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Greenwich and the story of time

Lines around the world
Measuring latitude and longitude
Finding longitude at sea
The first accurate chronometer
The Prime Meridian at Greenwich
Greenwich Mean Time (GMT)
Greenwich Meridian Trail
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Lines around the world

A worldwide grid

Longitude Latitude and longitude
View full size imageLatitude and longitude. © NMM

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.

Greenwich Observatory.
View full size imageGreenwich Observatory. © NMM

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.

World chart showing the Greenwich Meridian
View full size imageWorld chart showing the Greenwich Meridian, 1860. © NMM
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.


Find out more
StoriesMaritime Greenwich: A World Heritage Site
A unique historical landscape
Hot spotMaritime Greenwich
Uncover the history of this unique historical landscape
TrailMeridian Trail
Follow the Meridian Line trail through Greenwich Park.
National Maritime Museum/Royal Observatory GreenwichNew Opportunities Fund 
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