A worldwide grid
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 Latitude and longitude. © NMM
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Since the time of the ancient Greeks, mapmakers have divided the surface of the Earth into an imaginary fixed grid of equally spaced horizontal and vertical lines called a graticule.
This allows any place in the world to be pinpointed using just two coordinates.
The horizontal lines of the grid, running parallel to the equator, are called lines of latitude.
The vertical lines, each running between the north and south poles, are called meridians of longitude.
Where is zero?
The lines of latitude use the equator as the natural reference 'zero'. Distances north or south of that line can be measured quite easily. However, there is no vertical equivalent of the equator, so lines of longitude have no natural reference zero.
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Greenwich Observatory. © NMM
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This means that measurements east or west can be made from any known vertical grid reference (or line of longitude).
When a known longitude is treated as a reference zero it is called a Reference Meridian.
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World chart showing the Greenwich Meridian, 1860. © NMM
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Different countries chose different lines as their own prime or first meridian. However, a Reference Meridian was established at Greenwich in 1675 with the building of the Royal Observatory. A later Reference Meridian was adopted as the international standard in 1884.
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