HOW COME?: Planet Earth's wobbly rotation

Like a spinning toy top -- or a Weeble -- our planet can get a bit wobbly. Thankfully, we don't feel the wobbles. Or the spinning itself: Even as you read this, the planet you're sitting on is turning like a merry-go-round, whirling at about 1,000 mph as measured at the equator.

Think about a colorful spinning top. As the smoothly turning top loses momentum and slows, it begins to wobble more and more violently. In the end, the top falls off its upright axis and onto its side. But the Earth is more like the popular roly-poly Weeble toy. Our planet wobbles, but it won't "fall down."

The Earth's wobbles aren't large, so there's no danger of our planet's toppling over. (The planet Uranus, however, does orbit on its side, north and south poles alternately pointed toward the sun.)

Any sphere that isn't perfectly round will wobble a bit as it turns, and the Earth is no exception. The Earth quivers on its axis in several different ways, but its most famous wobble is The Chandler. Measured at the North Pole, the Earth Chandler wobbles about 20 feet over 433 days (about 14 months). As the wobble repeats over the years, it varies from 10 to 50 feet. Just by studying the night sky, astronomers can detect the recurring wobble, as stars gradually shift from their expected positions.

No one knows if the Earth has always done the Chandler wobble. But the wobble has been known about since the 19th century; self-taught astronomer Seth Chandler worked out its motion in 1891. It took more than 100 years for scientists to figure out what likely keeps the wobble repeating. Most of the Chandler wobbliness, they think, is due to fluctuating pressure on the ocean floor.

How does it work? Shifting winds and variations in the saltiness of the sea trigger small shifts in ocean currents. This changes water pressure on the ocean floor (the Earth's crust). Like changing the length of a violin string, the new pressure on the crust changes the frequency of one of our planet's natural vibrations. Wobbling is the result.

But the Chandler wobble is just one of many. Our planet shudders as it spins in cycles ranging from a few minutes to a billion years. For example, the Earth has an "annual wobble" of about 10 feet from the straight-and-narrow, thought to be caused by the circulation of its atmosphere. Scientists have also tracked wobbles lasting a few months, in which the planet jiggles an extra 3 to 8 inches off its axis as it turns. Researchers blame the weather -- high- and low-pressure systems over the Northern Hemisphere -- and ocean currents.

And there are more wobble triggers. Earthquakes cause the crust to heave and resettle, the sea level rises and falls, material shifts inside the Earth's core, the continents slowly drift apart. Meanwhile, the moon, sun and other planets tug relentlessly on our own. It's no wonder, then, that Earth has the nervous jiggles.