When explorers began travelling far from Europe, they discovered to their horror that compasses often pointed quite far from true north. Queen Elizabeth offered a substantial prize to anyone who could solve the problem. The court physician, William Gilbert, began experimenting with magnets and in 1600 published De Magnete, considered the first great work on magnetism and also the first great work on geophysics.
By experimenting with spheres of lodestone, a natural magnet, Gilbert deduced the overall form of magnetic fields and concluded that the Earth had two magnetic poles. By mapping the magnetic field at enough places, the angle between a compass needle and true north, called the variation, could be predicted for any place on Earth. If you knew roughly where you were, you could correct your compass and find true north even on a cloudy night.
|
|
The map at left shows why compasses don't point exactly north. The north magnetic pole is not at the geographic pole, but hundreds of kilometers away in northern Canada. Wisconsin is presently in a region where compass variation is almost negligible, but in Maine, compass needles point 20 degrees west of north and in Seattle, 20 degrees east. [My comment: Coming from a similar longitude, that would be true of Florida. Plus compare the angle of the red line on the right with the angle of Florida. Florida itself is "20 degrees west of north". So it's probably a toss of the coin as to whether the pond is aligned with the magnetic compass or with the angle at which Florida lies. Either way, that's quite an achivement for a society that had little technological improvements.] |
