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1 Your web browser (Safari 7) is out of date. For more security, comfort and the best experience on this site: Update your browser Ignore RO TATIO N For the complete encyclopedic entry with media resources, visit: 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. 1 of 5

2 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. 2 of 5

3 Vocabulary Term amusement recreational facility with games, food, and mechanical park rides. irregularly shaped planetary body, ranging from 6 meters asteroid (20 feet) to 933 kilometers (580 miles) in diameter, orbiting the sun between Mars and Jupiter. axis an invisible line around which an object spins. axle horizontal bar connecting wheels on a vehicle. beater kitchen device with rotating blades used for mixing ingredients. broccoli plant eaten as a vegetable. cabbage leafy green vegetable. carousel amusement park ride consisting of a flat, rotating circle with seats or sculptures of animals, such as horses. ceiling fan mechanical tool with rotating blades fixed to a ceiling, used to circulate air and cool the room. clubroot disease ailment of the roots of cabbages caused by a slime mold. comet celestial object made up of ice, gas, and dust that orbits the sun and leaves a tail of debris. crop agricultural produce. crop the system of changing the type of crop in a field over rotation time, mainly to preserve the productivity of the soil. encourage verb to inspire or support a person or idea. farmer person who cultivates land and raises crops. amusement park ride consisting of a single large wheel, Ferris rotating vertically, around which compartments or seats wheel are fixed. fertilize verb to make productive or fertile. 3 of 5

4 Term food material, usually of plant or animal origin, that living organisms use to obtain nutrients. globe scale model of the Earth, or sometimes used to mean the Earth itself. horizontal adjectiveleft-right direction or parallel to the Earth and the horizon. momentum speed, direction, or velocity at which something moves. moon natural satellite of a planet. North Pole fixed point that, along with the South Pole, forms the axis on which the Earth spins. nutrient substance an organism needs for energy, growth, and life. particle small piece of material. planet 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. rotation object's complete turn around its own axis. season period of the year distinguished by special climatic conditions. soil top layer of the Earth's surface where plants can grow. solar the sun and the planets, asteroids, comets, and other system bodies that orbit around it. South Pole fixed point that, along with the North Pole, forms the axis on which the Earth spins. spherical adjectiverounded and three-dimensional. sprinkler mechanical tool used for irrigation. sprouts young shoot of some plants, such as soybeans or alfalfa, eaten as a vegetable. sun star at the center of our solar system. theorize verb to formulate and propose a group of ideas to explain a scientific question. 4 of 5

5 Term threedimensional adjectivehaving the appearance of width, height, and depth. vertical up-down direction, or at a right angle to Earth and the horizon. weather state of the atmosphere, including temperature, atmospheric pressure, wind, humidity, precipitation, and cloudiness. Articles & Profiles National Geographic News: Six End-of-the-World Myths Debunked Websites National Geographic Science: Solar System Department of Energy: Ask a Scientist Earth s Rotation/Revolution National Geographic Society. All rights reserved. 5 of 5