Rotation describes the circular motion of an object around its center. There are different ways things can rotate.

Rotation of Earth

A very familiar kind of rotation is when a spherical, three-dimensional object turns around an invisible line inside its center. This center is called an axis. Spinning basketballs turn around an axis. Globes turn around an axis. Earth spins on an axis.

Earth's axis is vertical, meaning it runs up and down. (A rolling log, on the other hand, has a horizontal axis, meaning it runs sideways.) Earth's axis runs from the North Pole to the South Pole. It takes Earth 24 hours to make one complete rotation around this invisible line.

As Earth rotates, each area of its surface gets a turn to face and be warmed by the sun. This is important to all life on Earth. The sun affects everything from the weather we experience to the food we eat, and even our health. If Earth did not rotate, one half of Earth would always be hot and bright, and the other part would be frozen and dark.

Earth also moves around the sun. This movement is called a revolution, which is different from rotation. Objects rotate around an axis, but revolve around other objects. So Earth rotates around its axis as it revolves around the sun. It takes Earth 365 days to complete a revolution.

Leftover momentum from when planets were forming makes Earth, and all planets in the solar system, rotate and revolve. As the solar system formed, many moving particles clustered together. They formed a spinning mass. This mass eventually split into different bodies—planets, moons, asteroids, and comets. All these bodies revolve around the sun. In addition, all planets kept their own rotating motion, including Earth.

Earth rotates around its axis at a rate of about 1,600 kilometers (1,000 miles) an hour. It revolves around the sun at a rate of 107,800 kilometers (67,000 miles) an hour.

Other Kinds of Rotation
Planets are not the only round objects that rotate. Amusement park rides, such as Ferris wheels or carousels, also rotate. The Ferris wheel rotates around a horizontal axis, and the carousel rotates around a vertical one. Wheels on a car rotate around a strong horizontal bar called an axle. The axle runs from one side of the vehicle to the other, connecting the front wheels to each other and the back wheels to each other.

Many household items rotate, including sprinkler heads, beaters on a mixer, blades on a ceiling fan, and doorknobs.

There are some things that rotate without a specific axis. Farmers use crop rotation to help the soil stay healthy and plants receive the right nutrients from season to season. Crop rotation means growing things in a different spot each year. This helps prevent disease and encourages the soil to be fertilized by different crops. For example, cabbage, broccoli, and sprouts are in the same plant family. If they were grown in the same place year after year, they could all develop clubroot disease, weakening or killing the crop and damaging the soil. The field itself acts as an axis, with different crops being rotated in at different times.