Longitude in the Renaissance

26 Jan 2011

As is clear from the banner at the top of the page, this research project focuses on the period during which the Board of Longitude was in existence (or at least, given what Alexi wrote previously, the period during which Commissioners of Longitude could, at least theoretically, meet to discuss potential solutions and rewards). However, the problem of finding longitude - particularly at sea - has a much longer history than this. Fortunately for us, Thony Christie has told some of this story over at his blog, Renaissance Mathematicus. His post is well-worth a perusal, especially in setting out the three main solutions touted during the Renaissance: lunar distances, chronometric and the use of Jupiter's satellites. This last was to prove, as Thony says, very effective on land but it was too small a measurment to make successfully at sea (although it was tried).

The two astronomical methods both, of course, were used to find longitude from scratch. The chronometrc, or timekeeping, method, on the other hand, could only ever be used to keep longitude. In other words your clock had to be correctly set to your reference position/time and then kept running either keeping perfect time or loosing/gaining at a predicable rate - something unachievable before Harrison's advances.

There is one more solution that was seriously considered, investigated and invested in over the course of several centuries: terrestrial magnetism. In the 16th century sailors identified the geographical variation in terrestrial magnetism known as declination, and in 1522 the Italian navigator Giovanni Cabato, or John Cabot, suggested that it might be a means of finding longitude. The idea stuck around (a useful summary can be found in this article by Alan Cook), and late in the 17th century, Edmond Halley investigated and mapped magnetic variation on two sea voyages. While it was important to be able to understand and factor variation into compass-navigation, Halley also hoped that the patterns would be predicable enough to use for position-finding.

Halley was not successful, but still the idea did not go away. It didn't even go away after Maskleyne began publishing the Nautical Almanac and chronometers were being made in greater numbers. In fact, it outlasted the Board of Longitude itself and is, therefore, a tale for another day.