Longitude

WestlongitudinalEWlongitudes90°Eeast longitudelongeastlong.
Longitude (, AU and UK also ), 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.wikipedia
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Geographic coordinate system

Coordinatesgeographic coordinateslatitude and longitude
Longitude (, AU and UK also ), 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.
A common choice of coordinates is latitude, longitude and elevation.

Prime meridian

GreenwichGreenwich meridianGreenwich Prime Zero meridian
By convention, one of these, the Prime Meridian, which passes through the Royal Observatory, Greenwich, England, was allocated the position of 0° longitude.
A prime meridian is a meridian (a line of longitude) in a geographic coordinate system at which longitude is defined to be 0°.

Meridian (geography)

meridianmeridiansmeridian line
Meridians (lines running from pole to pole) connect points with the same longitude.
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.

Marine chronometer

chronometerchronometersChronoscope
John Harrison, a self-educated English clockmaker, invented the marine chronometer, the key piece in solving the problem of accurately establishing longitude at sea, thus revolutionising and extending the possibility of safe long distance sea travel.
A marine chronometer is a timepiece that is precise and accurate enough to be used as a portable time standard; it can therefore be used to determine longitude by means of accurately measuring the time of a known fixed location, for example Greenwich Mean Time (GMT) and the time at the current location.

Royal Observatory, Greenwich

Royal Greenwich ObservatoryRoyal ObservatoryGreenwich Observatory
By convention, one of these, the Prime Meridian, which passes through the Royal Observatory, Greenwich, England, was allocated the position of 0° longitude.
The basis of longitude, the meridian that passes through the Airy transit circle, first used in 1851, was adopted as the world's Prime Meridian at the International Meridian Conference on 22 October 1884 (voting took place on 13 October).

Degree (angle)

°degreesdegree
It is an angular measurement, usually expressed in degrees and denoted by the Greek letter lambda .
This property has many useful applications, such as dividing the world into 24 time zones, each of which is nominally 15° of longitude, to correlate with the established 24-hour day convention.

Lunar distance (navigation)

lunar distance methodlunar distancemethod of lunar distances
See Lunar distance (navigation).
In celestial navigation, knowledge of the time at Greenwich and the measured positions of one or more celestial objects allows the navigator to calculate latitude and longitude.

John Harrison

H-4H4H4 and H5
John Harrison, a self-educated English clockmaker, invented the marine chronometer, the key piece in solving the problem of accurately establishing longitude at sea, thus revolutionising and extending the possibility of safe long distance sea travel.
Longitude fixes the location of a place on Earth east or west of a north-south line called the prime meridian.

Right ascension

RAR.A.α
The vertical north–south plane still intersects the plane of the Greenwich meridian at some angle; that angle is the astronomical longitude, calculated from star observations.
Right ascension is the celestial equivalent of terrestrial longitude.

International Meridian Conference

1884 prime meridian systemabandonedcontemporaneously
While British cartographers had long used the Greenwich meridian in London, other references were used elsewhere, including El Hierro, Rome, Copenhagen, Jerusalem, Saint Petersburg, Pisa, Paris (see the article Paris meridian), Philadelphia, and Washington D.C. In 1884 the International Meridian Conference adopted the Greenwich meridian as the universal Prime Meridian or zero point of longitude.
It resulted in the recommendation of the Greenwich Meridian as the international standard for zero degrees longitude.

Longitude Act

Longitude Act 1714
In 1714 the British government passed the Longitude Act which offered large financial rewards to the first person to demonstrate a practical method for determining the longitude of a ship at sea.
It established the Board of Longitude and offered monetary rewards (Longitude rewards) for anyone who could find a simple and practical method for the precise determination of a ship's longitude.

Time zone

Sri Lanka Standard Time Zonetime zoneslocal time
So if the time zone a person is in is three hours ahead of UTC then that person is near 45° longitude (3 hours × 15° per hour = 45°).
Before clocks were invented, it was common practice to mark the time of day with apparent solar time (also called "true" solar time) – for example, the time on a sundial – which was typically different for every location and dependent on longitude.

Celestial sphere

celestialcelestial hemispherehemisphere
The vertical north–south plane still intersects the plane of the Greenwich meridian at some angle; that angle is the astronomical longitude, calculated from star observations.
Similar to geographic longitude and latitude, the equatorial coordinate system specifies positions relative to the celestial equator and celestial poles, using right ascension and declination.

Lambda

λLamdaλ
It is an angular measurement, usually expressed in degrees and denoted by the Greek letter lambda .

Geographical pole

polespolepolar
Meridians (lines running from pole to pole) connect points with the same longitude.
As cartography requires exact and unchanging coordinates, the averaged locations of geographical poles are taken as fixed cartographic poles and become the points where the body's great circles of longitude intersect.

180th meridian

180°antimeridianAnti-Meridian
Also the discontinuity at the ±180° meridian must be handled with care in calculations.
It is common to both east longitude and west longitude.

Horizontal position representation

Horizontal positionhorizontal gridrepresentation
To avoid these complexities, consider replacing latitude and longitude with another horizontal position representation in calculation.
Latitude/longitude and UTM are common horizontal position representations.

Galileo Galilei

GalileoGalileanGalilei
In 1612 Galileo Galilei demonstrated that with sufficiently accurate knowledge of the orbits of the moons of Jupiter one could use their positions as a universal clock and this would make possible the determination of longitude, but the method he devised was impracticable for navigators on ships because of their instability.
In 1612, having determined the orbital periods of Jupiter's satellites, Galileo proposed that with sufficiently accurate knowledge of their orbits, one could use their positions as a universal clock, and this would make possible the determination of longitude.

Latitude

latitudesSouthlatitudinal
A location's north–south position along a meridian is given by its latitude, which is approximately the angle between the local vertical and the equatorial plane.
Latitude is used together with longitude to specify the precise location of features on the surface of the Earth.

North Pole

NorthGeographic North Pole90
(This forms a right-handed coordinate system with the z-axis (right hand thumb) pointing from the Earth's center toward the North Pole and the x-axis (right hand index finger) extending from the Earth's center through the Equator at the Prime Meridian.)
At the North Pole all directions point south; all lines of longitude converge there, so its longitude can be defined as any degree value.

Navigation

nauticalnavigatenavigational
The measurement of longitude is important both to cartography and for ocean navigation.
A GNSS allow small electronic receivers to determine their location (longitude, latitude, and altitude) to within a few metres using time signals transmitted along a line of sight by radio from satellites.

Equation of time

difference of timeeccentricity of the Earth's orbitEquation of Time Tables
The details are more complex than described here: see the articles on Universal Time and on the equation of time for more details.
These calculated the mean time, albeit local to a point of longitude.

Plate tectonics

tectonic platesplate tectonictectonic
The Earth's tectonic plates move relative to one another in different directions at speeds on the order of 50 to 100mm per year.
For these mechanisms to be overall valid, systematic relationships should exist all over the globe between the orientation and kinematics of deformation and the geographical latitudinal and longitudinal grid of the Earth itself.

Geographical mile

mile (geographical)miles
A geographical mile is defined to be the length of one minute of arc 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.
In any ellipsoid, the length of a degree of longitude at the equator is thus exactly 60 geographical miles.

Minute and second of arc

masarcsecondarc second
A geographical mile is defined to be the length of one minute of arc 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. Each degree of longitude is sub-divided into 60 minutes, each of which is divided into 60 seconds.
Positions are traditionally given using degrees, minutes, and seconds of arcs for latitude, the arc north or south of the equator, and for longitude, the arc east or west of the Prime Meridian.