Related page "The wobbly(?) Earth" here
What would make the Earth stop spinning?
As
the Earth spins, "bulges" move across the Earth's surface like a
wave, pushing against the Earth's spin. This slows down the Earth's
spin. It means that Earth's day lengthens by one second every 50,000
years. The only thing that could stop the Earth's spin would be if another planet crashed into it.
The slowdown of earth's rotation will continue for 4 billion years—as
long as we can imagine. The slowdown changes the globe's shape. The result of these adjustments is that the earth is slowly becoming
more and more like a sphere.
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Earth might look like a sphere, -- the planet is slightly squished, making the circumference of the equator bigger than the circumference through the poles. Clouds, ocean currents, and long-range missiles all would behave differently if Earth were perfectly spherical.
This bulge exists because of a force created by Earth's rotation. As a result, a person on the equator is over 13 miles farther from Earth's centre than a person on the North Pole is. That difference is more than double the distance between sea level and the top of Mount Everest, but on a planetary scale, this bulge is virtually imperceptible. It's roughly equivalent to a single layer of duct tape around a football.
Earth is so close to being spherical that physicists sometimes will treat it as such in their models,
This bulge exists because of a force created by Earth's rotation. As a result, a person on the equator is over 13 miles farther from Earth's centre than a person on the North Pole is. That difference is more than double the distance between sea level and the top of Mount Everest, but on a planetary scale, this bulge is virtually imperceptible. It's roughly equivalent to a single layer of duct tape around a football.
Earth is so close to being spherical that physicists sometimes will treat it as such in their models,
But this simplification can cause significant errors when calculating motion on Earth's surface.
edited from source
related page "The wobbly(?) Earth" here