What's the time?

The Royal Observatory, GreenwichThe Royal Observatory, Greenwich by John Varley, circa 1800. Repro ID:  PU8920. ©National Maritime Museum, Greenwich, London Nowadays we can easily find out the time by looking at a watch or clock, but it has not always been so easy.

How can I tell if my watch is telling the right time?

You could compare your watch with a friend's. If they both say the same, you could check with another friend just to make sure. If the different watches show different times, you could check them by listening to the Greenwich time signal on the radio. At certain times of the day, usually on the hour, six pips are broadcast in quick succession. The last pip sounds slightly different from the rest. If your minute and second hand are both on the twelve when the last pip starts, then your watch is telling the right time.

But how do they know when to broadcast the six pips?

The pips are controlled by the time service at the BBC. Nowadays the time of transmission is worked out by comparing the time shown on more than 200 extremely accurate atomic clocks in different parts of the world. In the past, the clocks at the Greenwich observatory were used to work out when to transmit the pips.

But how did they know that the Greenwich clocks were right?

The astronomers at Greenwich used the stars to check the accuracy of their clocks. Each day the earth spins once on its axis, and because of this we see the sun seeming to move across the sky. It rises higher during the morning and sinks lower during the afternoon. The sun is only one of many stars which we see moving in this way. From their records, the astronomers were able to predict when each of the brightest stars would reach its highest point, and this gave them a reliable means of checking their clocks.

How did they actually do it?

They used special telescopes. Although these could point up and down, they could only point directly north or south along a meridian line. Instead of altering their clocks, the astronomers simply made a note of how fast or slow they were, and added or subtracted that amount when they recorded the time. Although no longer in use, the telescopes can be seen in their original positions at the Royal Observatory Greenwich.

Who built the Observatory, and why in Greenwich?

The Observatory was set up in Greenwich by Charles II in 1675. He was short of money at the time, so he chose land in his Royal Park in Greenwich and used recycled bricks and timber.

Why was the Observatory built at all?

In the 17th century, sailors at sea could work out their latitude (how far north or south they were), but couldn't work out their longitude (how far east or west they were). As a result, many ships got lost. Some of these then sank or ran out of supplies. Scientists believed that sailors would be able to work out their longitude by observing the position of the stars and the moon. To do this, they needed to know where the stars and moon were going to be at different times. King Charles had the Observatory built to provide this information.

Why did the Greenwich astronomers need good clocks?

To be able to predict where the stars and moon would be at different times in the future, the astronomers had to study where they were at different times in the present. To do this, they needed accurate telescopes and the best possible clocks.

What did people do before pips were broadcast on the radio?

Shepherd Gate ClockShepherd Gate Clock, 1852. Repro ID: D5601-2 ©National Maritime Museum, Greenwich, London Before television and radio, it was much harder for people to check their clocks and watches. There were a number of clocks around the country which people could rely on because they were checked against a special signal sent from Greenwich by telegraph wires.

In 1852, the Observatory had its own special clock installed, the Shepherd master clock. This was connected to a slave clock installed at the gates to the Observatory and was the first clock ever to show Greenwich Mean Time (GMT) directly to the public. People would bring their clocks or watches to Greenwich and check them, and in turn used their clock or watch to check those of other people. The Observatory gate clock is still there for us to use today.

Before the gate clock, the only way the public could check the time at Greenwich was by using the time ball.

Does it matter how accurate our clocks and watches are?

To catch the start of a television programme, or a particular train, then you do need to know the time fairly accurately. Two hundred years ago, before there were trains and televisions, most people didn't need to know the time so precisely. People who worked in factories were usually woken by 'knocker-uppers' who went from house to house with long poles knocking on the windows.

When was the first clock made?

Until the 13th century, there were no mechanical clocks as we know them. The oldest mechanical clock in England that still exists is the Salisbury Cathedral clock. It was made in 1386, and is still in the cathedral today. The second oldest clock can be seen working in the Science Museum, and was made for Wells Cathedral in 1392. Like other early clocks it does not have a face or hands, but tells the time by striking each quarter hour. Before mechanical clocks, people relied on other types of clocks such as sundials and water clocks.