Comments on: Magnetic Deviation: Comprehension, Compensation and Computation (Part I)

By: Correcting the Compass | We Other Historians

Correcting the Compass | We Other Historians — Thu, 22 Jul 2010 16:34:31 +0000

[…] For a thorough summary of the technical details of compass deviation, see Magnetic Deviation: Comprehension, Compensation and Computation. […]

By: John D.

John D. — Wed, 21 Jul 2010 05:16:45 +0000

For just plain fun with magnets, there’s a series of articles on www.evilmadscientist.com that shows how much easier it is to use with modern powerful magnets. Here’s they’re simplest design:

http://www.evilmadscientist.com/article.php/EasyCompass

While it took those mariners a while to realize that iron aboard the ship could affect the direction, with these materials a floating magnet will ignore north and head straight toward any nearby magnetic metal, quickly!

Little mathematics, I’m afraid, but neat to read about.

Thanks, a nice article. After reading it, I now have a new appreciation for the risks of placing a powerful magnet anywhere near a knife, too! — Ron

By: george huxtable

george huxtable — Thu, 10 Dec 2009 21:46:22 +0000

About Halley’s magnetic surveys in the North and South Atlantic. He did not determine longitude at sea from Jupiter satellites. That is possible only with high-magnification telescopes which are unusable at sea due to vessel’s motion.
Halley’s method was to estimate time from Lunar appulses, near-conjunctions of the Moon with a star, as it was passed by the midline bisector of the Moon. That was possible only for an astronomer such as Halley, very familiar with stars around the ecliptic; not for an ordinary mariner. It was the first successful measurement of longitude at sea by astronomical means. Halley also stated that he had taken Newton’s reflecting quadrant to sea. See “Astronomical Minds”, by Ted Gerrard, Samos Books, 2007.

Very interesting, thanks! I’ll go back and find where I saw references to Halley using eclipses of Jupiter’s satellites. I’ve also ordered the book you mention from Amazon. It looks fascinating (there is a sample PDF not on Amazon but at the Samos Books site at http://www.samosbooks.org/Astronomical_minds.pdf). At one point in the sample it says that Halley tried to sight Jupiter’s satellites on Fernando de Noronha but could not, and instead relied on a lunar method as you say. After I get the book I’ll update the essay according to the details in the rest of the book. Thank you very much, George! — Ron

By: Jagger

Jagger — Sun, 19 Apr 2009 06:01:20 +0000

Very intriguing! I had absolutely no idea of the intricacies of navigating a vessel on the high seas. I was aware of pounded metal taking on the magnetic pole of the location where the pounding occurred, but for some reason never put the impact of metal warships on pre-GPS navigation. So the question remains, as ordinance is fired during battle, does the captain have to understand the exact amount of cannonballs that were fired in order to re-compensate for the change in metal?

Fascinating.. great job!

Thanks! I had no idea either, and I only entered this realm while searching for the source of formulas for magnetic deviation on Lallemand's hexagonal chart for another essay I'm writing---stumbling across dygograms was an unexpected bonus.

In the case of a wooden ship, differing numbers of cannonballs would cause a definite change in the magnetic deviation, particularly for any compass located near the cannonballs. At the time of these wooden ships most compasses were of fairly shoddy construction anyway, according to Gurney's book. Nonetheless, it was found very early on that the armory and any metal stores had to be kept away from the compasses, so this effect must have been quite noticeable. Once ships with iron decks appeared, any ammunition below deck would be somewhat shielded by the deck.

I might mention that compasses for submarines were a real problem! Early submarines mounted the compass outside the sub and used sighting tubes to see the needle, but even then the presence of the hull would have had to be greatly compensated. The problem was essentially solved by the invention of the gyrocompass, which maintains a direction in space by its angular momentum rather than by a magnetic force. The development of these was largely driven by their need in submarines. --- Ron