magnetic heading was 226, and the heading decreased by only 2 degrees to 224, the runway should become Runway 22. Because the drift itself is slow, runway designation changes are uncommon
12.
North Pole
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The North Pole, also known as the Geographic North Pole or Terrestrial North Pole, is defined as the point in the Northern Hemisphere where the Earths axis of rotation meets its surface. The North Pole is the northernmost point on the Earth, lying diametrically opposite the South Pole and it defines geodetic latitude 90° North, as well as the direction of true north. At the North Pole all directions point south, all lines of longitude converge there, along tight latitude circles, counterclockwise is east and clockwise is west. The North Pole is at the center of the Northern Hemisphere, while the South Pole lies on a continental land mass, the North Pole is located in the middle of the Arctic Ocean amid waters that are almost permanently covered with constantly shifting sea ice. This makes it impractical to construct a permanent station at the North Pole, however, the Soviet Union, and later Russia, constructed a number of manned drifting stations on a generally annual basis since 1937, some of which have passed over or very close to the Pole. Since 2002, the Russians have also established a base, Barneo. This operates for a few weeks during early spring, studies in the 2000s predicted that the North Pole may become seasonally ice-free because of Arctic ice shrinkage, with timescales varying from 2016 to the late 21st century or later. The sea depth at the North Pole has been measured at 4,261 m by the Russian Mir submersible in 2007 and at 4,087 m by USS Nautilus in 1958. The nearest land is said to be Kaffeklubben Island, off the northern coast of Greenland about 700 km away. The nearest permanently inhabited place is Alert in the Qikiqtaaluk Region, Nunavut, Canada, around the beginning of the 20th century astronomers noticed a small apparent variation of latitude, as determined for a fixed point on Earth from the observation of stars. Part of this variation could be attributed to a wandering of the Pole across the Earths surface, the wandering has several periodic components and an irregular component. The component with a period of about 435 days is identified with the eight-month wandering predicted by Euler and is now called the Chandler wobble after its discoverer and it is desirable to tie the system of Earth coordinates to fixed landforms. Of course, given plate tectonics and isostasy, there is no system in all geographic features are fixed. Yet the International Earth Rotation and Reference Systems Service and the International Astronomical Union have defined a framework called the International Terrestrial Reference System. As early as the 16th century, many eminent people correctly believed that the North Pole was in a sea and it was therefore hoped that passage could be found through ice floes at favorable times of the year. Several expeditions set out to find the way, generally with whaling ships, one of the earliest expeditions to set out with the explicit intention of reaching the North Pole was that of British naval officer William Edward Parry, who in 1827 reached latitude 82°45′ North. In 1871 the Polaris expedition, a US attempt on the Pole led by Charles Francis Hall, another British Royal Navy attempt on the pole, part of the British Arctic Expedition, by Commander Albert H. Markham reached a then-record 83°2026 North in May 1876 before turning back. An 1879–1881 expedition commanded by US naval officer George W. DeLong ended tragically when their ship, over half the crew, including DeLong, were lost
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