How Come: Why does a compass point north

If you've ever held a compass in the palm of your hand, you know how it works: Turn the compass, and the needle, strangely, doesn't turn with it. Feeling an attraction we're unaware of, the floating needle moves on its own, like the pointer on a spooky Ouija board. The needle seems drawn toward some distant beacon, feeling the pull even through walls.

But a compass needle isn't made of just any metal. It's a metal that's been magnetized. This slender, lightweight magnet has its own north and south poles, which are attracted to the opposite poles of other magnets. Meanwhile, the magnet is free to pivot in any direction.

Luckily for lost travelers (especially before GPS tracking in cellphones and cars), the planet we travel on is itself a magnet. Pocket compasses respond to Earth's magnetism by lining up in its magnetic field. So even on a cloudy night, with no stars to guide him, a sailor adrift in a dark sea can find "north" -- and thus south, east and west.

(The Earth's field reverses itself every 500,000 years or so, but for now, Earth's "south" pole is in the north, its "north" pole in the south. Which is why a magnetized needle's north pole, attracted to its opposite, will point north.)

Human beings have used compasses to navigate for more than 2,000 years. The earliest compasses were made of wood, topped with a bit of lodestone, a naturally magnetic iron ore. Floated on water or other liquid, the wood compass was free to move, the lodestone turning until it aligned itself with the Earth's field.

Scientists think that the Earth's magnetic field is generated by looping electric currents in our planet's (superhot) liquid metal core. The result? Imagine a bar magnet stuck vertically through the center of the Earth, its invisible field arcing out into space like a horizontal figure 8. Although we don't notice it, magnetized objects feel its pull.

But while the Earth is large, its magnetic field is rather feeble. Our planetary magnet is much weaker than the magnets on your refrigerator, which can keep class photos and shopping lists clasped to the metal door, or grab a stray paper clip if it comes too near.

The Earth's magnetic field varies across the planet, but it's strongest at the poles. Magnetic strength is usually measured in "Gauss" or "Tesla" units. At its weakest, in parts of South America, the Earth's field strength is about .3 Gauss (30 microteslas). Near the north and south magnetic poles, the strength increases to about .6 gauss (60 microteslas).