As any police officer can tell you, measuring the straight line speed of a car -- or most any object -- is a fairly simple and reliable process. Measuring the speed of a rotating object like the Earth is slightly more complicated. After all, if you stand at one of the poles, you'll spin right along with the rest of the Earth, but you'll be stationary with respect to its center. Stand on the equator, though, and you'll have a linear speed of 1,036 miles per hour (1,667 kilometers per hour) [source: Esri]. That's faster than the speed of sound, and one of the reasons we tend to launch rockets toward the east [source: NASA]!
The difference between linear speed at the poles and at the equator produces an interesting phenomenon called the Coriolis effect. The effect is easiest to visualize if you think about someone setting out in a plane straight for the North Pole from the equator. Since the plane retains the lateral speed of the equator, it appears to curve with respect to the Earth as it approaches the slower moving poles.
Is there anything slowing the Earth's rotation down? Sure, but don't adjust your watches just yet. The forces changing the speed of the Earth's rotation make an extremely small impact. The tides, which are caused by the gravitational forces between the Earth, the sun and the moon, produce tidal friction as they interact with the Earth. That drag adds about 2.3 milliseconds to our day every century [sources: Lunar and Planetary Institute, Ray]. Weather systems can change the Earth's rotation, with winds applying a braking force to the planet's surface. Finally, earthquakes can mess with the length of the day by actually redistributing the Earth's mass. The 2011 earthquake that struck Japan actually accelerated the Earth's spin (because it shifted the mass toward the equator) and shortened the day by 1.8 microseconds [source: CBS News].
So, the next time you complain about the day being too long or too short, don't despair: It's changing all the time.