“I have been extremely ill used by the gentlemen who I might have expected better treatment from.” — John Harrison, on his struggle to receive the reward for solving the Longitude Problem
For centuries, the Royal Navy was the mightiest maritime force in the world, helping a small country on the edge of Europe establish a global empire. For a navy to function effectively, shipboard navigators need to know where they are when far from land, that is, their latitude and longitude. Finding one’s latitude is comparatively easy: Measure the sun’s altitude at local noon (when it’s at its highest), correct for the date using the well-known “equation of time,” and you’ve got it. Latitude is zero at the equator, 90 degrees at the poles.
Longitude — your angular distance around the globe from an arbitrary “prime meridian” stretching from pole to pole — is essentially the time difference between you and the meridian. This isn’t new; Hipparchus, who lived in the second century B.C., understood the connection between time and longitude. By convention, the international prime meridian — zero degrees longitude— runs through the Royal Observatory in Greenwich, outside London, England. If you know that your local time is, say, eight hours behind Greenwich time, you’re 8/24, or one-third of a day, west of Greenwich, on longitude 120 degrees (one-third of 360 degrees). Hence the “Longitude Problem” faced by ocean navigators before radio: How to know the time in Greenwich?
The night sky can be thought of as a celestial clock. Soon after discovering Jupiter’s four “Galilean moons,” Galileo realized that eclipses of these moons by the giant planet could serve as a precise way of coordinating time from anywhere on Earth, since these events are simultaneous for all observers. This trick is workable on land with a decent telescope but not on a moving ship. Later, astronomers published tables giving the time in Greenwich at any position of Earth’s moon relative to a handful of bright stars. However, this “lunar distance” method was clumsy, especially for a shipborne navigator, and difficult to master. In “A Question of Longitude” (Feb. 6 and 13, 2020), I discuss finding one’s latitude on land. What follows is the much harder matter of latitude at sea.
In 1685, with the future fortunes of Britain’s island nation at stake, King Charles II commissioned a Royal Observatory to be built just outside London to “find out the so much desired longitude of places for the perfecting of the art of navigation.” A few years later, on Oct. 22, 1707, the Shovell Disaster was the tragic catalyst that accelerated the quest for an accurate method to figure out longitude at sea. A navigation error had resulted in four British warships, under the command of Adm. Sir Cloudesley Shovell, striking rocks off the Scilly Isles west of Cornwall, England. Nearly 2,000 men drowned, including Shovell himself. As a result, Parliament offered 20,000 pounds (millions, today) to anyone solving the Longitude Problem.
After discounting several impracticable ideas for a navigator to know Greenwich time, it came down to either (1) using celestial navigation, such as the lunar distance method, or (2) carrying an accurate clock set to Greenwich time before sailing. It took John Harrison, a Yorkshire carpenter-turned-horologist, 40 years to design a clock that kept accurate time on a ship over a long voyage. (Dava Sobel’s Longitude, and the BBC drama based on her book, discusses Harrison’s triumphs and failures, albeit much dramatized.) In 1761, his fourth attempt, “H4” — a large pocket-watch-style marine chronometer — lost just 5.1 seconds (equivalent to about one mile of longitude) on an 81-day test voyage from Britain to Jamaica.
Next week, we’ll see why the prime meridian line in Greenwich isn’t.
Barry Evans (he/him, barryevans9@yahoo.com) understands why governments are distrusted when it took Parliament 22 years to award John Harrison most — not all — of his well-deserved £20,000 reward.
This article appears in The Eureka Chinatown Monument.