Longitude Answers 2



1. Clocks and pocket watches were made before 1714. Why would they not have been suitable for telling the time at sea?

The £20,000 prize was for a method of measuring longitude to half a degree.  Remember that the Earth rotates at 1º42' per minute of time,  so a watch would need to be accurate to 30 seconds of time over a typical voyage time of weeks. That accuracy was simply unheard of in those days.

Pendulum clocks will not work on a moving ship

2. Astronomical observations can be used to measure time, for example the timing of the orbits of the moons of Jupiter. Why would making those observations be difficult at sea?

Galileo knew that his telescopes could be used to observe moons and planets, but even Sir Isaac Newton had to admit that it is not possible to use a telescope to watch tiny astronomical objects on a moving ship.

Worse still ships would have to carry to carry huge almanacs of astronomical data prepared by hundreds of astronomers working over many years and  the ship's navigator would have to be a highly skilled astronomer and mathematician to make sense of the data and work out very difficult calculations-and all at night!

3. Is this a scientific or a technical problem?

Well it was both. It quickly became clear that astronomical observations would be to slow, too costly and too inaccurate to meet the target of half a degree of longitude over an Atlantic crossing.

The greatest hope was that someone would make a clock that would work very accurately at sea.


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Smartphones know where they are.

This long case clock uses hanging weights to store energy and a pendulum to keep time. How well do you think it would work on a sailing ship?