Which way is north? Beware: Earth's natural compass may be turning upside down.

Will your trusty compass needle ever go haywire? It could, use Earth's magnetic poles may be about to flip-flop.

Earth is surrounded by a magnetic field--an invisible shield that protects the planet from energized particles spewed forth by the sun. Using satellites to spy on the field's strength, scientists recently noted that our steadfast shield has become wimpier over the last 150 years. Some scientists say that this weakening may be the beginning of a major geological event: a reversal (flipping) of Earth's magnetic poles, an occurrence that happens roughly every 250,000 years. And Earth is long overdue: The last flip occurred more than 750,000 years ago.

CREATING THE MAGNETIC FIELD

Earth's magnetic field is generated in the planet's center, or core. Scientists think that the inner core is made of iron and nickel, which heats the liquid outer core, also made of iron and nickel (see diagram, right). This warming of the liquid layer causes convection, or the movement of heat in a liquid or gas. Like a pot of boiling water, the scorching iron and nickel begins churning. The swooshing creates a magnetic field, sending invisible lines of magnetic force around Earth. These lines join near the North and South Poles.

PREDICTING PROBLEMS

Scientists aren't sure why magnetic reversals happen. But they think the flips result from changing currents in the outer core.

The latest weakening of the magnetic field suggests that Earth's unseen armor may end up growing so weak that it will disappear briefly and then strengthen again in the opposite direction. The result? Compasses directed northward would have needles pointing southward. However, the magnetic field may also just be taking a break and may return to normal strength without flipping.

DANGER ZONE

Still, scientists keep watch because a severe magnetic change would spell disaster.

"If magnetic shielding is lower [over time], charged particles from the sun would penetrate deeper [into Earth's atmosphere] and create large holes in the ozone," says John Tarduno, a scientist at the University of Rochester who studies physical forces on Earth. The ozone layer protects you from invisible energy waves called ultraviolet (UV) rays. Without it, radiation would zap your skin, causing increased cancer rates.

Televisions and cell phones would stop working, too. Why? The solar radiation would affect satellites that beam the gadgets' signals to Earth.

How would birds and turtles fare, since many of them use Earth's magnetic field to navigate? Not to worry, says Gary Glatzmaier, an earth scientist at the University of California. A reversal would happen slowly, giving animals time to adapt to the change.

SIGH OF RELIEF

As for your safety, breathe easy. According to Glatzmaier, severe magnetic changes would not occur for another 1,000 years. He says: "By then we will have the technology to deal with it."

DID YOU KNOW?

* The earliest-known compass dates from the Hen Dynasty (2nd century B.C. to 2nd century A.D.) in China. The spoon-shaped device was made of lodestone, a magnetic ore, and rested on a bronze plate etched with directional markings. The compass was not used for navigation, though. It was used for geomancy, or the practice of placing objects in certain configurations to help create a harmonious flow of Earth's energy. Geomancy is now commonly referred to as feng shui, Chinese for "wind" and "water."

CRITICAL THINKING:

* How is the magnetic field involved in everyday activities? Have students come up with examples.

CROSS-CURRICULAR CONNECTIONS:

LANGUAGE ARTS: A shape poem, or concrete poem, is a poem in which the shape of the poem adds to the poem's meaning. For example, a poem about a tree might be in the shape of a tree. Create a shape poem about Earth and its magnetic field. Be sure to include terminology from the article.

RESOURCES

* "Will Compasses Point South?" by William J. Broad, The New York Times, July 13, 2004:

* This site has a wealth of information relating to the magnetic field: http://image.gsfc.nasa.gov/poetry/

* Read about Earth's inconstant magnetic field at: http://science.nasa.gov/headlines/y2003/29dec_magneticfietd.htm

DIRECTIONS: Fill in the blanks to complete the following sentences.

1. The -- --is an invisible shield that protects Earth from charged particles released by the--.

2. Earth's interior has these four main layers: --,--and--

3. Earth's inner two layers are made of--and--.

4. Some scientists believe a weakening of Earth's magnetic field may lead to a reversal of the--.

5. If Earth's magnetic field weakens, the amount of--radiation hitting Earth's surface will--.

1. magnetic field, sun

2. inner core, outer core, mantle, crust

3. iron, nickel

4. magnetic poles

5. ultraviolet, increase