The prime meridian is the reference for all lines of longitude, the imaginary vertical lines that divide the Earth's surface. The opposite meridian is the 180-degree longitude line, which is 180 degrees both east and west of the prime meridian. The 180-degree longitude line also serves as an approximate basis for drawing the international date line.
Lines of Longitude
Lines of longitude are imaginary, vertical lines along the surface of the Earth that are used as coordinates to determine locations east and west of a reference line. All lines of longitude join at the poles.
The prime meridian, also known as the Greenwich meridian line, is labeled zero degrees and runs through Western Europe and Western Africa. It is the reference on which all lines of longitude are based. For example, 30 degrees west longitude means 30 degrees west of the prime meridian.
180 Degrees Longitude
Because the Earth is spherical, traveling 180 degrees in either direction from the prime meridian will lead to 180 degrees longitude, the opposite meridian. The 180-degree meridian runs through the western Pacific Ocean.
The Earth is divided into 24 time zones, each approximately 15 degrees longitude in width. The prime meridian marks the Greenwich (or Zulu) time zone, with other time zones labeled plus or minus hours from this zone.
International Date Line
The international date line roughly corresponds to the 180-degree meridian, although it deviates with respect to national borders. Because the Earth is divided into 24 time zones, each plus or minus a given number of hours from Greenwich, there arises a line on the Earth where the plus and minus zones meet. By tradition, this occurs at the international date line. Traveling east across the line decreases the date by one (for example, from Wednesday to Tuesday) and traveling west across the line advances the date (Tuesday to Wednesday).
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