# Longitude - Page 4

## Longitude and telling the time

This method of telling how far west we have travelled is in essence the method of determining longitude at sea. If having got to Bristol we then set sail in a ship, we can tell local time by working out the moment when the sun is at its highest in the sky. We can also know London time (or Greenwich time) if we have an accurate watch with us. And the difference between the two will tell us how far west we have travelled, i.e. our longitude.

In 1860, we would not have had a problem with doing this. The problem of longitude had been solved by then. But back in 1741 when the Centurion was battling her way up the west coast of Chile, they would have had a problem with this technique. They could work out local noon easily enough - remember that is simply when the sun reaches the highest point in the sky. The problem is with knowing London time. You need an accurate clock, or chronometer, on board. There were very good clocks in the 18th century, but they were pendulum clocks. Not much use at sea.

## Clocks and accuracy

There were watches of course, with a spring rather than a pendulum. But these were not sufficiently accurate. How accurate does a watch need to be to be of any use at sea? Well, the earth turns on its axis in 24 hours, which is 360 degrees. So it turns through 1 degree in 4 minutes. So, if we know London time to within 4 minutes, we can work out our longitude to within 1 degree of longitude, which is about 70 miles. Which would be a start. And surely an accuracy of 4 minutes is not so hard to achieve?

Well, if you are a guard on the Royal Mail coach, and you set your watch in London and travel west for 24 hours, everything should be fine. After 24 hours your watch will still be fairly accurate, even if it is not perfect. But if you are at sea for 6 weeks say, and you want your chronometer to be accurate to within 4 minutes at the end of that time, then the watch has to be accurate to with 6 seconds a day. This kind of accuracy was unimaginable at the time that Centurion was 'all at sea'.

## The Board of Longitude

The problems that faced the Centurion were just one example of the problems that could arise when you could not work out your longitude. As a maritime nation, Britain had numerous examples of disasters far worse than the problems of the Centurion. In 1707 the British Admiral Sir Cloudesley Shovell led the entire British Mediterranean fleet onto rocks off the Scilly Isles, just off the UK mainland, with the loss of several ships and around 2,000 lives.

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