The 180th meridian is the line of longitude exactly opposite the Prime Meridian. 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 through the open waters of the Pacific Ocean. However, it does pass through a few countries: Map of Earth showing lines of longitude, which appear curved and vertical in this projection, but are actually halves of great circles Longitude, sometimes denoted by the Greek letter λ, describes the location of a place on Earth east or west of a north-south line called the Prime Meridian. ... Location of the Prime Meridian Prime Meridian in Greenwich The Prime Meridian is the meridian (line of longitude) passing through the Royal Greenwich Observatory, Greenwich, England; it is the meridian at which longitude is 0 degrees. ... The International Date Line around 180° The International Date Line is an imaginary line on the surface of the Earth, which offsets the hours that are added or subtracted as one travels east or west through successive time zones. ...
Longitude is seen to be a measure of the angle between the planes of two meridian circles, one of which is the prime meridian.
Lines of longitude are numbered east of the prime meridian from 0 to 180 east longitude and west from 0 to 180 west longitude.
Before a prime meridian was agreed upon, map makers usually began numbering the lines of longitude on their maps at whichever meridian passed through the site of their national observatory.
Bounding meridians are evenly divisible by 6°, and zones are numbered from 1 to 60 proceeding east from the 180thmeridian from Greenwich with minor exceptions.
The non-central meridians are drawn from pole to pole, intersecting the meridians at the proper point.
Since the spacings on the parallels are proportional to the cosine of the latitude, and since parallels are equally spaced, the meridians form curves which may be called cosine, sine, or sinusoidal curves; hence, the name of the projection.
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