Meridian (geography)

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

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). [1] 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 20,003.93 km (12,429.87 mi) on a modern ellipsoid (WGS 84). [2]

Contents

Pre-Greenwich

The first prime meridian was set by Eratosthenes in 200 BC. This prime meridian was used to provide measurement of the earth, but had many problems because of the lack of latitude measurement. [1] Many years later around the 19th century there were still concerns of the prime meridian. 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. [3] 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 be only one prime meridian, the prime 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. [4]

There were two main reasons for this. The first was that the USA had already chosen Greenwich as the basis for its own national time zone system. The second was that in the late 19th century, 72% of the world's commerce depended on sea-charts which used Greenwich as the Prime Meridian. The recommendation was based on the argument that naming Greenwich as Longitude 0º would be of advantage to the largest number of people. [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 Greenwich (0°) west to the antimeridian (180°) define the Western Hemisphere and the meridians from Greenwich (0°) east 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] [9] The position of the current geodetic prime meridian is not identified at all by any kind of sign or marking at Greenwich (as the older astronomic position was), 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.[ clarification needed ] United States 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. [10] 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 chord that goes from one pole to the other, passing through the observer, and is contrasted with the magnetic meridian, which goes through the magnetic poles and the observer. The true meridian can be found by careful astronomical observations, and the magnetic meridian is simply parallel to the compass needle. The arithmetic difference between the true and magnetic meridian is called the magnetic declination, which is important for the calibration of compasses. [16]

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] The meridian passage will not occur exactly at 12 hours because of the Earth orbit excentricity (see Equation of time).

Standard meridian

A standard meridian is a meridian used for determining standard time. [20] For instance, the 30th meridian east (UTC+02:00) is the standard meridian for Eastern European Time. Since the adoption of time zones – as opposed to local mean time or solar time – in the late 19th century and early 20th century, most countries have adopted the standard time of one of the 24 meridians closest to their geographical position, as decided by the International Meridian Conference in 1884. [21] [22] [23] Although, a few time zones are offset by an additional 30 or 45 minutes, such as in the Chatham Islands, South Australia and Nepal. [24]

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 affected by local gravity, which paired well with existing technologies such as the magnetic meridian. [9]

See also

Related Research Articles

<span class="mw-page-title-main">Latitude</span> Geographic coordinate specifying north–south position

In geography, latitude is a coordinate that specifies the north–south position of a point on the surface of the Earth or another celestial body. Latitude is given as an angle that ranges from −90° at the south pole to 90° at the north pole, with 0° at the Equator. Lines of constant latitude, or parallels, run east–west as circles parallel to the equator. Latitude and longitude are used together as a coordinate pair to specify a location on the surface of the Earth.

<span class="mw-page-title-main">Longitude</span> 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 surface of the Earth, or another celestial body. It is an angular measurement, usually expressed in degrees and denoted by the Greek letter lambda (λ). Meridians are imaginary semicircular lines running from pole to pole that connect points with the same longitude. The prime meridian defines 0° longitude; by convention the International Reference Meridian for the Earth passes near the Royal Observatory in Greenwich, south-east London on the island of Great Britain. Positive longitudes are east of the prime meridian, and negative ones are west.

<span class="mw-page-title-main">Right ascension</span> Astronomical equivalent of longitude

Right ascension is the angular distance of a particular point measured eastward along the celestial equator from the Sun at the March equinox to the point in question above the Earth. When paired with declination, these astronomical coordinates specify the location of a point on the celestial sphere in the equatorial coordinate system.

<span class="mw-page-title-main">Geographic coordinate system</span> System to specify locations on Earth

A geographic coordinate system (GCS) is a spherical or geodetic coordinate system for measuring and communicating positions directly on Earth as latitude and longitude. It is the simplest, oldest and most widely used of the various spatial reference systems that are in use, and forms the basis for most others. Although latitude and longitude form a coordinate tuple like a cartesian coordinate system, the geographic coordinate system is not cartesian because the measurements are angles and are not on a planar surface.

<span class="mw-page-title-main">Equatorial coordinate system</span> Celestial coordinate system used to specify the positions of celestial objects

The equatorial coordinate system is a celestial coordinate system widely used to specify the positions of celestial objects. It may be implemented in spherical or rectangular coordinates, both defined by an origin at the centre of Earth, a fundamental plane consisting of the projection of Earth's equator onto the celestial sphere, a primary direction towards the March equinox, and a right-handed convention.

<span class="mw-page-title-main">Sidereal time</span> Timekeeping system on Earth relative to the celestial sphere

Sidereal time is a system of timekeeping used especially by astronomers. Using sidereal time and the celestial coordinate system, it is easy to locate the positions of celestial objects in the night sky. Sidereal time is a "time scale that is based on Earth's rate of rotation measured relative to the fixed stars".

<span class="mw-page-title-main">Hour angle</span> Coordinates used in the equatorial coordinate system

In astronomy and celestial navigation, the hour angle is the dihedral angle between the meridian plane and the hour circle.

<span class="mw-page-title-main">Prime meridian</span> Line of longitude, at which longitude is defined to be 0°

A prime meridian is an arbitrarily chosen 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, like Earth, into two hemispheres: the Eastern Hemisphere and the Western Hemisphere. 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 Greenwich Meridian, the previous standard.

<span class="mw-page-title-main">North</span> 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.

<span class="mw-page-title-main">Sundial</span> Device that tells the time of day by the apparent position of the Sun in the sky

A sundial is a horological device that tells the time of day when direct sunlight shines 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 through 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.

<span class="mw-page-title-main">Prime meridian (Greenwich)</span> Meridian passing through Greenwich, London

The Greenwich meridian is a prime meridian, a geographical reference line that passes through the Royal Observatory, Greenwich, in London, England. From 1884 to 1974, the Greenwich meridian was the international standard prime meridian, used worldwide for timekeeping and navigation. The modern standard, the IERS Reference Meridian, is based on the Greenwich meridian, but differs slightly from it. This prime meridian was first established by Sir George Airy in 1851, and by 1884, over two-thirds of all ships and tonnage used it as the reference meridian on their charts and maps. In October of that year, at the invitation of the President of the United States, 41 delegates from 25 nations met in Washington, D.C., United States, for the International Meridian Conference. This conference selected the meridian passing through Greenwich as the world standard prime meridian due to its popularity. However, France abstained from the vote, and French maps continued to use the Paris meridian for several decades. In the 18th century, London lexicographer Malachy Postlethwayt published his African maps showing the "Meridian of London" intersecting the Equator a few degrees west of the later meridian and Accra, Ghana.

<span class="mw-page-title-main">Paris meridian</span> Meridian line in Paris, France

The Paris meridian is a meridian line running through the Paris Observatory in Paris, France – now longitude 2°20′14.02500″ East. It was a long-standing rival to the Greenwich meridian as the prime meridian of the world. The "Paris meridian arc" or "French meridian arc" is the name of the meridian arc measured along the Paris meridian.

<span class="mw-page-title-main">Longitude by chronometer</span>

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 altitude angle of an astronomical object above the horizon at culmination.

The navigational triangle or PZX triangle is a spherical triangle used in astronavigation to determine the observer's position on the globe. It is composed of three reference points on the celestial sphere:

Mecca Time was a proposed time standard that uses the line of longitude that goes through Mecca, Saudi Arabia as its Prime Meridian. A clock based on this meridian would be at approximately UTC+02:39:18.3.

<span class="mw-page-title-main">Astronomical rings</span> Early astronomical instrument

Astronomical rings, also known as Gemma's rings, are an early astronomical instrument. The instrument consists of three rings, representing the celestial equator, declination, and the meridian.

<span class="mw-page-title-main">Burt's solar compass</span> Surveying instrument that uses the Suns direction instead of magnetism

Burt's solar compass or astronomical compass/sun compass is a surveying instrument that makes use of the Sun's direction instead of magnetism. William Austin Burt invented his solar compass in 1835. The solar compass works on the principle that the direction to the Sun at a specified time can be calculated if the position of the observer on the surface of the Earth is known, to a similar precision. The direction can be described in terms of the angle of the Sun relative to the axis of rotation of the planet.

<span class="mw-page-title-main">Planetary coordinate system</span> Coordinate system for planets

A planetary coordinate system is a generalization of the geographic, geodetic, and the geocentric coordinate systems for planets other than Earth. Similar coordinate systems are defined for other solid celestial bodies, such as in the selenographic coordinates for the Moon. The coordinate systems for almost all of the solid bodies in the Solar System were established by Merton E. Davies of the Rand Corporation, including Mercury, Venus, Mars, the four Galilean moons of Jupiter, and Triton, the largest moon of Neptune. A planetary datum is a generalization of geodetic datums for other planetary bodies, such as the Mars datum; it requires the specification of physical reference points or surfaces with fixed coordinates, such as a specific crater for the reference meridian or the best-fitting equigeopotential as zero-level surface.

References

  1. 1 2 Withers, Charles W. J. (2017). "Absurd Vanity". Zero Degrees. Harvard University Press. pp. 25–72. doi:10.4159/9780674978935-004. ISBN   978-0-674-97893-5. JSTOR   j.ctt1n2ttsj.6.
  2. ↑ Weintrit, Adam (2013). "So, What is Actually the Distance from the Equator to the Pole? – Overview of the Meridian Distance Approximations". TransNav. 7 (2): 259–272. doi: 10.12716/1001.07.02.14 .
  3. ↑ First Teachings about the Earth; its lands and waters; its countries and States, etc. 1870.
  4. ↑ Rosenburg, Matt. "The Prime Meridian: Establishing Global Time and Space".
  5. ↑ "Royal Museum Greenwitch".
  6. ↑ Withers, Charles W. J. (2017). "Ruling Space, Fixing Time". Zero Degrees. Harvard University Press. pp. 263–274. doi:10.4159/9780674978935-011. ISBN   978-0-674-97893-5. JSTOR   j.ctt1n2ttsj.12.
  7. ↑ "What is the Prime Meridian? - Definition, Facts & Location - Video & Lesson Transcript | Study.com". study.com. Retrieved 2018-07-25.
  8. ↑ "Where exactly is the prime meridian?". Astronomy & Geophysics. 56 (5): 5.5.2–5.5. October 2015. doi: 10.1093/astrogeo/atv151 .
  9. 1 2 Malys, Stephen; Seago, John H.; Pavlis, Nikolaos K.; Seidelmann, P. Kenneth; Kaplan, George H. (December 2015). "Why the Greenwich meridian moved". Journal of Geodesy. 89 (12): 1263–1272. Bibcode:2015JGeod..89.1263M. doi: 10.1007/s00190-015-0844-y .
  10. 1 2 Smith, Humphry M. (January 1976). "Greenwich time and the prime meridian". Vistas in Astronomy. 20: 219–229. Bibcode:1976VA.....20..219S. doi:10.1016/0083-6656(76)90039-8.
  11. ↑ Doumbia, Vafi; Boka, Kouadio; Kouassi, Nguessan; Grodji, Oswald Didier Franck; Amory-Mazaudier, Christine; Menvielle, Michel (4 January 2017). "Induction effects of geomagnetic disturbances in the geo-electric field variations at low latitudes". Annales Geophysicae. 35 (1): 39–51. Bibcode:2017AnGeo..35...39D. doi: 10.5194/angeo-35-39-2017 .
  12. ↑ Haughton, Graves Champney (1 January 1851). "On the relative dynamic value of the degrees of the compass; and on the cause of the needle resting in the magnetic meridian". Abstracts of the Papers Communicated to the Royal Society of London. 5: 626. doi:10.1098/rspl.1843.0275. S2CID   118467818.
  13. ↑ Obuchovski, Romuald; Petroškevičius, Petras; Būga, Arūnas (1 January 2008). "Research on magnetic declination in Lithuanian territory". Aviation. 12 (2): 51–56. doi: 10.3846/1648-7788.2008.12.51-56 .
  14. ↑ "Definition of AZIMUTH". www.merriam-webster.com. Retrieved 2018-07-28.
  15. ↑ "The Sun and the Moon". The Lost Art of Finding Our Way. 2013. pp. 161–192. doi:10.4159/harvard.9780674074811.c8. ISBN   978-0-674-07481-1. S2CID   133042730.
  16. ↑ U.S. Coast and Geodetic Survey (1919). Principal Facts of the Earth's Magnetism and Methods of Determining the True Meridian and the Magnetic Declination. U.S. Government Printing Office. LCCN   19014371. OCLC   5426732.[ page needed ]
  17. ↑ McLean, Albert F. (1968). "Thoreau's True Meridian: Natural Fact and Metaphor". American Quarterly. 20 (3): 567–579. doi:10.2307/2711017. JSTOR   2711017.
  18. ↑ "Maps and Compasses". The Lost Art of Finding Our Way. 2013. pp. 99–124. doi:10.4159/harvard.9780674074811.c6. ISBN   978-0-674-07481-1.
  19. ↑ "Stars". The Lost Art of Finding Our Way. 2013. pp. 125–160. doi:10.4159/harvard.9780674074811.c7. ISBN   978-0-674-07481-1.
  20. ↑ "standard meridian". Merriam-Webster.com Dictionary . Merriam-Webster.
  21. ↑ Harper's School Geography: With Maps and Illustrations. American Book Company. 1885. p. 129. Standard meridians … They are 15° apart. The focal time on any one of them, therefore, differs exactly one hour from the local time on either of the two others between which it is situated. Each standard meridian is the middle line of a belt extending 15° wide.
  22. ↑ Gould, Ralph Edgar (1925). Standard Time Throughout the World (1 ed.). United States National Bureau of Standards. p. 2.
  23. ↑ Committee on Standard Time (1884). What Shall be the Prime Meridian for the World?. International Institute for Preserving and Perfecting Weights and Measures. p. 22. It is intended that local time at any place on the surface of the globe shall generally be regulated by the standard meridian nearest or most convenient to such place in longitude.
  24. ↑ "How time zones confused the world". BBC News . 7 August 2015. Retrieved 9 April 2022.
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