OK, so you are in a ship in the middle of the sea, with no GPS (it is the eighteenth century after all) and you want to know where you are. It’s a tricky problem – get it wrong and you end up, like Admiral Sir Cloudesley Shovell and a fair portion of the English fleet in 1707, embarrassingly and fatally wrecked on the Isles of Scilly.
Knowing latitude is easy. If you know what day of year it is (and surely every self-respecting mariner can follow a calendar, then the elevation of the sun at midday – where it is the highest – allows you to work out your latitude. For example, if it is 21 June and the sun happens to be directly overhead at midday, you know you must lie on the tropic of cancer (23 and a half degrees north).
But longitude is a problem. The reference is the Greenwich meridian, and what you need to know is the time difference between midday at Greenwich and midday where you are. Then for every hour difference, there is fifteen degrees of longitude (so 24 hours equals 360 degrees). There are essentially two ways to do this. One is to have on board a clock that gives you the time at Greenwich; the other is to observe astronomical phenomena (such as occultations of stars by the moon) that happen at known times in Greenwich.
The problem with the clock method in the eighteen century was that there wasn’t a clock reliable enough to do the job. People could build reasonable clocks that were just fine when sitting in someone’s lounge, but, on board a ship that is pitching and rolling, and subject to extremes of temperature, they just did not keep time well enough to find longitude with the required accuracy.
It was John Harrison, a clock maker, who eventually ‘solved’ this problem, with a series of every more accurate clocks, denoted H1 – H4. These are on display at Greenwich and make for a fascinating piece of history. Those, in themselves, make a visit to Greenwich worthwhile for a physicist. A copy of H4 was trialled by Captain Cook on his voyage to New Zealand, and it was Cook’s endorsement of it that partly contributed to its acceptance as a reliable method to find longitude.
For a full description you can do a lot worse than Dava Sobel’s excellent and very readable book ‘Longitude’.