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

Rotation of the Earthre

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. The Earth itself spins on an axis.

The 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.) The Earth's axis runs from the North Pole to the South Pole. It takes the Earth 24 hours, or one day, to make one complete rotation around this invisible line.

As the 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 the Earth did not rotate, one half of the Earth would always be hot and bright, and the other part would be frozen and dark.

The 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 the Earth rotates around its axis as it revolves around the sun. It takes the Earth 365 days, or one year, to complete a revolution.

Leftover momentum from when planets were forming makes the 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 the Earth.

The 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.

rotation
One of the first steps a potter takes when creating a bowl is to "center" the lump of clay on the wheel's axis. This helps ensure an even, round shape.

The Tipped Earth
Did you know that the Earth is tipped over? The Earth's axis is not exactly up and down. It is actually a little sideways, like a tipped-over spinning top. It is tipped about 23.5 degrees.

How did the Earth get tipped over? Scientists are not quite sure. They think it happened when the solar system had just been formed. They theorize that a huge object the size of the planet Mars crashed into the Earth, tipping it on its side.

amusement park
Noun

recreational facility with games, food, and mechanical rides.

asteroid
Noun

irregularly shaped planetary body, ranging from 6 meters (20 feet) to 933 kilometers (580 miles) in diameter, orbiting the sun between Mars and Jupiter.

Noun

an invisible line around which an object spins.

axle
Noun

horizontal bar connecting wheels on a vehicle.

beater
Noun

kitchen device with rotating blades used for mixing ingredients.

broccoli
Noun

plant eaten as a vegetable.

cabbage
Noun

leafy green vegetable.

carousel
Noun

amusement park ride consisting of a flat, rotating circle with seats or sculptures of animals, such as horses.

ceiling fan
Noun

mechanical tool with rotating blades fixed to a ceiling, used to circulate air and cool the room.

clubroot disease
Noun

ailment of the roots of cabbages caused by a slime mold.

comet
Noun

celestial object made up of ice, gas, and dust that orbits the sun and leaves a tail of debris.

Noun

agricultural produce.

crop rotation
Noun

the system of changing the type of crop in a field over time, mainly to preserve the productivity of the soil.

encourage
Verb

to inspire or support a person or idea.

farmer
Noun

person who cultivates land and raises crops.

Ferris wheel
Noun

amusement park ride consisting of a single large wheel, rotating vertically, around which compartments or seats are fixed.

fertilize
Verb

to make productive or fertile.

Noun

material, usually of plant or animal origin, that living organisms use to obtain nutrients.

Noun

scale model of the Earth, or sometimes used to mean the Earth itself.

horizontal
Adjective

left-right direction or parallel to the Earth and the horizon.

momentum
Noun

speed, direction, or velocity at which something moves.

Noun

natural satellite of a planet.

Noun

fixed point that, along with the South Pole, forms the axis on which the Earth spins.

Noun

substance an organism needs for energy, growth, and life.

particle
Noun

small piece of material.

Noun

large, spherical celestial body that regularly rotates around a star.

revolve
Verb

to orbit or spin around something.

rotate
Verb

to turn around a center point or axis.

Noun

object's complete turn around its own axis.

Noun

period of the year distinguished by special climatic conditions.

soil
Noun

top layer of the Earth's surface where plants can grow.

solar system
Noun

the sun and the planets, asteroids, comets, and other bodies that orbit around it.

Noun

fixed point that, along with the North Pole, forms the axis on which the Earth spins.

spherical
Adjective

rounded and three-dimensional.

sprinkler
Noun

mechanical tool used for irrigation.

sprouts
Noun

young shoot of some plants, such as soybeans or alfalfa, eaten as a vegetable.

Noun

star at the center of our solar system.

theorize
Verb

to formulate and propose a group of ideas to explain a scientific question.

three-dimensional
Adjective

having the appearance of width, height, and depth.

vertical
Noun

up-down direction, or at a right angle to Earth and the horizon.

Noun

state of the atmosphere, including temperature, atmospheric pressure, wind, humidity, precipitation, and cloudiness.