In the past, clocks were no good at sea. Their pendulums swung wildly in storms.

But this one is different. It looks like a pocket watch. It keeps perfect time. So it can be used to work out longitude. Like this ...

1 Before the voyage the clock is set to Greenwich time.

2 Captain Cook reaches a new island.

3 When the sun is directly overhead, he knows it's midday on the island ...

4 ... so he checks his clock. It is just 7 am in Greenwich.

5 Now he does a calculation to work out his longitude. It is shown on the right.