指南針是幫助人們辨別方向的好工具。有了它，不必再擔(dān)心在陰雨的天氣和在茫茫大海上迷失方向。很久以來，人們都認(rèn)為指南針是指向正南正北，實際上并不是這樣。

A compass in the Northern Hemisphere truely does point in a northerly direction, but not to the North Pole. Instead, the compass points to the North Magnetic Pole, which, as Sir James Clark Ross discovered in 1831, is located at the northernmost point of the Artic coast of North America. Similarly, a compass in the Southern hemisphere always points to the South Magnetic Pole, which is firmly planted south of Australia, in Antarctica.

The different directions their compasses pointed, when traversing the high-seas of the Northern Hemisphere, baffled ancient mariners. Their modern counterparts understand, and compensate for, the differences in the North Pole and the Northern Magnetic Pole, and chart their courses accordingly. The differences in the poles prove minor, in comparison to the tricks the Northern Magnetic Pole pulls from its home of Boothia. The bane of boyscouts, as they attempt to navigate with, or without, the benefit of their trusty compasses, is the fact that this Pole chooses to roam about in a 20-mile circle, and to shift its course between day and night.

This 20-mile variance, however, is not one of global proportions. Modern sea-farers compensate for the Northern Magnetic Pole's perpetual motion, by using charts, and tools other than the compass. All things considered, 20 miles is a minor measure for distant travelers to take into account in adjusting their travel agenda.

Thankfully, the Southern Magnetic Pole spares sailors the navigational nightmare its Northern nemesis does. In the south, compass needles actually do point true South, to the South Magnetic field.

（來源：coolquiz.com 英語點津 Annabel 編輯）