TheInfoList

Longitude (, ) is a
geographic coordinate A geographic coordinate system (GCS) is a coordinate system associated with position (geometry), positions on Earth (geographic position). A GCS can give positions: *as Geodetic coordinates, spherical coordinate system using latitude, long ...
that specifies the
west 250px, A compass rose with west highlighted in black West or Occident is one of the four cardinal directions or points of the compass The points of the compass are the vectors by which planet-based directions are conventionally defined. A co ...

position of a point on the
Earth Earth is the third planet from the Sun and the only astronomical object known to harbour and support life. 29.2% of Earth's surface is land consisting of continents and islands. The remaining 70.8% is Water distribution on Earth, covered wi ...

's surface, or the surface of a celestial body. It is an angular measurement, usually expressed in
degrees Degree may refer to: As a unit of measurement * Degree symbol (°), a notation used in science, engineering, and mathematics * Degree (angle), a unit of angle measurement * Degree (temperature), any of various units of temperature measurement ...
and denoted by the
Greek letter The Greek alphabet has been used to write the Greek language Greek (modern , romanized: ''Elliniká'', Ancient Greek, ancient , ''Hellēnikḗ'') is an independent branch of the Indo-European languages, Indo-European family of languages, nat ...

lambda Lambda (; uppercase , lowercase ; el, λάμ(β)δα, ''lám(b)da'') is the 11th letter of the Greek alphabet, representing the sound Dental, alveolar and postalveolar lateral approximants, /l/. In the system of Greek numerals, lambda has a ...

(λ). Meridians (lines running from
pole Pole may refer to: Astronomy *Celestial pole, the projection of the planet Earth's axis of rotation onto the celestial sphere; also applies to the axis of rotation of other planets *Pole star, a visible star that is approximately aligned with the ...
to pole) connect points with the same longitude. The
prime meridian #REDIRECT Prime meridian#REDIRECT Prime meridian A prime meridian is the meridian (geography), meridian (a line of longitude) in a geographic coordinate system at which longitude is defined to be 0°. Together, a prime meridian and its anti-meri ...

, which passes near the
Royal Observatory, Greenwich The Royal Observatory, Greenwich (ROG; known as the Old Royal Observatory from 1957 to 1998, when the working Royal Greenwich Observatory, RGO, temporarily moved south from Greenwich to Herstmonceux Herstmonceux ( , ) is a village and ci ...

, England, is defined as 0° longitude by convention. Positive longitudes are east of the prime meridian, and negative ones are west. Because of the Earth's rotation, there is a close connection between longitude and time. Local time (for example from the position of the Sun) varies with longitude: a difference of 15° longitude corresponds to a one-hour difference in local time. Comparing local time to an absolute measure of time allows longitude to be determined. Depending on the era, the absolute time might be obtained from a celestial event visible from both locations, such as a lunar eclipse, or from a time signal transmitted by telegraph or radio. The principle is straightforward, but in practice finding a reliable method of determining longitude took centuries and required the effort of some of the greatest scientific minds. A location's north–south position along a meridian is given by its
latitude In geography Geography (from Greek: , ''geographia'', literally "earth description") is a field of science devoted to the study of the lands, features, inhabitants, and phenomena of the Earth and planets. The first person to use the ...

, which is approximately the angle between the local vertical and the equatorial plane. Longitude is generally given using the geometrical or astronomical vertical. This can differ slightly from the gravitational vertical because of .

# History

The concept of longitude was first developed by ancient Greek astronomers.
Hipparchus Hipparchus of Nicaea (; el, Ἵππαρχος, ''Hipparkhos'';  BC) was a Greek astronomer, geographer, and mathematician. He is considered the founder of trigonometry, but is most famous for his incidental discovery of precession of the ...
(2nd century BCE) used a coordinate system that assumed a spherical Earth, and divided it into 360° as we still do today. His
prime meridian #REDIRECT Prime meridian#REDIRECT Prime meridian A prime meridian is the meridian (geography), meridian (a line of longitude) in a geographic coordinate system at which longitude is defined to be 0°. Together, a prime meridian and its anti-meri ...

passed through
Alexandria Alexandria ( or ; ar, الإسكندرية ; arz, اسكندرية ; Coptic language, Coptic: Rakodī; el, Αλεξάνδρεια ''Alexandria'') is the List of cities and towns in Egypt, third-largest city in Egypt after Cairo and Giza, ...

. He also proposed a method of determining longitude by comparing the local time of a
lunar eclipse A lunar eclipse occurs when the moves into the . This can occur only when the , Earth, and Moon are exactly or very closely aligned (in ) with Earth between the other two, and only on the night of a . The type and length of a lunar eclipse dep ...

at two different places, thus demonstrating an understanding of the relationship between longitude and time..
Claudius Ptolemy Claudius Ptolemy (; grc-koi, Κλαύδιος Πτολεμαῖος, ''Klaúdios Ptolemaîos'' ; la, Claudius Ptolemaeus; AD) was a mathematician A mathematician is someone who uses an extensive knowledge of mathematics Mathematics ...
(2nd century CE) developed a mapping system using curved parallels that reduced distortion. He also collected data for many locations, from Britain to the Middle East. He used a prime meridian through the Canary Islands, so that all longitude values would be positive. While Ptolemy's system was sound, the data he used were often poor, leading to a gross over-estimate (by about 70%) of the length of the Mediterranean. After the fall of the Roman Empire, interest in geography greatly declined in Europe. Hindu and Muslim astronomers continued to develop these ideas, adding many new locations and often improving on Ptolemy's data. For example used simultaneous observations of two lunar eclipses to determine the difference in longitude between
Antakya Antakya (), historically known as Antioch (Greek: Αντιόχεια), is the capital of Hatay Province Hatay Province ( tr, Hatay ili, ,'' پارێزگای خاتای, ar, لواء إسكندرون, translit=Liwa Iskenderun, lit=District of ...

and
Raqqa Raqqa ( ar, ٱلرَّقَّة, ar-Raqqah, also , and ) is a city in Syria Syria ( ar, سُورِيَا or ar, سُورِيَة, ''Sūriyā''), officially the Syrian Arab Republic ( ar, ٱلْجُمْهُورِيَّةُ ٱلْعَرَ ...
with an error of less than 1°. This is considered to be the best that can be achieved with the methods then available: observation of the eclipse with the naked eye, and determination of local time using an
astrolabe An astrolabe ( grc, ἀστρολάβος ; ar, ٱلأَسْطُرلاب ; persian, ستاره‌یاب ) is an ancient astronomical instrument that was a handheld model of the universe. Its various functions also make it an elaborate inclinom ...

to measure the altitude of a suitable "clock star". In the later Middle Ages, interest in geography revived in the west, as travel increased, and Arab scholarship began to be known through contact with Spain and North Africa. In the 12th century, astronomical tables were prepared for a number of European cities, based on the work of al-Zarqālī in
Toledo Toledo most commonly refers to: * Toledo, Spain, a city in Spain * Province of Toledo, Spain * Toledo, Ohio, a city in the United States Toledo may also refer to: Places Belize * Toledo District * Toledo Settlement Bolivia * Toledo, Oruro ...
. The lunar eclipse of September 12, 1178 was used to establish the longitude differences between Toledo,
Marseille Marseille ( , , ; also spelled in English as Marseilles; oc, Marselha ) is the prefecture A prefecture (from the Latin Latin (, or , ) is a classical language belonging to the Italic languages, Italic branch of the Indo-European langua ...

s, and
Hereford Hereford () is a cathedral city City status in the United Kingdom is granted by the monarch of the United Kingdom The monarchy of the United Kingdom, commonly referred to as the British monarchy, is the constitutional monarchy of the ...

.
Christopher Columbus Christopher Columbus * lij, Cristoffa C(or)ombo * es, Cristóbal Colón * pt, Cristóvão Colombo * ca, Cristòfor (or ) * la, Christophorus Columbus. (; born between 25 August and 31 October 1451, died 20 May 1506) was an Italian ...

made two attempts to use lunar eclipses to discover his longitude, the first in
Saona Island Saona Island ( es, Isla Saona) is a tropical island located a short distance from the mainland on the south-east tip of the Dominican Republic. It is a government-protected nature reserve and is part of ''Parque Nacional Cotubanamá''. The island ...

, on 14 September 1494 (second voyage), and the second in
Jamaica Jamaica (; ) is an island country An island country or an island nation is a country A country is a distinct territory, territorial body or political entity. It is often referred to as the land of an individual's birth, residence or ...

on 29 February 1504 (fourth voyage). It is assumed that he used astronomical tables for reference. His determinations of longitude showed large errors of 13° and 38° W respectively. Randles (1985) documents longitude measurement by the Portuguese and Spanish between 1514 and 1627 both in the Americas and Asia. Errors ranged from 2° to 25°. The telescope was invented in the early 17th century. Initially an observation device, developments over the next half century transformed it into an accurate measurement tool. The
pendulum clock A pendulum clock is a clock that uses a pendulum, a swinging weight, as its timekeeping element. The advantage of a pendulum for timekeeping is that it is a harmonic oscillator: It swings back and forth in a precise time interval dependent on it ...
was patented by
Christiaan Huygens Christiaan Huygens ( , also , ; la, Hugenius; 14 April 1629 – 8 July 1695), also spelled Huyghens, was a Dutch mathematician A mathematician is someone who uses an extensive knowledge of mathematics Mathematics (from Greek: ) i ...

in 1657 and gave an increase in accuracy of about 30 fold over previous mechanical clocks. These two inventions would revolutionise observational astronomy and cartography. On land, the period from the development of telescopes and pendulum clocks until the mid-18th century saw a steady increase in the number of places whose longitude had been determined with reasonable accuracy, often with errors of less than a degree, and nearly always within 2° to 3°. By the 1720s errors were consistently less than 1°. At sea during the same period, the situation was very different. Two problems proved intractable. The first was the need of a navigator for immediate results. The second was the marine environment. Making accurate observations in an ocean swell is much harder than on land, and pendulum clocks do not work well in these conditions.

## The chronometer

In response to the problems of navigation, a number of European maritime powers offered prizes for a method to determine longitude at sea. The best-known of these is the
Longitude Act The Longitude Act 1714 was an Act of Parliament Acts of parliament, sometimes referred to as primary legislation, are texts of law passed by the Legislature, legislative body of a jurisdiction (often a parliament or council). In most countries, act ...
passed by the British parliament in 1714. It offered two levels of rewards, for solutions within 1° and 0.5°. Rewards were given for two solutions: lunar distances, made practicable by the tables of
Tobias Mayer Birthplace of Tobias Mayer Tobias Mayer (17 February 172320 February 1762) was a German astronomer An astronomer is a scientist in the field of astronomy who focuses their studies on a specific question or field outside the scope of Earth. Th ...

developed into an
nautical almanac A nautical almanac is a publication describing the positions of a selection of celestial bodies for the purpose of enabling navigators to use celestial navigation to determine the position of their ship while at sea. The Almanac specifies for each ...
by the
Astronomer Royal Astronomer Royal is a senior post in the Royal Households of the United Kingdom. There are two officers, the senior being the Astronomer Royal dating from 22 June 1675; the second is the Astronomer Royal for Scotland dating from 1834. The post ...
Nevil Maskelyne The Rev Dr Nevil Maskelyne DD FRS FRSE (; 6 October 1732 – 9 February 1811) was the fifth British Astronomer Royal. He held the office from 1765 to 1811. He was the first person to scientifically measure the mass of the planet Earth. Bio ...

; and for the chronometers developed by the Yorkshire carpenter and clock-maker
John Harrison John Harrison ( – 24 March 1776) was a self-educated English carpenter Carpenters in an Indian village Carpentry is a skilled trade and a craft in which the primary work performed is the cutting, shaping and installation of building mate ...

. Harrison built five chronometers over more than three decades. This work was supported and rewarded with thousands of pounds from the Board of Longitude, but he fought to receive money up to the top reward of £20,000, finally receiving an additional payment in 1773 after the intervention of parliament. It was some while before either method became widely used in navigation. In the early years, chronometers were very expensive, and the calculations required for lunar distances were still complex and time-consuming. Lunar distances came into general use after 1790. Chronometers had the advantages that both the observations and the calculations were simpler, and as they became cheaper in the early 19th century they started to replace lunars, which were seldom used after 1850. The first working telegraphs were established in Britain by Wheatstone and Cooke in 1839, and in the US by in 1844. It was quickly realised that the telegraph could be used to transmit a time signal for longitude determination. The method was soon in practical use for longitude determination, especially in North America, and over longer and longer distances as the telegraph network expanded, including western Europe with the completion of transatlantic cables. The US Coast Survey was particularly active in this development, and not just in the United States. The Survey established chains of mapped locations through Central and South America, and the West Indies, and as far as Japan and China in the years 1874–90. This contributed greatly to the accurate mapping of these areas. While mariners benefited from the accurate charts, they could not receive telegraph signals while under way, and so could not use the method for navigation. This changed when wireless telegraphy (radio) became available in the early 20th century. Wireless time signals for the use of ships were transmitted from
Halifax, Nova Scotia Halifax (Miꞌkmaq The Miꞌkmaq (also ''Mi'gmaq'', ''Lnu'', ''Miꞌkmaw'' or ''Miꞌgmaw''; ; ) are a First Nations people of the Indigenous peoples of the Northeastern Woodlands, Northeastern Woodlands, indigenous to the areas now known as C ...
, starting in 1907 and from the
Eiffel Tower The Eiffel Tower ( ; french: links=yes, tour Eiffel ) is a wrought-iron Wrought iron is an iron Iron () is a chemical element with Symbol (chemistry), symbol Fe (from la, Wikt:ferrum, ferrum) and atomic number 26. It is a metal th ...

in Paris from 1910. These signals allowed navigators to check and adjust their chronometers frequently.
systems came into general use after
World War II World War II or the Second World War, often abbreviated as WWII or WW2, was a global war A world war is "a war War is an intense armed conflict between states State may refer to: Arts, entertainment, and media Literatur ...
. The systems all depended on transmissions from fixed navigational beacons. A ship-board receiver calculated the vessel's position from these transmissions. They allowed accurate navigation when poor visibility prevented astronomical observations, and became the established method for commercial shipping until replaced by
GPS The Global Positioning System (GPS), originally Navstar GPS, is a satellite-based radionavigation system owned by the United States government The federal government of the United States (U.S. federal government) is the national ...
in the early 1990s.

# Determination

The main methods for determining longitude are listed below. With one exception (magnetic declination) they all depend on a common principle, which was to determine an absolute time from an event or measurement and to compare the corresponding local time at two different locations. * Lunar distances. In its orbit around the Earth, the Moon moves relative to the stars at a rate of just over 0.5°/hour. The angle between the Moon and a suitable star is measured with a
sextant A sextant is a doubly reflecting navigation instrument that measures the angular distance Angular distance \theta (also known as angular separation, apparent distance, or apparent separation) is the angle In Euclidean geometry, an angle is ...

, and (after consulting tables and lengthy calculations) gives a value for absolute time. *Satellites of Jupiter.
Galileo Galileo di Vincenzo Bonaiuti de' Galilei ( , ; 15 February 1564 – 8 January 1642), commonly referred to as Galileo, was an astronomer An astronomer is a scientist in the field of astronomy who focuses their studies on a specific qu ...

proposed that with sufficiently accurate knowledge of the orbits of the satellites, their positions could provide a measure of absolute time. The method requires a telescope, as the moons are not visible to the naked eye. *Appulses, occultations, and eclipses. An appulse is the least apparent distance between two objects (the Moon, a star or a planet); an
occultation An occultation is an event that occurs when one object is hidden by another object that passes between it and the observer. The term is often used in astronomy, but can also refer to any situation in which an object in the foreground blocks fro ...

occurs when a star or planet passes behind the Moon — essentially a type of eclipse. Lunar eclipses continued to be used. The times of any of these events can be used as the measure of absolute time. * Chronometers. A clock is set to the local time of a starting point whose longitude is known, and the longitude of any other place can be determined by comparing its local time with the clock time. *Magnetic declination. A compass needle does not in general point exactly north. The
variation Variation or Variations may refer to: Science and mathematics * Variation (astronomy), any perturbation of the mean motion or orbit of a planet or satellite, particularly of the moon * Genetic variation thumb File:Genetic Variation and Inhe ...

from true north varies with location, and it was suggested that this could provide a basis for determination of longitude. With the exception of magnetic declination, all proved practicable methods. Developments on land and sea, however, were very different. There is no other physical principle determining longitude directly but with time. Longitude at a point may be determined by calculating the time difference between that at its location and
Coordinated Universal Time Coordinated Universal Time or UTC is the primary time standard A time standard is a specification for measuring time: either the rate at which time passes; or points in time; or both. In modern times, several time specifications have been o ...
(UTC). Since there are 24 hours in a day and 360 degrees in a circle, the sun moves across the sky at a rate of 15 degrees per hour (360° ÷ 24 hours = 15° per hour). So if a location's
time zone A time zone is an area that observes a uniform standard time Standard time is the synchronization of clock A clock or a timepiece is a device used to Measurement, measure and indicate time. The clock is one of the oldest Invent ...
is three hours ahead of UTC then that location is near 45° longitude (3 hours × 15° per hour = 45°). The word ''near'' is used because the point might not be at the centre of the time zone; also the time zones are defined politically, so their centres and boundaries often do not lie on meridians at multiples of 15°. In order to perform this calculation, however, one needs a chronometer (watch) set to UTC and needs to determine local time by solar or astronomical observation. The details are more complex than described here: see the articles on
Universal Time#REDIRECT Universal Time Universal Time (UT) is a time standard based on Earth's rotation. There are several versions of Universal Time, which differ by up to a few seconds. The most commonly used are Coordinated Universal Time (UTC) and UT1 (see ...

and on the
equation of time The equation of time describes the discrepancy between two kinds of solar time Solar time is a calculation of the passage of time Time is the continued of and that occurs in an apparently succession from the , through the , int ...

for more details.

# Values

Longitude is given as an
angular measurement In Euclidean geometry Euclidean geometry is a mathematical system attributed to Alexandria ) , name = Alexandria ( or ; ar, الإسكندرية ; arz, اسكندرية ; Coptic language, Coptic: Rakod ...

ranging from 0° at the Prime Meridian to +180° eastward and −180° westward. The Greek letter λ (lambda) is used to denote the location of a place on Earth east or west of the Prime Meridian. Each degree of longitude is sub-divided into 60
minutes Minutes, also known as minutes of meeting (abbreviation MoM), protocols or, informally, notes, are the instant written record of a meeting A meeting is when two or more people A people is a plurality of person A person (plural people ...
, each of which is divided into 60 seconds. A longitude is thus specified in
sexagesimal Sexagesimal, also known as base 60 or sexagenary, is a numeral system A numeral system (or system of numeration) is a writing system A writing system is a method of visually representing verbal communication Communication (from Latin ...
notation as, for example, 23° 27′ 30″ E. For higher precision, the seconds are specified with a
decimal fraction The decimal numeral system A numeral system (or system of numeration) is a writing system A writing system is a method of visually representing verbal communication Communication (from Latin ''communicare'', meaning "to share") is t ...
. An alternative representation uses degrees and minutes, and parts of a minute are expressed in decimal notation, thus: 23° 27.5′ E. Degrees may also be expressed as a decimal fraction: 23.45833° E. For calculations, the angular measure may be converted to
radian The radian, denoted by the symbol \text, is the SI unit The International System of Units, known by the international abbreviation SI in all languages and sometimes pleonastically as the SI system, is the modern form of the metric sy ...

s, so longitude may also be expressed in this manner as a signed fraction of (), or an unsigned fraction of 2. For calculations, the West/East suffix is replaced by a negative sign in the
western hemisphere The Western Hemisphere is the half of Earth Earth is the third planet from the Sun and the only astronomical object known to harbour and support life. 29.2% of Earth's surface is land consisting of continents and islands. The remaining ...
. The international standard convention (
ISO 6709 ISO 6709, ''Standard representation of geographic point location by coordinates'', is the international standard for representation of latitude, longitude and altitude for geographic point locations. The first edition (ISO 6709:1983) was develop ...
)—that East is positive—is consistent with a right-handed
Cartesian coordinate system A Cartesian coordinate system (, ) in a plane Plane or planes may refer to: * Airplane An airplane or aeroplane (informally plane) is a fixed-wing aircraft A fixed-wing aircraft is a heavier-than-air flying machine Early fly ...
, with the North Pole up. A specific longitude may then be combined with a specific latitude (positive in the
northern hemisphere The Northern Hemisphere is the half of Earth Earth is the third planet from the Sun and the only astronomical object known to harbour and support life. 29.2% of Earth's surface is land consisting of continents and islands. The remain ...

) to give a precise position on the Earth's surface. Confusingly, the convention of negative for East is also sometimes seen, most commonly in the
United States The United States of America (U.S.A. or USA), commonly known as the United States (U.S. or US) or America, is a country Continental United States, primarily located in North America. It consists of 50 U.S. state, states, a Washington, D.C., ...

; the
Earth System Research Laboratory The Earth System Research Laboratory (ESRL) is a laboratory in National Oceanic and Atmospheric Administration's (NOAA) Office of Oceanic and Atmospheric Research (OAR). It is one of seven NOAA Research Laboratories (RLs) and is located in Boulder, ...
used it on an older version of one of their pages, in order "to make coordinate entry less awkward" for applications confined to the
Western Hemisphere The Western Hemisphere is the half of Earth Earth is the third planet from the Sun and the only astronomical object known to harbour and support life. 29.2% of Earth's surface is land consisting of continents and islands. The remaining ...
. They have since shifted to the standard approach.NOAA ESRL Sunrise/Sunset Calculator
(deprecated). ''
Earth System Research Laboratory The Earth System Research Laboratory (ESRL) is a laboratory in National Oceanic and Atmospheric Administration's (NOAA) Office of Oceanic and Atmospheric Research (OAR). It is one of seven NOAA Research Laboratories (RLs) and is located in Boulder, ...
''. Retrieved October 18, 2019. Note that the longitude is
singular Singular may refer to: * Singular, the grammatical number In linguistics, grammatical number is a grammatical category of nouns, pronouns, adjectives, and verb agreement (linguistics), agreement that expresses count distinctions (such as "one", ...
at the
Poles The Poles,, ; singular masculine: ''Polak'', singular feminine: ''Polka'' or Polish people, are a nation A nation is a community A community is a social unitThe term "level of analysis" is used in the social sciences to point to the loc ...
and calculations that are sufficiently accurate for other positions may be inaccurate at or near the Poles. Also the discontinuity at the ± 180° meridian must be handled with care in calculations. An example is a calculation of east displacement by subtracting two longitudes, which gives the wrong answer if the two positions are on either side of this meridian. To avoid these complexities, consider replacing latitude and longitude with another
horizontal position representation A position representation is the parameters used to express a position relative to a reference. When representing positions relative to the Earth, it is often most convenient to represent vertical position (height or depth) separately, and to use s ...
in calculation.

# Length of a degree of longitude

The length of a degree of longitude (east–west distance) depends only on the radius of a circle of latitude. For a sphere of radius that radius at latitude is , and the length of a one-degree (or
radian The radian, denoted by the symbol \text, is the SI unit The International System of Units, known by the international abbreviation SI in all languages and sometimes pleonastically as the SI system, is the modern form of the metric sy ...

) arc along a circle of latitude is :$\Delta^1_= \fraca \cos \phi$ When the Earth is modelled by an
ellipsoid An ellipsoid is a surface that may be obtained from a sphere by deforming it by means of directional Scaling (geometry), scalings, or more generally, of an affine transformation. An ellipsoid is a quadric surface;  that is, a Surface (mathemat ...

this arc length becomes :$\Delta^1_=\frac$ where , the eccentricity of the ellipsoid, is related to the major and minor axes (the equatorial and polar radii respectively) by :$e^2=\frac$ An alternative formula is :$\Delta^1_= \fraca \cos \beta \quad \mbox\tan \beta = \frac \tan \phi$; here $\beta$ is the so-called parametric or reduced latitude. cos decreases from 1 at the equator to 0 at the poles, which measures how circles of latitude shrink from the equator to a point at the pole, so the length of a degree of longitude decreases likewise. This contrasts with the small (1%) increase in the length of a degree of latitude (north–south distance), equator to pole. The table shows both for the
WGS84 The World Geodetic System (WGS) is a standard for use in cartography Cartography (; from Greek χάρτης ''chartēs'', "papyrus, sheet of paper, map"; and γράφειν ''graphein'', "write") is the study and practice of making and us ...
ellipsoid with = and = . Note that the distance between two points 1 degree apart on the same circle of latitude, measured along that circle of latitude, is slightly more than the shortest (
geodesic In geometry Geometry (from the grc, γεωμετρία; ' "earth", ' "measurement") is, with , one of the oldest branches of . It is concerned with properties of space that are related with distance, shape, size, and relative position o ...

) distance between those points (unless on the equator, where these are equal); the difference is less than . A
geographical mile The geographical mile is a unit of length determined by 1 minute of arc along the Earth Earth is the third planet from the Sun and the only astronomical object known to harbor life. About 29% of Earth's surface is land consisting of conti ...
is defined to be the length of one
minute of arc A minute of arc, arcminute (arcmin), arc minute, or minute arc, denoted by the symbol , is a unit of angular measurement equal to of one degree Degree may refer to: As a unit of measurement * Degree symbol (°), a notation used in scie ...
along the equator (one equatorial minute of longitude) therefore a degree of longitude along the equator is exactly 60 geographical miles or 111.3 kilometers, as there are 60 minutes in a degree. The length of 1 minute of longitude along the equator is 1 geographical mile or , while the length of 1 second of it is 0.016 geographical mile or .

*
American Practical Navigator ''The American Practical Navigator'' (colloquially often referred to as ''Bowditch''), originally written by Nathaniel Bowditch, is an encyclopedia of navigation Navigation is a field of study that focuses on the process of monitoring and control ...
*
Cardinal direction The four cardinal directions, or cardinal points, are the directions , , , and , commonly denoted by their initials N, E, S, and W. East and west are (at s) to north and south, with east being in the direction of rotation from north and west ...
*
Ecliptic longitude The ecliptic coordinate system is a celestial coordinate system commonly used for representing the apparent place, apparent positions and orbits of Solar System objects. Because most planets (except Mercury (planet), Mercury) and many Small Solar S ...
*
Geodesy Geodesy ( ) is the Earth science of accurately measuring and understanding Earth's geometric shape, orientation in space, and gravitational field. The field also incorporates studies of how these properties change over time and equivalent measu ...
*
Geodetic system A geodetic datum or geodetic system (also: geodetic reference datum, geodetic reference system, or geodetic reference frame) is a global or for precisely measuring locations on or other planetary bodies. are crucial to any technology or techni ...
*
Geographic coordinate system A geographic coordinate system (GCS) is a coordinate system associated with positions on Earth (geographic position). A GCS can give positions: *as spherical coordinate system using latitude In geography, latitude is a geographic c ...
*
Geographical distance Geographical distance is the distance measured along the surface of the earth Earth is the third planet from the Sun and the only astronomical object known to harbor life. About 29% of Earth's surface is land consisting of continent ...
*
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*
Great-circle distance The great-circle distance, orthodromic distance, or spherical distance is the shortest distance between two points on the surface of a sphere of a sphere A sphere (from Greek language, Greek —, "globe, ball") is a Geometry, geometrical ob ...
*
History of longitude The history of longitude is a record of the effort, by astronomers, cartographers and navigators over the centuries, to discover a means of determining longitude Longitude (, ), is a geographic coordinate that specifies the east East is on ...
* '' The Island of the Day Before'' *
Latitude In geography Geography (from Greek: , ''geographia'', literally "earth description") is a field of science devoted to the study of the lands, features, inhabitants, and phenomena of the Earth and planets. The first person to use the ...

*
Meridian arc In geodesy Geodesy () is the Earth science Earth science or geoscience includes all fields of natural science Natural science is a branch of science Science (from the Latin word ''scientia'', meaning "knowledge") is a systematic ...
*
Natural Area Code The Natural Area Code (or Universal Address) is a proprietary geocode A geocode is a code In communication Communication (from Latin ''communicare'', meaning "to share") is the act of developing Semantics, meaning among Subject (philoso ...
*
Navigation Navigation is a field of study that focuses on the process of monitoring and controlling the movement of a craft or vehicle from one place to another.Bowditch, 2003:799. The field of navigation includes four general categories: land navigation, ...

*
Orders of magnitude An order of magnitude is an approximation of the logarithm In mathematics Mathematics (from Ancient Greek, Greek: ) includes the study of such topics as quantity (number theory), mathematical structure, structure (algebra), space (geome ...
* *
Right ascension Right ascension (abbreviated RA; symbol ) is the angular distance of a particular point measured eastward along the celestial equator from the Sun at the equinox (celestial coordinates), March equinox to the (hour circle of the) point in questio ...

on
celestial sphere In astronomy Astronomy (from el, ἀστρονομία, literally meaning the science that studies the laws of the stars) is a natural science that studies astronomical object, celestial objects and celestial event, phenomena. It uses ma ...

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World Geodetic System The World Geodetic System (WGS) is a standard for use in cartography Cartography (; from χάρτης ''chartēs'', "papyrus, sheet of paper, map"; and γράφειν ''graphein'', "write") is the study and practice of making and using ...

# References

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Resources for determining your latitude and longitude

* [http://entertainment.timesonline.co.uk/tol/arts_and_entertainment/the_tls/article5136819.ece "Longitude forged"]: an essay exposing a hoax solution to the problem of calculating longitude, undetected in Dava Sobel's Longitude, fro
TLS
November 12, 2008.
Board of Longitude Collection, Cambridge Digital Library
– complete digital version of the Board's archive
Longitude And Latitude Of Points of Interest

A land beyond the stars - Museo Galileo
{{Authority control Meridians (geography), * Navigation Geodesy