• 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.

Photograph by George F. Mobley

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.

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.

• Term Part of Speech Definition Encyclopedic Entry
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.

axis Noun

an invisible line around which an object spins.

Encyclopedic Entry: axis
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 of matter surrounded by ice and dust that orbits the sun and leaves a tail of debris.

crop Noun

agricultural produce.

Encyclopedic Entry: crop
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.

food Noun

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

Encyclopedic Entry: food
globe Noun

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

Encyclopedic Entry: globe

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

momentum Noun

speed, direction, or velocity at which something moves.

moon Noun

natural satellite of a planet.

Encyclopedic Entry: moon
North Pole Noun

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

Encyclopedic Entry: North Pole
nutrient Noun

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

Encyclopedic Entry: nutrient
particle Noun

small piece of material.

planet Noun

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

Encyclopedic Entry: planet
revolve Verb

to orbit or spin around something.

rotate Verb

to turn around a center point or axis.

rotation Noun

object's complete turn around its own axis.

Encyclopedic Entry: rotation
season Noun

period of the year distinguished by special climatic conditions.

Encyclopedic Entry: season
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.

South Pole Noun

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

Encyclopedic Entry: South Pole

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.

sun Noun

star at the center of our solar system.

theorize Verb

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

having the appearance of width, height, and depth.

vertical Noun

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

weather Noun

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

Encyclopedic Entry: weather

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