What with GPS and radio navigation aids available to pilots and seafarers, we take for granted the ability to determine our current location in the world.  It wasn’t always that way.  Up until the end of the 18th century, there was no provably accurate method of determining one’s longitude.  Determining latitude is relatively easy by observing the angle of the sun at local noon, or the altitude of any number of known “fixed” stars.  Determining longitude is something else entirely.  By the beginning of the 18th century, people knew that they could use time to determine latitude:  if you know what time it is at some known position (London, for example) when local noon occurs at your position, then you can infer your longitude very easily.  The earth rotates at 15 degrees per hour.  So if it’s 3:00 pm in London when local noon occurs at your position, then you’re 45 degrees west of London.  Simple.  Except that clocks that could keep accurate time on board a ship didn’t exist in 1700.  At the Equator, a four minute error in time keeping adds up to a 60 mile error in position.

Longitude, by Dava Sobel explains the problem in great detail, and goes on to tell how one man (John Harrison) spent most of his adult life trying to solve it (and eventually succeeding).  At 175 pages, it’s an easy afternoon’s reading, and quite an engaging story.  The author also discusses some of the crackpot ideas that people came up with.  I nearly split a gut laughing when I read about the wounded dog theory.  This isn’t a new book—it was originally published in 1995.  I read about it on Slashdot some months ago, and finally remembered to pick it up the last time I was at the book store.  They only had the hard bound edition, which was pretty pricey at $19.00.  I don’t know if there’s a paperback edition available.  In any case, it’s well worth the read.