Shepherd Gate Dial. © NMMGreenwich Mean Time (GMT)
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| Shepherd Gate Dial. © NMM |
Greenwich Mean Time or GMT is an expression known across the world.
However, although it obviously refers to the time at Greenwich, the expression ‘Mean Time’ is often not understood.
Why Mean Time?
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| Sundial. © NMM |
The rotation of the Earth gives us day and night. The Sun appears to cross the sky from east to west by 15 degrees per hour. A sundial shows this as Apparent Solar Time but, unfortunately, the Sun is not as regular as one might think. Throughout the year, at certain times, it appears to speed up slightly and slow down.
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| The equation of time. © NMM |
Why Greenwich Mean Time?
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| Map of Britain showing east-west time difference. © NMM |
All places that are separated eastward or westward around the world will have a different local time from each other, depending on how far apart they are.
For example, a clock set to local time at Lowestoft on the east coast of England, will be half an hour ahead of a clock set to local time at Lands End, at the far western tip of England.
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| Pocket watch. © NMM |
Until the mid-19th century all communities set their clocks to local time. So Victorian travellers moving eastward or westward simply adjusted their watches to local time when they arrived in each new town.
Problems started to occur in the 1840s, with the development of the railways and the invention of the electric telegraph. People needed one agreed time, independent of location, to be able to use the telegraph and draw up the railway timetables.
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| Shepherd master clock. © NMM |
GMT for the world
At the Washington Meridian Conference of 1884, GMT was accepted as the time standard for the world.
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| Map of 24-hour time-zones. © NMM |
Today, GMT - with slight refinements to keep it in step with atomic clocks - is known as Coordinated Universal Time and is still the world’s time standard.