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36-650: Broadly speaking, the vilayet was bounded by the 20th parallel north to the north, the Aden protectorate to the south, the Red Sea to the west and the 45th meridian east to the east. The southern border was demarcated by the Anglo-Turkish Boundary Commission of 1902–1905, while the limit of the eastern border was left vague. Since the Ottoman conquest of Yemen in 1517, it had been known as

72-662: A geographic coordinate system at which longitude is defined to be 0°. Together, a prime meridian and its anti-meridian (the 180th meridian in a 360°-system ) form a great circle . This great circle divides a spheroid, like Earth, into two hemispheres : the Eastern Hemisphere and the Western Hemisphere (for an east-west notational system). For Earth's prime meridian, various conventions have been used or advocated in different regions throughout history. Earth's current international standard prime meridian

108-493: A spherical Earth , the book described the thousands years old customs of the prime meridian , or zero longitude, as passing through Avanti , the ancient name for the historic city of Ujjain , and Rohitaka , the ancient name for Rohtak ( 28°54′N 76°38′E  /  28.900°N 76.633°E  / 28.900; 76.633  ( Rohitaka (Rohtak) ) ), a city near the Kurukshetra . Ptolemy's Geographia

144-560: A line of longitude. In 1541, Mercator produced his famous 41 cm terrestrial globe and drew his prime meridian precisely through Fuerteventura (14°1'W) in the Canaries. His later maps used the Azores, following the magnetic hypothesis. But by the time that Ortelius produced the first modern atlas in 1570, other islands such as Cape Verde were coming into use. In his atlas longitudes were counted from 0° to 360°, not 180°W to 180°E as

180-626: Is roughly 43 metres (47 yards) to the west of the Airy Transit Circle, a distance equivalent to roughly 2 seconds of longitude. It was Airy's transit circle that was adopted in principle (with French delegates, who pressed for adoption of the Paris meridian abstaining) as the Prime Meridian of the world at the 1884 International Meridian Conference. All of these Greenwich meridians were located via an astronomic observation from

216-589: Is the IERS Reference Meridian . It is derived, but differs slightly, from the Greenwich Meridian , the previous standard. A prime meridian for a planetary body not tidally locked (or at least not in synchronous rotation) is entirely arbitrary, unlike an equator , which is determined by the axis of rotation. However, for celestial objects that are tidally locked (more specifically, synchronous), their prime meridians are determined by

252-630: Is the same as that of its orbit. East longitudes are used if the rotation is retrograde . The notion of longitude for Greeks was developed by the Greek Eratosthenes (c.   276 – 195   BCE) in Alexandria , and Hipparchus (c.   190 – 120   BCE) in Rhodes , and applied to a large number of cities by the geographer Strabo (64/63   BCE – c.   24   CE). But it

288-448: Is usual today. This practice was followed by navigators well into the 18th century. In 1634, Cardinal Richelieu used the westernmost island of the Canaries, El Hierro , 19° 55' west of Paris, as the choice of meridian. The geographer Delisle decided to round this off to 20°, so that it simply became the meridian of Paris disguised. In the early 18th century the battle was on to improve the determination of longitude at sea, leading to

324-857: The Anglo-Turkish Treaty delimited the border between Yemen and the Aden Protectorate. When World War I broke out, Imam Yahya remained nominally loyal to the Sultan, but tried to negotiate with Britain at the same time. The Asir, on the other hand, joined Britain as soon as the war began. The Arab Revolt in Hejaz cut off Yemen from the rest of the Ottoman Empire, and the imam took the opportunity to establish his power over all of Yemen. Turkish forces withdrew in 1918, and Imam Yahya strengthened his control over northern Yemen creating

360-740: The International Meridian Conference in Washington, D.C. , 22 countries voted to adopt the Greenwich meridian as the prime meridian of the world. The French argued for a neutral line, mentioning the Azores and the Bering Strait , but eventually abstained and continued to use the Paris meridian until 1911. The current international standard Prime Meridian is the IERS Reference Meridian . The International Hydrographic Organization adopted an early version of

396-876: The Mutawakkilite Kingdom of Yemen . Governors of the Yemen Vilayet: Sanjaks, circa 1876: 20th parallel north Download coordinates as: The 20th parallel north is a circle of latitude that is 20 degrees north of the Earth's equatorial plane . It crosses Africa , Asia , the Indian Ocean , the Pacific Ocean , North America , the Caribbean , and the Atlantic Ocean . The parallel defines part of

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432-717: The Royal Observatory, Greenwich . "Maskelyne's tables not only made the lunar method practicable, they also made the Greenwich meridian the universal reference point. Even the French translations of the Nautical Almanac retained Maskelyne's calculations from Greenwich – in spite of the fact that every other table in the Connaissance des Temps considered the Paris meridian as the prime." In 1884, at

468-790: The Yemen Eyalet . After the Tanzimat reforms in the Ottoman Empire, Yemen Vilayet was established from most of the former Eyalet in 1872. In the 1830s, aided by the collapse of the Zaidi Imamate due to internal division and the adoption of modern weaponry after the Crimean War , the Ottomans moved into northern Yemen, eventually taking San'a and making it the capital of the Yemen Vilayet in 1872. Even then, Ottoman control

504-530: The border between Libya and Sudan . Within Sudan it defines the border between the Northern and North Darfur states. At this latitude the sun is visible for 13 hours, 21 minutes during the summer solstice and 10 hours, 55 minutes during the winter solstice . On 21 June, the maximum altitude of the sun is 93.44 degrees and 46.56 degrees on 21 December. In this case an angle larger than 90° means that

540-859: The 18th century most countries in Europe adapted their own prime meridian, usually through their capital, hence in France the Paris meridian was prime, in Prussia it was the Berlin meridian, in Denmark the Copenhagen meridian, and in United Kingdom the Greenwich meridian . Between 1765 and 1811, Nevil Maskelyne published 49 issues of the Nautical Almanac based on the meridian of

576-605: The IERS Reference Meridian, the plane of which passes through the centre of mass of the Earth. This differs from the plane established by the Airy transit, which is affected by vertical deflection (the local vertical is affected by influences such as nearby mountains). The change from relying on the local vertical to using a meridian based on the centre of the Earth caused the modern prime meridian to be 5.3″ east of

612-626: The IRM in 1983 for all nautical charts. It was adopted for air navigation by the International Civil Aviation Organization on 3 March 1989. Since 1984, the international standard for the Earth's prime meridian is the IERS Reference Meridian. Between 1884 and 1984, the meridian of Greenwich was the world standard. These meridians are very close to each other. In October 1884 the Greenwich Meridian

648-621: The Ottomans. By 1910 they controlled most of Asir, but they were ultimately defeated by Turkish and Hejazi forces. Ahmet İzzet Pasha concluded a treaty with Imam Yahya in October 1911, by which he was recognised as temporal and spiritual head of the Zaidis, was given the right to appoint officials over them, and collect taxes from them. The Ottomans maintained their system of government in the Sunni-majority parts of Yemen. In March 1914,

684-572: The Zaidis rebelled against the Turks, and Imam Mohammed ibn Yahya laid the foundation of a hereditary dynasty. When he died in 1904, his successor Imam Yahya ibn Mohammed led the revolt against the Turks in 1904–1905, and forced them to grant important concessions to the Zaidis. The Ottoman agreed to withdraw the civil code and restore sharia in Yemen. In 1906, the Idrisi leaders of Asir rebelled against

720-512: The astronomic Greenwich prime meridian through the Airy Transit Circle. At the latitude of Greenwich, this amounts to 102 metres (112 yards). This was officially accepted by the Bureau International de l'Heure (BIH) in 1984 via its BTS84 (BIH Terrestrial System) that later became WGS84 (World Geodetic System 1984) and the various International Terrestrial Reference Frames (ITRFs). Due to the movement of Earth's tectonic plates ,

756-553: The culmination takes place at an maximum altitude of 86.56 degrees in the opposite cardinal direction. In the northern torrid zone, the Sun remains in the south during winter, but can reach over the zenith to the north in midsummer. Starting at the Prime Meridian and heading eastwards, the parallel 20° north passes through: Prime Meridian A prime meridian is an arbitrarily chosen meridian (a line of longitude ) in

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792-423: The development of the marine chronometer by John Harrison . But it was the development of accurate star charts, principally by the first British Astronomer Royal , John Flamsteed between 1680 and 1719 and disseminated by his successor Edmund Halley , that enabled navigators to use the lunar method of determining longitude more accurately using the octant developed by Thomas Godfrey and John Hadley . In

828-475: The face always inward of the orbit (a planet facing its star, or a moon facing its planet), just as equators are determined by rotation. Longitudes for the Earth and Moon are measured from their prime meridian (at 0°) to 180° east and west. For all other Solar System bodies, longitude is measured from 0° (their prime meridian) to 360°. West longitudes are used if the rotation of the body is prograde (or 'direct', like Earth), meaning that its direction of rotation

864-780: The ideal International Terrestrial Reference System (ITRS) and its realization, the International Terrestrial Reference Frame (ITRF). A current convention on the Earth uses the line of longitude 180° opposite the IRM as the basis for the International Date Line . Download coordinates as: On Earth, starting at the North Pole and heading south to the South Pole , the IERS Reference Meridian (as of 2016) passes through 8 countries, 4 seas, 3 oceans and 1 channel: As on

900-456: The line of 0° longitude along the surface of the Earth has slowly moved toward the west from this shifted position by a few centimetres (inches); that is, towards the Airy Transit Circle (or the Airy Transit Circle has moved toward the east, depending on your point of view) since 1984 (or the 1960s). With the introduction of satellite technology, it became possible to create a more accurate and detailed global map. With these advances there also arose

936-620: The link between longitude and time. Based on observations to satellites and celestial compact radio sources (quasars) from various coordinated stations around the globe, Airy's transit circle drifts northeast about 2.5 centimetres (1 inch) per year relative to this Earth-centred 0° longitude. It is also the reference meridian of the Global Positioning System operated by the United States Department of Defense , and of WGS84 and its two formal versions,

972-629: The location of the Airy Transit Circle ever since the first observation he took with it. Prior to that, it was defined by a succession of earlier transit instruments, the first of which was acquired by the second Astronomer Royal , Edmond Halley in 1721. It was set up in the extreme north-west corner of the Observatory between Flamsteed House and the Western Summer House. This spot, now subsumed into Flamsteed House,

1008-529: The necessity to define a reference meridian that, whilst being derived from the Airy Transit Circle, would also take into account the effects of plate movement and variations in the way that the Earth was spinning. As a result, the IERS Reference Meridian was established and is commonly used to denote the Earth's prime meridian (0° longitude) by the International Earth Rotation and Reference Systems Service , which defines and maintains

1044-432: The surface of the Earth, oriented via a plumb line along the direction of gravity at the surface. This astronomic Greenwich meridian was disseminated around the world, first via the lunar distance method , then by chronometers carried on ships, then via telegraph lines carried by submarine communications cables , then via radio time signals. One remote longitude ultimately based on the Greenwich meridian using these methods

1080-500: The western tip of Africa (17.5° W) as negative numbers were not yet in use. His prime meridian corresponds to 18° 40' west of Winchester (about 20°W) today. At that time the chief method of determining longitude was by using the reported times of lunar eclipses in different countries. One of the earliest known descriptions of standard time in India appeared in the 4th century CE astronomical treatise Surya Siddhanta . Postulating

1116-602: Was Ptolemy (c.   90 – 168   CE) who first used a consistent meridian for a world map in his Geographia . Ptolemy used as his basis the " Fortunate Isles ", a group of islands in the Atlantic , which are usually associated with the Canary Islands (13° to 18°W), although his maps correspond more closely to the Cape Verde islands (22° to 25° W). The main point is to be comfortably west of

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1152-487: Was eventually settled at 370 leagues (2,193 kilometers, 1,362 statute miles, or 1,184 nautical miles) west of Cape Verde . This is shown in the copies of Spain's Padron Real made by Diogo Ribeiro in 1527 and 1529. São Miguel Island (25.5°W) in the Azores was still used for the same reason as late as 1594 by Christopher Saxton , although by then it had been shown that the zero magnetic declination line did not follow

1188-483: Was first printed with maps at Bologna in 1477, and many early globes in the 16th century followed his lead. But there was still a hope that a "natural" basis for a prime meridian existed. Christopher Columbus reported (1493) that the compass pointed due north somewhere in mid-Atlantic, and this fact was used in the important Treaty of Tordesillas of 1494, which settled the territorial dispute between Spain and Portugal over newly discovered lands. The Tordesillas line

1224-515: Was largely confined to cities, and the Zaidi imam 's rule over Upper Yemen was formally recognized. Starting in 1872, after the Sana'a region was firmly under control, Ahmed Muhtar Pasha set about restructuring the administration of the Yemen vilayet, dividing it into four sanjaks, with San'a' city serving as capital of the vilayet. Asir became a sanjak of Yemen in 1872. In the late 19th century,

1260-413: Was selected by delegates (forty-one delegates representing twenty-five nations) to the International Meridian Conference held in Washington, D.C. , United States to be the common zero of longitude and standard of time reckoning throughout the world. The position of the historic prime meridian, based at the Royal Observatory, Greenwich , was established by Sir George Airy in 1851. It was defined by

1296-693: Was that of the North American Datum 1927 or NAD27, an ellipsoid whose surface best matches mean sea level under the United States . Beginning in 1973 the International Time Bureau and later the International Earth Rotation and Reference Systems Service changed from reliance on optical instruments like the Airy Transit Circle to techniques such as lunar laser ranging , satellite laser ranging , and very-long-baseline interferometry . The new techniques resulted in

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