GMT was also crucial to the other great solution to the ‘longitude problem’, represented by John Harrison’s famous timekeepers.īritish mariners started keeping at least one chronometer set to GMT. This data enabled navigators to find their position at sea. These were tables of ‘lunar distance’ data based on observations at Greenwich and using GMT as the time standard. In 1767 Maskelyne introduced the Nautical Almanac as part of the great 18th century quest to determine longitude. In the 1700s, the fifth Astronomer Royal Nevil Maskelyne brought Greenwich Mean Time to a wider audience. At first though, Greenwich time was only really important to astronomers. This was Greenwich Mean Time, or the average time when the Sun crossed the meridian at Greenwich. Here he had the best pendulum clocks installed and set them to the local time. Soon after, he was appointed as the first Astronomer Royal and moved into the new Royal Observatory in Greenwich. John Flamsteed came up with the formula for converting solar time to mean time, and published a set of conversion tables in the early 1670s. It wasn’t until the invention of the pendulum clock in the 1650s that it was possible to work out the relationship between mean (clock) time and solar time. Greenwich Mean Time is also the name of the time zone used by some countries in Africa and Western Europe, including in Iceland all year round. Though it has now been replaced by Coordinated Universal Time (UTC), GMT is still the legal time in Britain in the winter, used by the Met Office, Royal Navy and BBC World Service. From 1884 until 1972, GMT was the international standard of civil time. GMT stands for Greenwich Mean Time, the local clock time at Greenwich. Today GMT is reckoned from one midnight to the next.įind out about the Prime Meridian What does GMT stand for? It’s a way of standardising and regularising time so we can all know exactly what time it is for our (or anyone’s) location. Solar time varies throughout the year, as the time interval between the Sun crossing a set meridian line changes.īut each day measured by a clock has the same length, equal to the average (mean) length of a solar day. Essentially, mean time is clock time rather than solar (astronomical) time.

Time zone gmt