Why Greenwich Mean Time?
Why do people all over the world use Greenwich Mean Time? To answer this question you need to know a bit about astronomy, navigation, history and what a meridian is. A meridian is just another name for a line of Longitude. Meridians of Longitude are imaginary lines that go round the earth from pole to pole, while Parallels of Latitude go round the earth parallel to the equator. The Greenwich, or prime, meridian is the line of Longitude going through Greenwich. Lines of longitude and latitude are marked on navigational charts just as grid lines are marked on maps.
How did people tell the time before they had clocks?
The history of time keeping goes back to hundreds of years BC. The ancient Greeks divided the day into 24 hours. What did they use to tell the time? Sundials and water clocks. If you use the sun to tell the time, then because the earth is turning, the time it is for you depends on where you are. Every body agrees that it is mid day when the sun is at its highest, but when its mid day here its mid night on the other side of the world! So for hundreds and hundreds of years everybody used sundials to set their own ‘local’ time. During the 14th Century people started to build large mechanical clocks, using weights and later pendulums, which grandfather clocks still use, but still each village and town would keep their own local time.
Why is Greenwich important?
In 1675 King Charles II appointed John Flamsteed as the first Astronomer Royal to provide astronomical data for use by the Royal Navy. The Kings astronomer needed an observatory and so one was built at Greenwich. When away from land, Sailors find their position by taking the baring of a star, but the stars move across the sky during the night (actually the earth turns beneath them). To get their position they need to know the exact position of the star, at the time they took the baring. This information is given in books called navigational tables. So it is very important that sailors know what the time is, but the mechanical clocks they had did not keep time, because of the motion of the ship. In 1761 a man called John Harrison won a competition by making a clock which was accurate to 1/5th of a second a day, even at sea. From now on sailors could reliably find their exact position while out at sea.
As time went by, travelling long distances became less dangerous, so people traveled more and trade routes developed. New inventions like the steam train (1815) and the telegraph (1831) improved communications and the fact that different places were using different times started to become a real problem. So in 1884 twenty-five different nations held a conference in Washington DC, to discuss what to do. They split the world up into time zones and twenty-two nations voted to use the local time at Greenwich as the base time for the rest of the world.
Why did they agree on Greenwich?
With so much trading going on, there were lots of sailing ships travelling all over the world. That meant a lot of sailors checking their position, and most of them were using British navigational tables. Where were the observations for these tables made? At Greenwich where the line of zero degrees longitude goes through the observatory’s main telescope.
To allow ships on the Thames to set their clocks accurately, there is a big red ball on a pole on the roof of the observatory. Every day at exactly 1.00pm the ball drops from the top to the bottom of the pole. This is the origin of the Greenwich Time Signal.