When I retired I received a very nice, and moderately expensive, watch. It is a “diver’s watch” in that it can withstand the pressure of water down to 300 feet and it has a movable dial so that I can set the time during my five-minute safety stop and not have to remember what time it was when I started. The only problem is that the watch loses about two–three minutes a day! Using this watch I wouldn’t have come close to winning a prize in 1714 of what would be the equivalent of $3.5 million dollars today.

That prize was offered to anyone who could build a watch or clock that could be carried on a ship and would remain so accurate that you could use it to calculate longitude to within ½ a degree after six weeks of leaving port. The English Parliament offered the prize as part of the Longitude Act.

It’s okay, however, that my Seiko automatic diver’s watch doesn’t keep time well enough for that, because the prize was never officially awarded, though multiple people met the criteria to win it.

To determine longitude you need to know the time in Greenwich, England on the Prime Meridian, and you need to know the time where you are. One degree of longitude is equal to about four minutes so to win the prize your clock or watch had to be accurate to within two minutes over a six-week period of time, about the time necessary to cross the Atlantic Ocean.

In the western hemisphere, your local time should trail Greenwich Mean Time by four minutes for every 66 miles (one degree). Gothenburg time is 6.677 hours different from Greenwich. That is 400.6428 minutes divided by four minutes/degree and you get a longitude reading of 100.1607*W.

Clocks back in 1714 were capable of that kind of precision, but they were large pendulum clocks that just wouldn’t work on a ship due to the motion of the ocean. A watch or small clock had to be built that would not be so affected. A carpenter turned watchmaker succeeded and then soon after a number of other people did as well.

The carpenter, John Harrison, and his son built four such clocks and Larcum Kendall built a fifth. Kendall’s clock went on James Cook’s second voyage around the world