What is GMT?

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?

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.

The equation of time. © NMM

The result is that the time read from a sundial can be as much as 16 minutes out from a truly accurate clock. That is why it is necessary to take an average, or mean, of the length of all the days in the year. It is this Mean Solar Time that we set our clocks too.

Why Greenwich Mean Time?

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.

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 with local time

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.

Shepherd master clock. © NMM

Greenwich Observatory, with its well-established record of accurate timekeeping for astronomical purposes, was the obvious supplier of this time standard. From 1880 Greenwich Mean Time, i.e. mean solar time at Greenwich, was adopted legally as the single time standard for the United Kingdom.

GMT for the world

At the Washington Meridian Conference of 1884, GMT was accepted as the time standard for the world.

Map of 24-hour time-zones. © NMM

This conference also established Universal Time, from which the international 24-hour time-zone system grew. In this, all zones refer back to GMT on the prime meridian.

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.