Meridian (geography)

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Meridians run between the North and South poles. Primemeridian.jpg
Meridians run between the North and South poles.

A (geographic) meridian (or line of longitude) is the half of an imaginary great circle on the Earth's surface, terminated by the North Pole and the South Pole, connecting points of equal longitude, as measured in angular degrees east or west of the Prime Meridian. [1] The position of a point along the meridian is given by that longitude and its latitude, measured in angular degrees north or south of the Equator. Each meridian is perpendicular to all circles of latitude. Each is also the same length, being half of a great circle on the Earth's surface and therefore measuring 20,003.93 km (12,429.9 miles).

Contents

Pre-Greenwich

The first prime meridian was set by Eratosthenes in 200 BCE. This prime meridian was used to provide measurement of the earth, but had many problems because of the lack of latitude measurement. [2] Many years later around the 19th century there was still concerns of the prime meridian. The idea of having one prime meridian came from William Parker Snow, because he realized the confusion of having multiple prime meridian locations. Many of these geographical locations were traced back to the ancient Greeks, and others were created by several nations. [3] Multiple locations for the geographical meridian meant that there was inconsistency, because each country had their own guidelines for where the prime meridian was located.

Etymology

The term meridian comes from the Latin meridies, meaning "midday"; the subsolar point passes through a given meridian at solar noon, midway between the times of sunrise and sunset on that meridian. [4] Likewise, the Sun crosses the celestial meridian at the same time. The same Latin stem gives rise to the terms a.m. (ante meridiem) and p.m. (post meridiem) used to disambiguate hours of the day when utilizing the 12-hour clock.

International Meridian Conference

Because of a growing international economy, there was a demand for a set international prime meridian to make it easier for worldwide traveling which would, in turn, enhance international trading across countries. As a result, a Conference was held in 1884, in Washington, D.C. Twenty-six countries were present at the International Meridian Conference to vote on an international prime meridian. Ultimately the outcome was as follows: there would only be a single meridian, the meridian was to cross and pass at Greenwich (which was the 0°), there would be two longitude direction up to 180° (east being plus and west being minus), there will be a universal day, and the day begins at the mean midnight of the initial meridian. [5]

Geographic

The astronomic prime meridian at Greenwich, England. The geodetic prime meridian is actually 102.478 meters east of this point since the adoption of WGS84. Prime meridian.jpg
The astronomic prime meridian at Greenwich, England. The geodetic prime meridian is actually 102.478 meters east of this point since the adoption of WGS84.

Toward the ending of the 12th century there were two main locations that were acknowledged as the geographic location of the meridian, France and Britain. These two locations often conflicted and a settlement was reached only after there was an International Meridian Conference held, in which Greenwich was recognized as the 0° location. [6]

The meridian through Greenwich (inside Greenwich Park), England, called the Prime Meridian, was set at zero degrees of longitude, while other meridians were defined by the angle at the center of the earth between where it and the prime meridian cross the equator. As there are 360 degrees in a circle, the meridian on the opposite side of the earth from Greenwich, the antimeridian, forms the other half of a circle with the one through Greenwich, and is at 180° longitude near the International Date Line (with land mass and island deviations for boundary reasons). The meridians from West of Greenwich (0°) to the antimeridian (180°) define the Western Hemisphere and the meridians from East of Greenwich (0°) to the antimeridian (180°) define the Eastern Hemisphere. [7] [ unreliable source? ] Most maps show the lines of longitude.

The position of the prime meridian has changed a few times throughout history, mainly due to the transit observatory being built next door to the previous one (to maintain the service to shipping). Such changes had no significant practical effect. Historically, the average error in the determination of longitude was much larger than the change in position. The adoption of World Geodetic System 84" (WGS84) as the positioning system has moved the geodetic prime meridian 102.478 metres east of its last astronomic position (measured at Greenwich). [8] The position of the current geodetic prime meridian is not identified at all by any kind of sign or marking (as the older astronomic position was) in Greenwich, but can be located using a GPS receiver.

Effect of Prime Meridian (Greenwich Time)

It was in the best interests of the nations to agree to one standard meridian to benefit their fast growing economy and production. The disorganized system they had before was not sufficient for their increasing mobility. The coach services in England had erratic timing before the GWT. U.S. and Canada were also improving their railroad system and needed a standard time as well. With a standard meridian, stage coach and trains were able to be more efficient. [9] The argument of which meridian is more scientific was set aside in order to find the most convenient for practical reasons. They were also able to agree that the universal day was going to be the mean solar day. They agreed that the days would begin at midnight and the universal day would not impact the use of local time. A report was submitted to the "Transactions of the Royal Society of Canada," dated 10 May 1894; on the "Unification of the Astronomical, Civil and Nautical Days"; which stated that:

civil day- begins at midnight and ends at midnight following,
astronomical day- begins at noon of civil day and continue until following noon, and
nautical day- concludes at noon of civil day, starting at preceding noon. [10]

Magnetic meridian

The magnetic meridian is an equivalent imaginary line connecting the magnetic south and north poles and can be taken as the horizontal component of magnetic force lines along the surface of the earth. [11] Therefore, a compass needle will be parallel to the magnetic meridian. However, a compass needle will not be steady in the magnetic meridian, because of the longitude from east to west being complete geodesic. [12] The angle between the magnetic and the true meridian is the magnetic declination, which is relevant for navigating with a compass. [13] Navigators were able to use the azimuth (the horizontal angle or direction of a compass bearing) [14] of the rising and setting Sun to measure the magnetic variation (difference between magnetic and true north). [15]

True meridian

The true meridian is the plane that passes through true north poles and true south poles at the spot of the observer. The difference between true meridian and magnetic meridian is that the true meridian is fixed while the magnetic meridian is formed through the movement of the needle. [16] [ unreliable source ] True bearing is the horizontal angle between true meridian and a line.

Henry D. Thoreau classified this true meridian versus the magnetic meridian in order to have a more qualitative, intuitive, and abstract function. He used the true meridian since his compass varied by a few degrees. There were some variations. When he noted the sight line for the True Meridian from his family's house to the depot, he could check the declination of his compass before and after surveying throughout the day. He noted this variation down. [17]

Meridian passage

The meridian passage is the moment when a celestial object passes the meridian of longitude of the observer. At this point, the celestial object is at its highest point. When the sun passes two times an altitude while rising and setting can be averaged to give the time of meridian passage. Navigators utilized the sun's declination and the sun's altitude at local meridian passage, in order to calculate their latitude with the formula. [18]

Latitude = (90° – noon altitude + declination)

The declination of major stars are their angles north and south from the celestial equator. [19] It is important to note that the Meridian passage will not occur exactly at 12 hours because of the inclination of the earth. The meridian passage can occur within a few minutes of variation. [20]

Measurement of Earth rotation

Many of these instruments rely on the ability to measure the longitude and latitude of the earth. These instruments also were typically effected by local gravity, which paired well with existing technologies such as the magnetic meridian. [21]

See also

Related Research Articles

Longitude geographic coordinate that specifies the east-west position of a point on the Earths surface

Longitude, is a geographic coordinate that specifies the east–west position of a point on the Earth's surface, or the surface of a celestial body. It is an angular measurement, usually expressed in degrees and denoted by the Greek letter lambda (λ). Meridians connect points with the same longitude. By convention, one of these, the Prime Meridian, which passes through the Royal Observatory, Greenwich, England, was allocated the position of 0° longitude. The longitude of other places is measured as the angle east or west from the Prime Meridian, ranging from 0° at the Prime Meridian to +180° eastward and −180° westward. Specifically, it is the angle between a plane through the Prime Meridian and a plane through both poles and the location in question.

Compass direction finding and navigation instrument

A compass is an instrument used for navigation and orientation that shows direction relative to the geographic cardinal directions. Usually, a diagram called a compass rose shows the directions north, south, east, and west on the compass face as abbreviated initials. When the compass is used, the rose can be aligned with the corresponding geographic directions; for example, the "N" mark on the rose points northward. Compasses often display markings for angles in degrees in addition to the rose. North corresponds to 0°, and the angles increase clockwise, so east is 90° degrees, south is 180°, and west is 270°. These numbers allow the compass to show magnetic North azimuths or true North azimuths or bearings, which are commonly stated in this notation. If magnetic declination between the magnetic North and true North at latitude angle and longitude angle is known, then direction of magnetic North also gives direction of true North.

Geographic coordinate system Coordinate system

A geographic coordinate system is a coordinate system that enables every location on Earth to be specified by a set of numbers, letters or symbols. The coordinates are often chosen such that one of the numbers represents a vertical position and two or three of the numbers represent a horizontal position; alternatively, a geographic position may be expressed in a combined three-dimensional Cartesian vector. A common choice of coordinates is latitude, longitude and elevation. To specify a location on a plane requires a map projection.

Hour angle unit of angle

In astronomy and celestial navigation, the hour angle is one of the coordinates used in the equatorial coordinate system to give the direction of a point on the celestial sphere. The hour angle of a point is the angle between two planes: one containing Earth's axis and the zenith, and the other containing Earth's axis and the given point.

Prime meridian A line of longitude, at which longitude is defined to be 0°

A prime meridian is the meridian in a geographic coordinate system at which longitude is defined to be 0°. Together, a prime meridian and its anti-meridian form a great circle. This great circle divides a spheroid into two hemispheres. If one uses directions of East and West from a defined prime meridian, then they can be called the Eastern Hemisphere and the Western Hemisphere.

North One of the four cardinal directions

North is one of the four compass points or cardinal directions. It is the opposite of south and is perpendicular to east and west. North is a noun, adjective, or adverb indicating direction or geography.

Sundial Device that tells the time of day by the apparent position of the Sun in the sky

A sundial is a device that tells the time of day when there is sunlight by the apparent position of the Sun in the sky. In the narrowest sense of the word, it consists of a flat plate and a gnomon, which casts a shadow onto the dial. As the Sun appears to move across the sky, the shadow aligns with different hour-lines, which are marked on the dial to indicate the time of day. The style is the time-telling edge of the gnomon, though a single point or nodus may be used. The gnomon casts a broad shadow; the shadow of the style shows the time. The gnomon may be a rod, wire, or elaborately decorated metal casting. The style must be parallel to the axis of the Earth's rotation for the sundial to be accurate throughout the year. The style's angle from horizontal is equal to the sundial's geographical latitude.

Magnetic declination the angle on the horizontal plane between magnetic north and true north

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Eastern Hemisphere half of the Earth that is east of the prime meridian and west of 180° longitude

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Longitude by chronometer is a method, in navigation, of determining longitude using a marine chronometer, which was developed by John Harrison during the first half of the eighteenth century. It is an astronomical method of calculating the longitude at which a position line, drawn from a sight by sextant of any celestial body, crosses the observer's assumed latitude. In order to calculate the position line, the time of the sight must be known so that the celestial position i.e. the Greenwich Hour Angle and Declination, of the observed celestial body is known. All that can be derived from a single sight is a single position line, which can be achieved at any time during daylight when both the sea horizon and the sun are visible. To achieve a fix, more than one celestial body and the sea horizon must be visible. This is usually only possible at dawn and dusk.

Ex-meridian is a celestial navigation method of calculating an observer’s position on Earth. The method gives the observer a position line on which the observer is situated. It is usually used when the Sun is obscured at noon, and as a result, a meridian altitude is not possible. The navigator measures the altitude of the Sun as close to noon as possible and then calculates where the position line lies.

Meridian altitude is a method of celestial navigation to calculate an observer's latitude. It notes the angular height of an astronomical object above the horizon at culmination.

In astronomy and astrology, the prime vertical is the vertical circle passing east and west through the zenith of a specific location, and intersecting the horizon in its east and west points. It is also known as the first vertical.

Solar compass navigational instrument

The solar compass, a surveying instrument that makes use of the sun's direction, was first invented and made by William Austin Burt. He patented it on February 25, 1836, in the United States Patent Office as No 9428X. It received a medal at the Great Exhibition of 1851.

The navigational triangle or PZX triangle is a term used in Astro-Navigation, or Celestial Navigation, and its solution can give you your position anywhere on the globe. The sky is described as a Celestial Sphere with a North and South Pole corresponding to our own North and South Pole. Conveniently, the 'Pole Star' lies at the North Pole of the Celestial Sphere.

Mecca Time Proposed prime meridian

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Circle of equal altitude

The circle of equal altitude also called circle of position, CoP, is the real line of position in celestial navigation. It is defined as the locus of points on the Earth on which an observer sees a star, at a given time, with the same observed altitude. It was discovered by the American sea-captain Thomas Hubbard Sumner.

References

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