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Meridian (geography)
In geography and geodesy, a meridian is the locus connecting points of equal longitude, which is the angle (in degrees or other units) east or west of a given prime meridian (currently, the IERS Reference Meridian). In other words, it is a coordinate line for longitudes, a line of longitude. The position of a point along the meridian at a given longitude is given by its latitude, measured in angular degrees north or south of the Equator. On a Mercator projection or on a Gall-Peters projection, each meridian is perpendicular to all circles of latitude. Assuming a spherical Earth, a meridian is a great semicircle on Earth's surface. Adopting instead a spheroidal or ellipsoid model of Earth, the meridian is half of a north-south great ellipse. The length of a meridian is twice the length of an Earth quadrant, equal to on a modern ellipsoid ( WGS 84). Pre-Greenwich The first prime meridian was set by Eratosthenes in 200 BC. This prime meridian was used to provide mea ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
Circle Of Latitude
A circle of latitude or line of latitude on Earth is an abstract east–west small circle connecting all locations around Earth (ignoring elevation) at a given latitude coordinate line. Circles of latitude are often called parallels because they are Parallel (geometry), parallel to each other; that is, planes that contain any of these circles never Intersection, intersect each other. A location's position along a circle of latitude is given by its longitude. Circles of latitude are unlike circles of longitude, which are all great circles with the centre of Earth in the middle, as the circles of latitude get smaller as the distance from the Equator increases. Their length can be calculated by a common sine or cosine function. For example, the 60th parallel north or 60th parallel south, south is half as long as the Equator (disregarding Earth's minor flattening by 0.335%), stemming from \cos(60^) = 0.5. On the Mercator projection or on the Gall-Peters projection, a circle of la ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
Prime Meridian
A prime meridian is an arbitrarily chosen meridian (geography), meridian (a line of longitude) in a geographic coordinate system at which longitude is defined to be 0°. On a spheroid, a prime meridian and its anti-meridian (the 180th meridian in a degree (angle), 360°-system) form a great ellipse. This divides the body (e.g. Earth) into hemispheres of Earth, 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 is the IERS Reference Meridian. It is derived, but differs slightly, from the Prime meridian (Greenwich), Greenwich Meridian, the previous standard. 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� ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
International Meridian Conference
The International Meridian Conference was a conference held in October 1884 in Washington, D.C., in the United States, to determine a prime meridian for international use. The conference was held at the request of President of the United States, U.S. President Chester A. Arthur. The subject to discuss was the choice of "a meridian to be employed as a common zero of longitude and standard of time reckoning throughout the world". It resulted in the recommendation of the Greenwich Meridian as the international standard for zero degrees longitude. Background By the 1870s there was pressure both to establish a prime meridian for worldwide navigation purposes and to unify local times for railway timetables. The first International Geographical Union, International Geographical Congress, held in Antwerp in 1871, passed a motion in favour of the use of the Prime meridian (Greenwich), Greenwich Meridian for (smaller scale) passage Nautical chart, charts, suggesting that it should become ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
12-hour Clock
The 12-hour clock is a time convention in which the 24 hours of the day are divided into two periods: a.m. (from Latin , translating to "before midday") and p.m. (from Latin , translating to "after midday"). Each period consists of 12 hours numbered: 12 (acting as 0), 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, and 11. The 12-hour clock has been developed since the second millennium BC and reached its modern form in the 16th century. The 12-hour time convention is common in several English-speaking nations and former British Empire, British colonies, as well as a few other countries. In English-speaking countries: "12 p.m." usually indicates noon, while "12 a.m." means midnight, but the reverse convention has also been used (see #Confusion at noon and midnight, § Confusion at noon and midnight). "Noon" and "midnight" are unambiguous. History and use The natural day-and-night division of a calendar day forms the fundamental basis as to why each day is split ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
Meridian (astronomy)
In astronomy, the meridian is the great circle passing through the celestial poles, as well as the zenith and nadir of an observer's location. Consequently, it contains also the north and south points on the horizon, and it is perpendicular to the celestial equator and horizon. Meridians, celestial and geographical, are determined by the pencil of planes passing through the Earth's rotation axis. For a location ''not'' on this axis, there is a unique meridian plane in this axial-pencil through that location. The intersection of this plane with Earth's surface defines two '' geographical meridians'' (either one east and one west of the prime meridian, or else the prime meridian itself and its anti-meridian), and the intersection of the plane with the celestial sphere is the celestial meridian for that location and time. There are several ways to divide the meridian into semicircles. In one approach, the observer's upper meridian extends from a celestial pole and passes t ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
Noon
Noon (also known as noontime or midday) is 12 o'clock in the daytime. It is written as 12 noon, 12:00 m. (for '' meridiem'', literally 12:00 midday), 12 p.m. (for ''post meridiem'', literally "after midday"), 12 pm, or 12:00 (using a 24-hour clock) or 1200 ( military time). Solar noon is the time when the Sun appears to contact the local celestial meridian. This is when the Sun reaches its apparent highest point in the sky, at 12 noon apparent solar time and can be observed using a sundial. The local or clock time of solar noon depends on the date, longitude, and time zone A time zone is an area which observes a uniform standard time for legal, Commerce, commercial and social purposes. Time zones tend to follow the boundaries between Country, countries and their Administrative division, subdivisions instead of ..., with Daylight Saving Time tending to place solar noon closer to 1:00pm. Etymology The word ''noon'' is derived from Latin ''nona hora'', the ninth c ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
Subsolar Point
The subsolar point on a planet or a moon is the point at which its Sun is perceived to be directly overhead (at the zenith); that is, where the Sun's rays strike the planet exactly perpendicular to its surface. The subsolar point occurs at the location on a planet or a moon where the Sun culminates at the location's zenith. This occurs at solar noon. At this point, the Sun's rays will fall exactly vertical relative to an object on the ground and thus cast no observable shadow. To an observer on a planet with an orientation and rotation similar to those of Earth, the subsolar point will appear to move westward with a speed of 1600 km/h, completing one circuit around the globe each day, approximately moving along the equator. However, it will also move north and south between the tropics over the course of a year, so will appear to spiral like a helix. The term subsolar point can also mean the point closest to the Sun on an astronomical object, even though the Sun might no ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
Eratosthenes
Eratosthenes of Cyrene (; ; – ) was an Ancient Greek polymath: a Greek mathematics, mathematician, geographer, poet, astronomer, and music theory, music theorist. He was a man of learning, becoming the chief librarian at the Library of Alexandria. His work is comparable to the study of geography, and he introduced some of the terminology, even coining the terms geography and geographer. He is best known for being the first person known to calculate the Earth's circumference, which he did by using the extensive survey results he could access in his role at the Library. His calculation was remarkably accurate (his error margin turned out to be less than 1%). He was the first to calculate Earth's axial tilt, which similarly proved to have remarkable accuracy. He created the Eratosthenes' Map of the World, first global projection of the world, incorporating Circle of latitude, parallels and Longitude, meridians based on the available geographic knowledge of his era. Eratosth ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
World Geodetic System
The World Geodetic System (WGS) is a standard used in cartography, geodesy, and satellite navigation including GPS. The current version, WGS 84, defines an Earth-centered, Earth-fixed coordinate system and a geodetic datum, and also describes the associated Earth Gravitational Model (EGM) and World Magnetic Model (WMM). The standard is published and maintained by the United States National Geospatial-Intelligence Agency. History Efforts to supplement the various national surveying systems began in the 19th century with Friedrich Robert Helmert, F.R. Helmert's book (''Mathematical and Physical Theories of Physical Geodesy''). Austria and Germany founded the (Central Bureau of International Geodesy), and a series of global ellipsoids of the Earth were derived (e.g., Helmert 1906, John Fillmore Hayford, Hayford 1910 and 1924). A unified geodetic system for the whole world became essential in the 1950s for several reasons: * International space science and the beginning of as ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
Earth Quadrant
In geodesy and navigation, a meridian arc is the curve between two points near the Earth's surface having the same longitude. The term may refer either to a segment of the meridian, or to its length. Both the practical determination of meridian arcs (employing measuring instruments in field campaigns) as well as its theoretical calculation (based on geometry and abstract mathematics) have been pursued for many years. Measurement The purpose of measuring meridian arcs is to determine a figure of the Earth. One or more measurements of meridian arcs can be used to infer the shape of the reference ellipsoid that best approximates the geoid in the region of the measurements. Measurements of meridian arcs at several latitudes along many meridians around the world can be combined in order to approximate a ''geocentric ellipsoid'' intended to fit the entire world. The earliest determinations of the size of a spherical Earth required a single arc. Accurate survey work beginning in th ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
Great Ellipse
150px, A spheroid A great ellipse is an ellipse passing through two points on a spheroid and having the same center as that of the spheroid. Equivalently, it is an ellipse on the surface of a spheroid and centered on the origin, or the curve formed by intersecting the spheroid by a plane through its center. For points that are separated by less than about a quarter of the circumference of the earth, about 10\,000\,\mathrm, the length of the great ellipse connecting the points is close (within one part in 500,000) to the geodesic distance. The great ellipse therefore is sometimes proposed as a suitable route for marine navigation. The great ellipse is special case of an earth section path. Introduction Assume that the spheroid, an ellipsoid of revolution, has an equatorial radius a and polar semi-axis b. Define the flattening f=(a-b)/a, the eccentricity e=\sqrt, and the second eccentricity e'=e/(1-f). Consider two points: A at (geographic) latitude \phi_1 and longitude \l ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
