Editing Longitude (section)

{{Short description|Geographic coordinate that specifies the east-west position of a point on the Earth's surface}}
{{other uses|Longitude (disambiguation)}}
{{Use British English|date=May 2021}}  
[[File:Division of the Earth into Gauss-Krueger zones - Globe.svg|thumb|upright=0.9|right|A [[Geographic coordinate system|graticule]] on the [[Earth]] as a [[sphere]] or an [[ellipsoid]]. The lines from pole to pole are lines of constant longitude, or [[meridian (geography)|meridians]]. The circles parallel to the [[Equator]] are circles of constant [[latitude]], or [[circle of latitude|parallels]]. The graticule shows the latitude and longitude of points on the surface. In this example, meridians are spaced at 6° intervals and parallels at 4° intervals.]]
{{longlat}}{{Geodesy|notshowfig=1}}

'''Longitude''' ({{IPAc-en|ˈ|l|ɒ|n|dʒ|ᵻ|tj|uː|d}}, {{small|AU and UK also}} {{IPAc-en|ˈ|l|ɒ|ŋ|ɡ|ᵻ|-}})<ref>{{cite web |url=http://www.merriam-webster.com/dictionary/longitude |title=Definition of LONGITUDE |publisher=[[Merriam-Webster]] |access-date=14 March 2018 |archive-date=16 June 2018 |archive-url=https://web.archive.org/web/20180616030146/https://www.merriam-webster.com/dictionary/longitude |url-status=live }}</ref><ref>[[Oxford English Dictionary]]</ref> is a [[geographic coordinate system|geographic coordinate]] that specifies the [[east]]-[[west]] position of a point on the surface of the [[Earth]], or another [[Celestial navigation|celestial]] body. It is an [[angular measurement]], usually expressed in [[degree (angle)|degrees]] and denoted by the [[Greek alphabet|Greek letter]] [[lambda]] (λ). [[meridian (geography)|Meridians]] are imaginary semicircular lines running from [[geographical pole|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, Greenwich|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.

Because of the [[Earth's rotation]], there is a close connection [[Time zone|between longitude and time measurement]]. Scientifically precise [[local time]] varies with longitude: a difference of 15° longitude corresponds to a one-hour difference in local time, due to the differing position in relation to the Sun. 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]]'', which is approximately the angle between the [[equatorial plane]] and the [[Normal (geometry)|normal]] from the ground at that location.

Longitude is generally given using the [[geodetic normal]] or the [[gravity direction]]. The '''astronomical longitude''' can differ slightly from the ordinary longitude because of ''[[vertical deflection]]'', small variations in Earth's gravitational field (see [[astronomical latitude]]).