Meridian (geography)
Encyclopedia
A meridian is an imaginary line on the Earth's surface from the North Pole
North Pole
The North Pole, also known as the Geographic North Pole or Terrestrial North Pole, is, subject to the caveats explained below, defined as the point in the northern hemisphere where the Earth's axis of rotation meets its surface...

to the South Pole
South Pole
The South Pole, also known as the Geographic South Pole or Terrestrial South Pole, is one of the two points where the Earth's axis of rotation intersects its surface. It is the southernmost point on the surface of the Earth and lies on the opposite side of the Earth from the North Pole...

that connects all locations along it with a given longitude
Longitude
Longitude is a geographic coordinate that specifies the east-west position of a point on the Earth's surface. It is an angular measurement, usually expressed in degrees, minutes and seconds, and denoted by the Greek letter lambda ....

. The position of a point along the meridian is given by its latitude
Latitude
In geography, the latitude of a location on the Earth is the angular distance of that location south or north of the Equator. The latitude is an angle, and is usually measured in degrees . The equator has a latitude of 0°, the North pole has a latitude of 90° north , and the South pole has a...

. Each meridian is perpendicular to all circles of latitude
Circle of latitude
A circle of latitude, on the Earth, is an imaginary east-west circle connecting all locations that share a given latitude...

. Each is also the same size, being half of a great circle
Great circle
A great circle, also known as a Riemannian circle, of a sphere is the intersection of the sphere and a plane which passes through the center point of the sphere, as opposed to a general circle of a sphere where the plane is not required to pass through the center...

on the Earth's surface and therefore measuring 20,003.93 km (12,429.9 miles).

The meridian through Greenwich, England, also called the Prime Meridian
Prime Meridian
The Prime Meridian is the meridian at which the longitude is defined to be 0°.The Prime Meridian and its opposite the 180th meridian , which the International Date Line generally follows, form a great circle that divides the Earth into the Eastern and Western Hemispheres.An international...

was set at zero degrees of longitude, with other meridians being 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, forming the other half of a circle with the one through Greenwich, is 180° longitude
180th meridian
The 180th meridian or antimeridian is the meridian which is 180° east or west of the Prime Meridian passing through the Royal Observatory, Greenwich. It is common to both east longitude and west longitude. It is used as the basis for the International Date Line because it for the most part passes...

, and the others lie between 0° and 180° of West longitude in the Western Hemisphere
Western Hemisphere
The Western Hemisphere or western hemisphere is mainly used as a geographical term for the half of the Earth that lies west of the Prime Meridian and east of the Antimeridian , the other half being called the Eastern Hemisphere.In this sense, the western hemisphere consists of the western portions...

(West of Greenwich) and between 0° and 180° of East longitude in the Eastern Hemisphere
Eastern Hemisphere
The Eastern Hemisphere, also Eastern hemisphere or eastern hemisphere, is a geographical term for the half of the Earth that is east of the Prime Meridian and west of 180° longitude. It is also used to refer to Europe, Asia, Africa, and Australasia, vis-à-vis the Western Hemisphere, which includes...

(East of Greenwich). Most maps show the lines of longitude.

The position of the 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 have had no significant effect as historically, the accuracy of the determination of longitude was much larger than the change in position. The adoption of WGS84 as the positioning system, has moved the meridian approximately 200 metres east of its last position (measured at Greenwich). The position of the current meridian is not identified at all in Greenwich but may easily be located using a GPS receiver.

The term "meridian" comes from the Latin meridies, meaning "midday"; the sun crosses a given meridian midway between the times of sunrise and sunset on that meridian. The same Latin stem gives rise to the terms a.m. (ante meridiem) and p.m. (post meridiem)
12-hour clock
The 12-hour clock is a time conversion convention in which the 24 hours of the day are divided into two periods called ante meridiem and post meridiem...

used to disambiguate hours of the day when using the 12-hour clock
12-hour clock
The 12-hour clock is a time conversion convention in which the 24 hours of the day are divided into two periods called ante meridiem and post meridiem...

.

The magnetic meridian is an equivalent imaginary line connecting the magnetic south and north poles and can be taken as the magnetic force lines along the surface of the earth. That is, a compass needle will be parallel to the magnetic meridian. The angle between the magnetic and the true meridian is the magnetic declination
Magnetic declination
Magnetic declination is the angle between magnetic north and true north. The declination is positive when the magnetic north is east of true north. The term magnetic variation is a synonym, and is more often used in navigation...

, which is relevant for navigating with a compass.

• Prime Meridian
Prime Meridian
The Prime Meridian is the meridian at which the longitude is defined to be 0°.The Prime Meridian and its opposite the 180th meridian , which the International Date Line generally follows, form a great circle that divides the Earth into the Eastern and Western Hemispheres.An international...

(includes other reference meridians)
• Meridian (astronomy)
Meridian (astronomy)
This article is about the astronomical concept. For other uses of the word, see Meridian.In the sky, a meridian is an imaginary great circle on the celestial sphere. It passes through the north point on the horizon, through the celestial pole, up to the zenith, through the south point on the...

• Parallels
Circle of latitude
A circle of latitude, on the Earth, is an imaginary east-west circle connecting all locations that share a given latitude...

• Meridian lines used with a gnomon to measure solar elevation and time of year
• Meridian arc
Meridian arc
In geodesy, a meridian arc measurement is a highly accurate determination of the distance between two points with the same longitude. Two or more such determinations at different locations then specify the shape of the reference ellipsoid which best approximates the shape of the geoid. This...

For meridians used as references in surveying:
• Public Land Survey System
Public Land Survey System
The Public Land Survey System is a method used in the United States to survey and identify land parcels, particularly for titles and deeds of rural, wild or undeveloped land. Its basic units of area are the township and section. It is sometimes referred to as the rectangular survey system,...

, United States
• Dominion Land Survey
Dominion Land Survey
The Dominion Land Survey is the method used to divide most of Western Canada into one-square-mile sections for agricultural and other purposes. It is based on the layout of the Public Land Survey System used in the United States, but has several differences...