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Anthracite
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Anthracite coal
Black coal, hard coal, stone coal, blind coal, Kilkenny coal, crow coal, craw coal, black diamond
Metamorphic rock
Anthracite coal
Composition
Carbon, 92–98%
Anthracite, often referred to as hard coal, is a hard, compact variety of coal that has a submetallic luster. It has the highest carbon content, the fewest impurities, and the highest energy density of all types of coal except for graphite and is the highest ranking of coal.
Anthracite is the most metamorphosed type of coal (but still represents low-grade metamorphism), in which the carbon content is between 92% and 98%.[1][2] The term is applied to those varieties of coal which do not give off tarry or other hydrocarbon vapours when heated below their point of ignition.[3] Anthracite ignites with difficulty and burns with a short, blue, and smokeless flame.
Anthracite is categorized into standard grade, which is used mainly in power generation, and high grade (HG) and ultra high grade (UHG), the principal uses of which are in the metallurgy sector. Anthracite accounts for about 1% of global coal reserves,[4] and is mined in only a few countries around the world. China accounts for the majority of global production; other producers are Russia, Ukraine, North Korea, South Africa, Vietnam, the UK, Australia, Canada and the US. Total production in 2010 was 670 million tons.[5]
Contents
[hide]
1 Names
2 Properties
3 History of mining and use
4 Anthracite today
4.1 Mining
5 Sizing and grading
5.1 High grade
6 Underground fires
7 Major reserves
8 See also
9 Notes
10 References
11 Further reading
11.1 Primary sources
12 External links
Names[edit]
An American culm pile.
Anthracite derives from the Greek anthrakítēs (ἀνθρακίτης), literally "coal-like".[6] Other terms which refer to anthracite are black coal, hard coal, stone coal,[7][8] dark coal, coffee coal, blind coal (in Scotland),[3] Kilkenny coal (in Ireland),[7] crow coal or craw coal, and black diamond. "Blue Coal" is the term for a once-popular and trademarked brand of anthracite, mined by the Glen Alden Coal Company in Pennsylvania, and sprayed with a blue dye at the mine before shipping to its northeastern U.S. markets to distinguish it from its competitors.
Culm has different meanings in British and American English. In British English, "culm" is the imperfect anthracite of north Devon and Cornwall, which was used as a pigment. The term is also used to refer to some Carboniferous rock strata found in both Britain and in the Rhenish hill countries (the Culm Measures).[3] Lastly, it may refer to coal exported from Britain during the 19th century.[7]. In American English, "culm" refers to the waste or slack from anthracite mining,[3] mostly dust and small pieces not suitable for use in home furnaces.
Properties[edit]
This section needs additional citations for verification. Please help improve this article by adding citations to reliable sources. Unsourced material may be challenged and removed. (February 2015) (Learn how and when to remove this template message)
Anthracite (Ibbenbüren, Germany)
Anthracite (near Bay City, Michigan)
Anthracite is similar in appearance to the mineraloid jet and is sometimes used as a jet imitation.
Anthracite differs from ordinary bituminous coal by its greater hardness (2.75–3 on the Mohs scale[9]), its higher relative density of 1.3–1.4, and luster, which is often semi-metallic with a mildly brown reflection. It contains a high percentage of fixed carbon and a low percentage of volatile matter. It is also free from included soft or fibrous notches and does not soil the fingers when rubbed.[3] Anthracitization is the transformation of bituminous coal into anthracite.
The moisture content of fresh-mined anthracite generally is less than 15 percent. The heat content of anthracite ranges from 22 to 28 million Btu per short ton (26 to 33 MJ/kg) on a moist, mineral-matter-free basis. The heat content of anthracite coal consumed in the United States averages 25 million Btu/ton (29 MJ/kg), on the as-received basis (i.e., containing both inherent moisture and mineral matter). Since the 1980s, anthracite refuse or mine waste has been used for coal power generation in a form of recycling. The practice known as reclamation is being applied to culm piles antedating laws requiring mine owners to restore lands to their approximate original condition.
Chemically, anthracite may be considered as a transition stage between ordinary bituminous coal and graphite, produced by the more or less complete elimination of the volatile constituents of the former, and it is found most abundantly in areas that have been subjected to considerable stresses and pressures, such as the flanks of great mountain ranges.[3] Anthracite is associated with strongly deformed sedimentary rocks that were subjected to higher pressures and temperatures (but short of metamorphic conditions) just as bituminous coal is generally associated with less deformed or flat-lying sedimentary rocks. For example, the compressed layers of anthracite that are deep mined in the folded Ridge and Valley Province of the Appalachian Mountains of the Coal Region of East-central Pennsylvania are extensions of the same layers of bituminous coal that are mined on the generally flat lying and undeformed sedimentary rocks further west on the Allegheny Plateau of Kentucky and West Virginia, Eastern Ohio, and Western Pennsylvania. In the same way the anthracite region of South Wales is confined to the contorted portion west of Swansea and Llanelli, the central and eastern portions producing steam coal, coking coal and domestic house coals.[10]
Structurally, anthracite shows some alteration by the development of secondary divisional planes and fissures so that the original stratification lines are not always easily seen. The thermal conductivity is also higher; a lump of anthracite feels perceptibly colder when held in the warm hand than a similar lump of bituminous coal at the same temperature. The chemical composition of some typical anthracites is given in the article coal.[3]
Anthracite has a history of use in blast furnaces for iron smelting; however, it lacked the pore space of metallurgical coke, which eventually replaced anthracite.[11]
History of mining and use[edit]
Anthracite coal breaker and power house buildings, New Mexico, circa 1935
In southwest Wales, anthracite has been burned as a domestic fuel since at least medieval times,[12] when it was mined near Saundersfoot. More recently, large-scale mining of anthracite took place right across the western part of the South Wales Coalfield until the late 20th century. Mining continues but now on a smaller scale.
In the United States, anthracite coal history began in 1790 in Pottsville, Pennsylvania, with the discovery of coal made by the hunter Necho Allen in what is now known as the Coal Region. Legend has it that Allen fell asleep at the base of Broad Mountain and woke to the sight of a large fire because his campfire had ignited an outcrop of anthracite coal. By 1795, an anthracite-fired iron furnace had been built on the Schuylkill River.
Anthracite was first experimentally burned as a residential heating fuel in the US on 11 February 1808, by Judge Jesse Fell in Wilkes-Barre, Pennsylvania, on an open grate in a fireplace. Anthracite differs from wood in that it needs a draft from the bottom, and Judge Fell proved with his grate design that it was a viable heating fuel.[citation needed]
In spring 1808, John and Abijah Smith shipped the first commercially mined load of anthracite down the Susquehanna River from Plymouth, Pennsylvania, marking the birth of commercial anthracite mining in the United States. From that first mine, production rose to an all-time high of over 100 million tons in 1917.[citation needed]
Anthracite usage was inhibited by the difficulty of igniting it. This was a particular concern in smelting iron using a blast furnace. With the invention of hot blast in 1828, which used waste heat to preheat combustion air, anthracite became a preferred fuel, accounting for 45% of US pig iron production within 15 years.[13] Anthracite for iron smelting was later displaced by coke.
From the late 19th century until the 1950s, anthracite was the most popular fuel for heating homes and other buildings in the northern US, until it was supplanted by oil-burning systems and more recently natural gas systems. Many large public buildings, such as schools, were heated with anthracite-burning furnaces through the 1980s.
Anthracite is a "fighting fuel", World War II poster
During the American Civil War, Confederate blockade runners used anthracite as a smokeless fuel for their boilers to avoid giving away their position to the blockaders.[14]
The invention of the Wootten firebox enabled locomotives to directly burn anthracite efficiently, particularly waste culm. In the early 20th century US, the Delaware, Lackawanna & Western Railroad started using only the more expensive anthracite coal in its passenger locomotives, dubbed themselves "The Road of Anthracite," and advertised widely that travelers on their line could make railway journeys without getting their clothing stained with soot. The advertisements featured a white-clad woman named Phoebe Snow and poems containing lines like "My gown stays white / From morn till night / Upon the road of Anthracite". Similarly, the Great Western Railway in the UK was able to use its access to anthracite (it dominated the anthracite region) to earn a reputation for efficiency and cleanliness unmatched by other UK companies.
Internal combustion motors driven by the so-called "mixed", "poor", "semi-water" or "Dowson gas" produced by the gasification of anthracite with air (and a small proportion of steam) were at one time the most economical method of obtaining power, consuming 1 pound of fuel per horsepower-hour, or less. Large quantities of anthracite for power purposes were formerly exported from South Wales to France, Switzerland and parts of Germany.[15] As of April 2013[update], widespread commercial anthracite mining in Wales has now ceased, although a few large open cast sites remain, along with some relatively small drift mining operations.[citation needed]
Anthracite today[edit]
American football trophy custom-made from anthracite
Anthracite generally costs two to three times as much as regular coal. In June 2008, the wholesale cost of anthracite was US$150/short ton.[16]
The principal use of anthracite today is for a domestic fuel in either hand-fired stoves or automatic stoker furnaces. It delivers high energy per its weight and burns cleanly with little soot, making it ideal for this purpose. Its high value makes it prohibitively expensive for power plant use. Other uses include the fine particles used as filter media, and as an ingredient in charcoal briquettes. Anthracite is an authorised fuel[17] in terms of the United Kingdom's Clean Air Act 1993, meaning that it can be used within a designated Smoke Control Area such as the central London boroughs.
Mining[edit]
China today mines by far the largest share of global anthracite production, accounting for more than three-quarters of global output.[5] Most Chinese production is of standard-grade anthracite, which is used in power generation. Increased demand in China has made that country into a net importer of the fuel, mostly from Vietnam, another major producer of anthracite for power generation, although increasing domestic consumption in Vietnam means that exports may be scaled back.[18]
Current U.S. anthracite production averages around 5 million tons per year. Of that, about 1.8 million tons were mined in the state of Pennsylvania.[19] Mining of anthracite coal continues to this day in eastern Pennsylvania, and contributes up to 1% to the gross state product. More than 2,000 people were employed in the mining of anthracite coal in 1995. Most of the mining as of that date involved reclaiming coal from slag heaps (waste piles from past coal mining) at nearby closed mines. Some underground anthracite coal is also being mined.
Countries producing HG and UHG anthracite include Russia and South Africa. HG and UHG anthracite are used as a coke or coal substitute in various metallurgical coal applications (sintering, PCI, direct BF charge, pelletizing). It plays an important role in cost reduction in the steel making process and is also used in production of ferro-alloys, silicon-manganese, calcium-carbide and silicon-carbide. South Africa exports lower-quality, higher-ash anthracite to Brazil to be used in steel-making.[citation needed]
Sizing and grading[edit]
Anthracite is processed into different sizes by what is commonly referred to as a breaker. The large coal is raised from the mine and passed through breakers with toothed rolls to reduce the lumps to smaller pieces. The smaller pieces are separated into different sizes by a system of graduated sieves, placed in descending order.[15] Sizing is necessary for different types of stoves and furnaces.
Anthracite is classified into three grades, depending on its carbon content. Standard grade is used as a domestic fuel and in industrial power-generation. The rarer higher grades of anthracite are purer – i.e., they have a higher carbon content – and are used in steel-making and other segments of the metallurgical industries. Technical characteristics of the various grades of anthracite are as follows:[citation needed]
Standard grade anthracite
High grade anthracite
Ultra High grade anthracite
Coke
Moisture (maximum) 15% 15% 13% 5%
Ash (maximum) 20% 15% 12% 14%
Volatiles (maximum) 10% 10% 5% 2%
Fixed carbon (minimum) 73% 80% 85% 84%
Sulfur (maximum) 1% 1% 0.6% 0.8%
Group of breaker boys, from a 1910 photograph by Lewis Hine
Anthracite is divided by size mainly into applications that need lumps (typically larger than 10 mm) – various industrial processes where it replaces metallurgical coke, and domestic fuel – and those that need fines (less than 10 mm), such as sintering and pelletising.[18]
The common American classification by size is as follows:[citation needed]
Lump, steamboat, egg and stove coals, the latter in two or three sizes, all three being above 1 1⁄2 in (38 mm) size on round-hole screens.
High grade[edit]
High grade (HG) and ultra high grade (UHG) anthracite are the highest grades of anthracite coal. They are the purest forms of coal, having the highest degree of coalification, the highest carbon count and energy content and the fewest impurities (moisture, ash and volatiles).
High grade and ultra high grade anthracite are harder than standard grade anthracite, and have a higher relative density. An example of a chemical formula for high-grade anthracite would be C240H90O4NS,[20] representing 94% carbon.[21] UHG anthracite typically has a minimum carbon content of 95%.
They also differ in usage from standard grade anthracite (used mainly for power generation), being employed mainly in metallurgy as a cost-efficient substitute for coke in processes such as sintering and pelletising, as well as pulverised coal injection (PCI) and direct injection into blast furnaces. They can also be used in water purification and domestically as a smokeless fuel.
HG and UHG anthracite account for a small percentage of the total anthracite market. The major producing countries are Russia, Ukraine, Vietnam, South Africa and the US.
Classification
Minimum Size (inches)
Maximum Size (inches)
Chestnut 7/8 1 1⁄2
Pea 9/16 7/8
Buckwheat 3/8 9/16
Rice 3/16 3/8
Barley 3/32 3/16
The primary sizes used in the United States for domestic heating are Chestnut, Pea, Buckwheat and Rice, with Chestnut and Rice being the most popular. Chestnut and Pea are used in hand fired furnaces while the smaller Rice and Buckwheat are used in automatic stoker furnaces. Rice is currently the most sought after size due to the ease of use and popularity of that type of furnace.
In South Wales a less elaborate classification is adopted, but great care is exercised in hand-picking and cleaning the coal from particles of pyrites in the higher qualities known as best malting coals, which are used for kiln-drying malt.[15]
Anthracite dust can be made into briquettes and is sold in the United Kingdom under trade names such as Phurnacite, Ancit and Taybrite.
Underground fires[edit]
See also: Coal seam fire
Culm dump of anthracite tailings on fire near Scranton, Pennsylvania
Historically from time to time, underground seams of coal have caught fire, often from careless or unfortunate mining activities. The pocket of ignited coal is fed oxygen by vent paths that have not yet been discovered. These can smolder for years. Commonly, exhaust vents in populated areas are soon sensed and are sealed while vents in uninhabited areas remain undiscovered. Occasionally, vents are discovered via fumes sensed by passers-by, often in forested areas. Attempts to extinguish those remaining have at times been futile, and several such combustion areas exist today. The existence of an underground combustion site can sometimes be identified in the winter where fallen snow is seen to be melted by the warmth conducted from below. Proposals for harnessing this heat as geothermal energy have not been successful.
A vein of anthracite that caught fire in Centralia, Pennsylvania in 1962 has been burning ever since, turning the once-thriving borough into a ghost town.[22]
Major reserves[edit]
Among current producers, Russia, China and Ukraine have the largest estimated recoverable reserves of anthracite. Other countries with substantial reserves include Vietnam and North Korea.[23]
Geologically, the largest most concentrated anthracite deposit in the world is found in northeastern Pennsylvania, United States. Locally called the Coal Region, the deposit contains 480 square miles of coal bearing rock which originally held 22.8 billion short tons (20.68 billion tonnes) of anthracite.[24] (The geographic region is roughly 100 miles (161 km) in length and 30 miles (48 km) in width.) Because of historical mining and development of the lands overlying the coal, it is estimated that 7 billion short tons (6.3 billion tonnes) of minable reserves remain. The United States also contains several smaller deposits of anthracite, such as those historically mined in Crested Butte, Colorado.
The Groundhog Anthracite Deposit, located in British Columbia, Canada, is the largest previously undeveloped anthracite deposit in the world. It is owned by Australian publicly listed company, Atrum Coal and boasts 1.57 billion tonnes of high grade anthracite.[25]
Anthracites of newer Tertiary or Cretaceous age are found in the Crowsnest Pass part of the Rocky Mountains in Canada and at various places in the Andes in Peru.[15]
See also[edit]
mining portal
Antratsyt, a town in Ukraine named after a large supply of anthracite found there
Bituminous coal, or "soft" coal; another kind of coal
History of coal miners
History of coal mining
Notes[edit]
Jump up ^ "MIN 454: Underground Mining Methods handout; from course at the University of Alaska Fairbanks". Archived from the original on 26 March 2009. Retrieved 2009-05-05.
Jump up ^ R. Stefanenko (1983). Coal Mining Technology: Theory and Practice. Society for Mining Metallurgy. ISBN 0-89520-404-5.
^ Jump up to: a b c d e f g h Bauerman 1911, p. 105.
Jump up ^ World Coal Association – The Coal Resource Archived October 15, 2009, at the Wayback Machine.
^ Jump up to: a b "International Energy Statistics". www.eia.gov.
Jump up ^ "anthracite", The Oxford English Dictionary. 2nd ed. 1989. OED Online. Oxford University Press. Retrieved 2010-06-26. [1]
^ Jump up to: a b c EB (1878).
Jump up ^ Not to be confused with the German Steinkohle[3] or Dutch steenkool which are broader terms meaning all varieties of coal of a stonelike hardness and appearance, like bituminous coal and often anthracite as well, as opposed to lignite, which is softer.
Jump up ^ US Geological Survey and US Department of Mines (1968). Mineral Resources of the Appalachian Region; USGS Professional Paper 580. USGS. p. 126.
Jump up ^ Bauerman 1911, pp. 105-106.
Jump up ^ Rosenberg 1982, pp. 89
Jump up ^ Owen, George, The Description of Pembrokeshire, Dillwyn Miles (Ed), Gomer Press, Llandysul, 1994, ISBN 1-85902-120-4, pp. 60, 69–70, 90–95, 139, 255
Jump up ^ Rosenberg, Nathan (1982). Inside the Black Box: Technology and Economics. Cambridge, New York: Cambridge University Press. p. 88. ISBN 0-521-27367-6.
Jump up ^ Underwood, Rodman L. (2008). Waters of Discord: The Union Blockade of Texas During the Civil War. McFarland. p. 55. ISBN 978-0-7864-3776-4.
^ Jump up to: a b c d Bauerman 1911, p. 106.
Jump up ^ Urbina, Ian (June 10, 2008). "King Coal Country Debates a Sacrilege, Gas Heat". The New York Times. Retrieved June 21, 2008.
Jump up ^ "uksmokecontrolareas.co.uk". Archived from the original on 2009-02-07.
^ Jump up to: a b Petmin Annual Report 2011 Archived May 5, 2012, at the Wayback Machine.
Jump up ^ "retrieved January 3, 2011". Coaldiver.org. Retrieved 2018-01-24.
Jump up ^ "Coal characteristics: Indiana Center for Coal Technology Research Basic Facts File #8" (PDF). Indiana Center for Coal Technology Research. Retrieved 15 May 2012.
Jump up ^ "Molar mass of C240H90O4NS". www.webqc.org.
Jump up ^ Bellows, Alan (2006) "The Smoldering Ruins of Centralia" DamnInteresting.com (accessed August 29, 2006)
Jump up ^ "Marston – Anthracite production and exports: A world map" (PDF).
Jump up ^ http://www.libraries.psu.edu/content/dam/psul/up/emsl/documents/pags_reports/PR%20no.130.pdf[permanent dead link]
Jump up ^ "Atrum Coal Groundhog Project - Atrum Coal". atrumcoal.com.
References[edit]
"Anthracite". Encyclopædia Britannica. 2 (9th ed.). 1878. p. 106.
Attribution:
Bauerman, Hilary (1911). "Anthracite". In Chisholm, Hugh. Encyclopædia Britannica. 2 (11th ed.). Cambridge University Press. pp. 105–106.
Further reading[edit]
Chandler, Alfred D. (1972). "Anthracite coal and the beginnings of the industrial revolution in the United States". Business History Review. 46 (2): 141–181.
Hudson Coal Company (1932). The Story of Anthracite. New York. p. 425. — Useful overview of the industry in the 20th century; fair-minded with an operators perspective
Primary sources[edit]
Report of the United states coal commission.... (5 vol in 3; 1925) Official US government investigation. online vol 1-2
Tryon, Frederick Gale, and Joseph Henry Willits, eds. What the Coal Commission Found: An Authoritative Summary by the Staff (1925).
General policies committee of anthracite operators. The anthracite coal strike of 1922: A statement of its causes and underlying purposes (1923); Official statement by the operators. online
External links[edit]
Wikimedia Commons has media related to Anthracite.
Wikisource has the text of the 1879 American Cyclopædia article Anthracite.
HD Video close up of what Anthracite looks like
The Distribution of Pennsylvania Coals
History of anthracite coal mining
"A Jewel In the Crown of Old King Coal Eckley Miners' Village" by Tony Wesolowsky, Pennsylvania Heritage Magazine, Volume XXII, Number 1 – Winter 1996
The Eastern Pennsylvania Coalition for Abandoned Mine Reclamation
The Anthracite Heritage Museum.
Pennsylvania's Northern Coal Field
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Earth's rotation
From Wikipedia, the free encyclopedia
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For the duration of daylight and night, see Daytime and Night.
An animation of Earth's rotation around the planet's axis
This long-exposure photo of the northern night sky over the Nepali Himalayas shows the apparent paths of the stars as Earth rotates.
Earth's rotation is the rotation of Planet Earth around its own axis. Earth rotates eastward, in prograde motion. As viewed from the north pole star Polaris, Earth turns counter clockwise.
The North Pole, also known as the Geographic North Pole or Terrestrial North Pole, is the point in the Northern Hemisphere where Earth's axis of rotation meets its surface. This point is distinct from Earth's North Magnetic Pole. The South Pole is the other point where Earth's axis of rotation intersects its surface, in Antarctica.
Earth rotates once in about 24 hours with respect to the Sun, but once every 23 hours, 56 minutes, and 4 seconds with respect to the stars (see below). Earth's rotation is slowing slightly with time; thus, a day was shorter in the past. This is due to the tidal effects the Moon has on Earth's rotation. Atomic clocks show that a modern day is longer by about 1.7 milliseconds than a century ago,[1] slowly increasing the rate at which UTC is adjusted by leap seconds. Analysis of historical astronomical records shows a slowing trend of about 2.3 milliseconds per century since the 8th century BCE.[2]
Contents
[hide]
1 History
1.1 Empirical tests
2 Periods
2.1 True solar day
2.2 Mean solar day
2.3 Stellar and sidereal day
2.4 Angular speed
3 Changes
3.1 In rotational axis
3.2 In rotational velocity
3.2.1 Tidal interactions
3.2.2 Resonant stabilization
3.2.3 Global events
3.3 Measurement
3.4 Ancient observations
4 Origin
5 See also
6 Notes
7 References
8 External links
History[edit]
Among the ancient Greeks, several of the Pythagorean school believed in the rotation of the earth rather than the apparent diurnal rotation of the heavens. Perhaps the first was Philolaus (470–385 BCE), though his system was complicated, including a counter-earth rotating daily about a central fire.[3]
A more conventional picture was that supported by Hicetas, Heraclides and Ecphantus in the fourth century BCE who assumed that the earth rotated but did not suggest that the earth revolved about the sun. In the third century BCE, Aristarchus of Samos suggested the sun's central place.
However, Aristotle in the fourth century BCE criticized the ideas of Philolaus as being based on theory rather than observation. He established the idea of a sphere of fixed stars that rotated about the earth.[4] This was accepted by most of those who came after, in particular Claudius Ptolemy (2nd century CE), who thought the earth would be devastated by gales if it rotated.[5]
In 499 CE, the Indian astronomer Aryabhata wrote that the spherical earth rotates about its axis daily, and that the apparent movement of the stars is a relative motion caused by the rotation of the Earth. He provided the following analogy: "Just as a man in a boat going in one direction sees the stationary things on the bank as moving in the opposite direction, in the same way to a man at Lanka the fixed stars appear to be going westward."[6][7]
In the 10th century, some Muslim astronomers accepted that the Earth rotates around its axis.[8] According to al-Biruni, Abu Sa'id al-Sijzi (d. circa 1020) invented an astrolabe called al-zūraqī based on the idea believed by some of his contemporaries "that the motion we see is due to the Earth's movement and not to that of the sky."[9][10] The prevalence of this view is further confirmed by a reference from the 13th century which states: "According to the geometers [or engineers] (muhandisīn), the earth is in constant circular motion, and what appears to be the motion of the heavens is actually due to the motion of the earth and not the stars."[9] Treatises were written to discuss its possibility, either as refutations or expressing doubts about Ptolemy's arguments against it.[11] At the Maragha and Samarkand observatories, the Earth's rotation was discussed by Tusi (b. 1201) and Qushji (b. 1403); the arguments and evidence they used resemble those used by Copernicus.[12]
In medieval Europe, Thomas Aquinas accepted Aristotle's view[13] and so, reluctantly, did John Buridan[14] and Nicole Oresme[15] in the fourteenth century. Not until Nicolaus Copernicus in 1543 adopted a heliocentric world system did the contemporary understanding of earth's rotation begin to be established. Copernicus pointed out that if the movement of the earth is violent, then the movement of the stars must be very much more so. He acknowledged the contribution of the Pythagoreans and pointed to examples of relative motion. For Copernicus this was the first step in establishing the simpler pattern of planets circling a central sun.[16]
Tycho Brahe, who produced accurate observations on which Kepler based his laws, used Copernicus's work as the basis of a system assuming a stationary earth. In 1600, William Gilbert strongly supported the earth's rotation in his treatise on the earth's magnetism[17] and thereby influenced many of his contemporaries.[18] Those like Gilbert who did not openly support or reject the motion of the earth about the sun are often called "semi-Copernicans".[19] A century after Copernicus, Riccioli disputed the model of a rotating earth due to the lack of then-observable eastward deflections in falling bodies;[20] such deflections would later be called the Coriolis effect. However, the contributions of Kepler, Galileo and Newton gathered support for the theory of the rotation of the Earth.
Empirical tests[edit]
The earth's rotation implies that the equator bulges and the poles are flattened. In his Principia, Newton predicted this flattening would occur in the ratio of 1:230, and pointed to the 1673 pendulum measurements by Richer as corroboration of the change in gravity,[21] but initial measurements of meridian lengths by Picard and Cassini at the end of the 17th century suggested the opposite. However measurements by Maupertuis and the French Geodesic Mission in the 1730s established the flattening, thus confirming both Newton and the Copernican position.[22]
In the Earth's rotating frame of reference, a freely moving body follows an apparent path that deviates from the one it would follow in a fixed frame of reference. Because of the Coriolis effect, falling bodies veer slightly eastward from the vertical plumb line below their point of release, and projectiles veer right in the northern hemisphere (and left in the southern) from the direction in which they are shot. The Coriolis effect is mainly observable at a meteorological scale, where it is responsible for the differing rotation direction of cyclones in the northern and southern hemispheres.
Hooke, following a 1679 suggestion from Newton, tried unsuccessfully to verify the predicted eastward deviation of a body dropped from a height of 8.2 meters, but definitive results were only obtained later, in the late 18th and early 19th century, by Giovanni Battista Guglielmini in Bologna, Johann Friedrich Benzenberg in Hamburg and Ferdinand Reich in Freiberg, using taller towers and carefully released weights.[n 1] A ball dropped from a height of 158.5 m (520 ft) departed by 27.4 mm (1.08 in) from the vertical compared with a calculated value of 28.1 mm (1.11 in).
The most celebrated test of Earth's rotation is the Foucault pendulum first built by physicist Léon Foucault in 1851, which consisted of a lead-filled brass sphere suspended 67 m from the top of the Panthéon in Paris. Because of the Earth's rotation under the swinging pendulum, the pendulum's plane of oscillation appears to rotate at a rate depending on latitude. At the latitude of Paris the predicted and observed shift was about 11 degrees clockwise per hour. Foucault pendulums now swing in museums around the world.
Periods [edit]
Starry circles arc around the south celestial pole, seen overhead at ESO's La Silla Observatory.[23]
True solar day[edit]
Main article: Solar time
Earth's rotation period relative to the Sun (solar noon to solar noon) is its true solar day or apparent solar day. It depends on the Earth's orbital motion and is thus affected by changes in the eccentricity and inclination of Earth's orbit. Both vary over thousands of years, so the annual variation of the true solar day also varies. Generally, it is longer than the mean solar day during two periods of the year and shorter during another two.[n 2] The true solar day tends to be longer near perihelion when the Sun apparently moves along the ecliptic through a greater angle than usual, taking about 10 seconds longer to do so. Conversely, it is about 10 seconds shorter near aphelion. It is about 20 seconds longer near a solstice when the projection of the Sun's apparent motion along the ecliptic onto the celestial equator causes the Sun to move through a greater angle than usual. Conversely, near an equinox the projection onto the equator is shorter by about 20 seconds. Currently, the perihelion and solstice effects combine to lengthen the true solar day near 22 December by 30 mean solar seconds, but the solstice effect is partially cancelled by the aphelion effect near 19 June when it is only 13 seconds longer. The effects of the equinoxes shorten it near 26 March and 16 September by 18 seconds and 21 seconds, respectively.[24][25][26]
Mean solar day[edit]
Main article: Mean solar day
The average of the true solar day during the course of an entire year is the mean solar day, which contains 86,400 mean solar seconds. Currently, each of these seconds is slightly longer than an SI second because Earth's mean solar day is now slightly longer than it was during the 19th century due to tidal friction. The average length of the mean solar day since the introduction of the leap second in 1972 has been about 0 to 2 ms longer than 86,400 SI seconds.[27][28][29] Random fluctuations due to core-mantle coupling have an amplitude of about 5 ms.[30][31] The mean solar second between 1750 and 1892 was chosen in 1895 by Simon Newcomb as the independent unit of time in his Tables of the Sun. These tables were used to calculate the world's ephemerides between 1900 and 1983, so this second became known as the ephemeris second. In 1967 the SI second was made equal to the ephemeris second.[32]
The apparent solar time is a measure of the Earth's rotation and the difference between it and the mean solar time is known as the equation of time.
Stellar and sidereal day[edit]
On a prograde planet like the Earth, the stellar day is shorter than the solar day. At time 1, the Sun and a certain distant star are both overhead. At time 2, the planet has rotated 360° and the distant star is overhead again but the Sun is not (1→2 = one stellar day). It is not until a little later, at time 3, that the Sun is overhead again (1→3 = one solar day).
Earth's rotation period relative to the fixed stars, called its stellar day by the International Earth Rotation and Reference Systems Service (IERS), is 86,164.098 903 691 seconds of mean solar time (UT1) (23h 56m 4.098 903 691s, 0.997 269 663 237 16 mean solar days).[33][n 3] Earth's rotation period relative to the precessing or moving mean vernal equinox, named sidereal day, is 86,164.090 530 832 88 seconds of mean solar time (UT1) (23h 56m 4.090 530 832 88s, 0.997 269 566 329 08 mean solar days).[33] Thus the sidereal day is shorter than the stellar day by about 8.4 ms.[35]
Both the stellar day and the sidereal day are shorter than the mean solar day by about 3 minutes 56 seconds. The mean solar day in SI seconds is available from the IERS for the periods 1623–2005[36] and 1962–2005.[37]
Recently (1999–2010) the average annual length of the mean solar day in excess of 86,400 SI seconds has varied between 0.25 ms and 1 ms, which must be added to both the stellar and sidereal days given in mean solar time above to obtain their lengths in SI seconds (see Fluctuations in the length of day).
Angular speed[edit]
Plot of latitude vs tangential speed. The dashed line shows that the Kennedy Space Center example. The dot-dash line denotes typical airliner cruise speed.
The angular speed of Earth's rotation in inertial space is (7.2921150 ± 0.0000001) ×10−5 radians per SI second (mean solar second).[33] Multiplying by (180°/π radians)×(86,400 seconds/mean solar day) yields 360.9856°/mean solar day, indicating that Earth rotates more than 360° relative to the fixed stars in one solar day. Earth's movement along its nearly circular orbit while it is rotating once around its axis requires that Earth rotate slightly more than once relative to the fixed stars before the mean Sun can pass overhead again, even though it rotates only once (360°) relative to the mean Sun.[n 4] Multiplying the value in rad/s by Earth's equatorial radius of 6,378,137 m (WGS84 ellipsoid) (factors of 2π radians needed by both cancel) yields an equatorial speed of 465.1 m/s (1,526 ft/s), or 1,674.4 km/h (1,040.4 mph).[38] Some sources state that Earth's equatorial speed is slightly less, or 1,669.8 km/h.[39] This is obtained by dividing Earth's equatorial circumference by 24 hours. However, the use of only one circumference unwittingly implies only one rotation in inertial space, so the corresponding time unit must be a sidereal hour. This is confirmed by multiplying by the number of sidereal days in one mean solar day, 1.002 737 909 350 795,[33] which yields the equatorial speed in mean solar hours given above of 1,674.4 km/h.
The tangential speed of Earth's rotation at a point on Earth can be approximated by multiplying the speed at the equator by the cosine of the latitude.[40] For example, the Kennedy Space Center is located at latitude 28.59° N, which yields a speed of: cos 28.59° × 1,674.4 km/h (1,040.4 mph; 465.1 m/s) = 1,470.23 km/h (913.56 mph; 408.40 m/s)
Changes[edit]
Main articles: Fluctuations in the length of day and ΔT
Earth's axial tilt is about 23.4°. It oscillates between 22.1° and 24.5° on a 41,000-year cycle and is currently decreasing.
In rotational axis[edit]
The Earth's rotation axis moves with respect to the fixed stars (inertial space); the components of this motion are precession and nutation. It also moves with respect to the Earth's crust; this is called polar motion.
Precession is a rotation of the Earth's rotation axis, caused primarily by external torques from the gravity of the Sun, Moon and other bodies. The polar motion is primarily due to free core nutation and the Chandler wobble.
In rotational velocity[edit]
Tidal interactions[edit]
Over millions of years, the Earth's rotation slowed significantly by tidal acceleration through gravitational interactions with the Moon. In this process, angular momentum is slowly transferred to the Moon at a rate proportional to r − 6 {\displaystyle r^{-6}} , where r {\displaystyle r} is the orbital radius of the Moon. This process gradually increased the length of day to its current value and resulted in the Moon being tidally locked with the Earth.
This gradual rotational deceleration is empirically documented with estimates of day lengths obtained from observations of tidal rhythmites and stromatolites; a compilation of these measurements[41] found the length of day to increase steadily from about 21 hours at 600Myr ago[42] to the current 24 hour value. By counting the microscopic lamina that form at higher tides, tidal frequencies (and thus day lengths) can be estimated, much like counting tree rings, though these estimates can be increasingly unreliable at older ages.[43]
Resonant stabilization[edit]
The current rate of tidal deceleration is anomalously high, implying the Earth's rotational velocity must have decreased more slowly in the past. Empirical data[41] tentatively shows a sharp increase in rotational deceleration about 600Myr ago. Some models suggest that the Earth maintained a constant day length of 21 hours throughout much of the Precambrian.[42] This day length corresponds to the semidiurnal resonant period of the thermally-driven atmospheric tide; at this day length, the decelerative lunar torque could have been canceled by an accelerative torque from the atmospheric tide, resulting in no net torque and a constant rotational period. This stabilizing effect could have been broken by a sudden change in global temperature. Recent computational simulations support this hypothesis and suggest the Marinoan or Sturtian glaciations broke this stable configuration about 600Myr ago, citing the resemblance of simulated results and existing paleorotational data.[44]
Deviation of day length from SI based day
Global events[edit]
Additionally, some large-scale events, such as the 2004 Indian Ocean earthquake, have caused the length of a day to shorten by 3 microseconds by affecting the Earth's moment of inertia.[45] Post-glacial rebound, ongoing since the last Ice age, is also changing the distribution of the Earth's mass thus affecting the moment of inertia of the Earth and, by the conservation of angular momentum, the Earth's rotation period.[46]
Measurement[edit]
The primary monitoring of the Earth's rotation is performed with very-long-baseline interferometry coordinated with the Global Positioning System, satellite laser ranging, and other satellite techniques. This provides an absolute reference for the determination of universal time, precession, and nutation.[47]
Ancient observations[edit]
There are recorded observations of solar and lunar eclipses by Babylonian and Chinese astronomers beginning in the 8th century BCE, as well as from the medieval Islamic world and elsewhere. These observations can be used to determine changes in the Earth's rotation over the last 27 centuries, since the length of the day is a critical parameter in the calculation of the place and time of eclipses. A change in day length of milliseconds per century shows up as a change of hours and thousands of kilometers in eclipse observations. The ancient data is consistent with a shorter day, meaning the Earth was turning faster throughout the past.[48][49]
Origin[edit]
An artist's rendering of the protoplanetary disk.
The Earth's original rotation was a vestige of the original angular momentum of the cloud of dust, rocks, and gas that coalesced to form the Solar System. This primordial cloud was composed of hydrogen and helium produced in the Big Bang, as well as heavier elements ejected by supernovas. As this interstellar dust is heterogeneous, any asymmetry during gravitational accretion resulted in the angular momentum of the eventual planet.[50]
However, if the giant-impact hypothesis for the origin of the Moon is correct, this primordial rotation rate would have been reset by the Theia impact 4.5 billion years ago. Regardless of the speed and tilt of the Earth's rotation before the impact, it would have experienced a day some five hours long after the impact.[51] Tidal effects would then have slowed this rate to its modern value.
See also[edit]
Allais effect
Axial precession
Diurnal cycle
Earth's orbit
Formation and evolution of the Solar System
Geodesic (in mathematics)
Geodesics in general relativity
Geodesy
Giant-impact hypothesis
History of Earth
History of geodesy, Space techniques
List of important publications in geology
Newcomb's Tables of the Sun
Nychthemeron
Spherical Earth
World Geodetic System
Notes[edit]
Jump up ^ See Fallexperimente zum Nachweis der Erdrotation (German Wikipedia article).
Jump up ^ When Earth's eccentricity exceeds 0.047 and perihelion is at an appropriate equinox or solstice, only one period with one peak balances another period that has two peaks.[24]
Jump up ^ Aoki, the ultimate source of these figures, uses the term "seconds of UT1" instead of "seconds of mean solar time".[34]
Jump up ^ In astronomy, unlike geometry, 360° means returning to the same point in some cyclical time scale, either one mean solar day or one sidereal day for rotation on Earth's axis, or one sidereal year or one mean tropical year or even one mean Julian year containing exactly 365.25 days for revolution around the Sun.
References[edit]
Jump up ^ Dennis D. McCarthy; Kenneth P. Seidelmann (18 September 2009). Time: From Earth Rotation to Atomic Physics. John Wiley & Sons. p. 232. ISBN 978-3-527-62795-0.
Jump up ^ Stephenson, F. Richard (2003). "Historical eclipses and Earth's rotation". Astronomy & Geophysics. 44 (2). pp. 2.22–2.27. Bibcode:2003A&G....44b..22S. doi:10.1046/j.1468-4004.2003.44222.x.
Jump up ^ Burch, George Bosworth (1954). "The Counter-Earth". Osiris. 11: 267–294. doi:10.1086/368583. JSTOR 301675.
Jump up ^ Aristotle. Of the Heavens. Book II, Ch 13. 1.
Jump up ^ Ptolemy. Almagest Book I, Chapter 8.
Jump up ^ http://www.new1.dli.ernet.in/data1/upload/insa/INSA_1/20005b61_51.pdf
Jump up ^ Kim Plofker (2009). Mathematics in India. Princeton University Press. p. 71. ISBN 0-691-12067-6.
Jump up ^ Alessandro Bausani (1973). "Cosmology and Religion in Islam". Scientia/Rivista di Scienza. 108 (67): 762.
^ Jump up to: a b Young, M. J. L., ed. (2 November 2006). Religion, Learning and Science in the 'Abbasid Period. Cambridge University Press. p. 413. ISBN 9780521028875.
Jump up ^ Nasr, Seyyed Hossein (1 January 1993). An Introduction to Islamic Cosmological Doctrines. SUNY Press. p. 135. ISBN 9781438414195.
Jump up ^ Ragep, Sally P. (2007). "Ibn Sīnā: Abū ʿAlī al‐Ḥusayn ibn ʿAbdallāh ibn Sīnā". In Thomas Hockey; et al. The Biographical Encyclopedia of Astronomers. New York: Springer. pp. 570–2. ISBN 978-0-387-31022-0. (PDF version)
Jump up ^ Ragep, F. Jamil (2001a), "Tusi and Copernicus: The Earth's Motion in Context", Science in Context, Cambridge University Press, 14 (1–2): 145–163, doi:10.1017/s0269889701000060
Jump up ^ Aquinas, Thomas. Commentaria in libros Aristotelis De caelo et Mundo. Lib II, cap XIV. trans in Grant, Edward, ed. (1974). A Source Book in Medieval Science. Harvard University Press. pages 496–500
Jump up ^ Buridan, John (1942). Quaestiones super libris quattuo De Caelo et mundo. pp. 226–232. in Grant 1974, pp. 500–503
Jump up ^ Oresme, Nicole. Le livre du ciel et du monde. pp. 519–539. in Grant 1974, pp. 503–510
Jump up ^ Copernicus, Nicolas. On the Revolutions of the Heavenly Spheres. Book I, Chap 5–8.
Jump up ^ Gilbert, William. De Magnete, On the Magnet and Magnetic Bodies, and on the Great Magnet the Earth. pp. 313–347.
Jump up ^ Russell, John L. "Copernican System in Great Britain". In J. Dobrzycki. The Reception of Copernicus' Heliocentric Theory. p. 208.
Jump up ^ J. Dobrzycki 1972, p. 221
Jump up ^ Almagestum novum, chapter nine, cited in Graney, Christopher M. (2012). "126 arguments concerning the motion of the earth. GIOVANNI BATTISTA RICCIOLI in his 1651 ALMAGESTUM NOVUM" (PDF). Journal for the History of Astronomy. volume 43, pages 215–226.
Jump up ^ Newton, Isaac (1846). Newton's Principia. Translated by A. Motte. p. 412.
Jump up ^ Shank, J. B. (2008). The Newton Wars and the Beginning of the French Enlightenment. U of Chicago Press. pp. 324, 355.
Jump up ^ "Starry Spin-up". Retrieved 24 August 2015.
^ Jump up to: a b Jean Meeus; J. M. A. Danby (January 1997). Mathematical Astronomy Morsels. Willmann-Bell. pp. 345–346. ISBN 978-0-943396-51-4.
Jump up ^ Equation of time in red and true solar day in blue
Jump up ^ The duration of the true solar day
Jump up ^ http://hpiers.obspm.fr/eoppc/eop/eopc04_05/eopc04.62-now
Jump up ^ Physical basis of leap seconds
Jump up ^ Leap seconds Archived 12 March 2015 at the Wayback Machine.
Jump up ^ Prediction of Universal Time and LOD Variations
Jump up ^ R. Hide et al., "Topographic core-mantle coupling and fluctuations in the Earth's rotation" 1993.
Jump up ^ Leap seconds by USNO Archived 12 March 2015 at the Wayback Machine.
^ Jump up to: a b c d IERS EOP Useful constants
Jump up ^ Aoki, et al., "The new definition of Universal Time", Astronomy and Astrophysics 105 (1982) 359–361.
Jump up ^ Explanatory Supplement to the Astronomical Almanac, ed. P. Kenneth Seidelmann, Mill Valley, Cal., University Science Books, 1992, p.48, ISBN 0-935702-68-7.
Jump up ^ IERS Excess of the duration of the day to 86,400s … since 1623 Archived 3 October 2008 at the Wayback Machine. Graph at end.
Jump up ^ IERS Variations in the duration of the day 1962–2005
Jump up ^ Arthur N. Cox, ed., Allen's Astrophysical Quantities p.244.
Jump up ^ Michael E. Bakich, The Cambridge planetary handbook, p.50.
Jump up ^ Butterworth & Palmer. "Speed of the turning of the Earth". Ask an Astrophysicist. NASA Goddard Spaceflight Center.
^ Jump up to: a b Williams, George E. (1 February 2000). "Geological constraints on the Precambrian history of Earth's rotation and the Moon's orbit". Reviews of Geophysics. 38 (1): 37–59. Bibcode:2000RvGeo..38...37W. doi:10.1029/1999RG900016. ISSN 1944-9208.
^ Jump up to: a b Zahnle, K.; Walker, J. C. (1 January 1987). "A constant daylength during the Precambrian era?". Precambrian Research. 37: 95–105. Bibcode:1987PreR...37...95Z. doi:10.1016/0301-9268(87)90073-8. ISSN 0301-9268. PMID 11542096.
Jump up ^ Scrutton, C. T. (1 January 1978). Brosche, Professor Dr Peter; Sündermann, Professor Dr Jürgen, eds. Periodic Growth Features in Fossil Organisms and the Length of the Day and Month. Springer Berlin Heidelberg. pp. 154–196. doi:10.1007/978-3-642-67097-8_12. ISBN 9783540090465.
Jump up ^ Bartlett, Benjamin C.; Stevenson, David J. (1 January 2016). "Analysis of a Precambrian resonance-stabilized day length". Geophysical Research Letters: 2016GL068912. arXiv:1502.01421  . Bibcode:2016GeoRL..43.5716B. doi:10.1002/2016GL068912. ISSN 1944-8007.
Jump up ^ Sumatran earthquake sped up Earth's rotation, Nature, 30 December 2004.
Jump up ^ Wu, P.; W.R.Peltier (1984). "Pleistocene deglaciation and the earth's rotation: a new analysis". Geophysical Journal of the Royal Astronomical Society. 76 (3): 753–792. Bibcode:1984GeoJ...76..753W. doi:10.1111/j.1365-246X.1984.tb01920.x.
Jump up ^ Permanent monitoring
Jump up ^ Sid Perkins (6 December 2016). "Ancient eclipses show Earth's rotation is slowing". Science. doi:10.1126/science.aa10469.
Jump up ^ FR Stephenson; LV Morrison; CY Hohonkerk (7 December 2016). "Measurement of the Earth's rotation: 720 BC to AD 2015". Proceedings of the Royal Society A. 472 (2196). Bibcode:2016RSPSA.47260404S. doi:10.1098/rspa.2016.0404. PMC 5247521  .
Jump up ^ "Why do planets rotate?". Ask an Astronomer.
Jump up ^ Stevenson, D. J. (1987). "Origin of the moon–The collision hypothesis". Annual Review of Earth and Planetary Sciences. 15 (1): 271–315. Bibcode:1987AREPS..15..271S. doi:10.1146/annurev.ea.15.050187.001415.
External links[edit]
USNO Earth Orientation new site, being populated
USNO IERS old site, to be abandoned
IERS Earth Orientation Center: Earth rotation data and interactive analysis
International Earth Rotation and Reference Systems Service (IERS)
If the Earth's rotation period is less than 24 hours, why don't our clocks fall out of sync with the Sun?
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