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Earth

 

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Earth is the third planet from the sun and the fifth largest planet of the solar system. and the only planet definitely known to support life. The distance from the earth to the sun is around 93 million miles. The force of gravity has molded the earth into a sphere. It is slightly flattened at the poles and bulges at the equator. The earth's diameter is around 7,926 miles or 12,760 kilometers and the polar diameter 7,900 miles or 12,720 kilometers. The circumference at the equator is around 25,000 miles or 40,000 kilometers. The surface of the earth is divided into dry land and oceans, The dry land takes up around 57.5 million square miles or 148.9 million square kilometers, and the oceans around 139.5 million square miles or 361.3 million square kilometers. The earth is surrounded by a band of gases called the atmosphere which is mostly nitrogen and oxygen.

The earth rotates or turns from west to east about a line called its axis . The period of one complete rotation is a day. Because of the rotation of the earth we experience light and darkness (day and night). The earth revolves about the sun once every 365 1/4 days which is a year. The path of this revolution, the earth's orbit, is an oval in shape. This means the earth is closer to the sun in January than it is in July.

The change in seasons is caused by the tilt of the earth on its axis. When the northern end of the earth's axis is toward the sun, the most direct rays of sunlight fall in the Northern Hemisphere. This causes its summer season. At the same time the Southern Hemisphere has winter since it is then receiving indirect rays. Halfway between, in spring and in autumn, there is a time called the equinox when all parts of the earth have equal day and night. When the northern end of the earth's axis is tilted away from the sun, the least direct sunlight falls on the Northern Hemisphere. This causes winter.

The earth is thought to be 4.5 billion to 5 billion years old. Scientists have based this on radioactive dating of lunar or moon rocks and meteorites. There are several ideas on how the earth was formed. The most accepted at this time is the gravitational condensation theory. This states that the whole solar system was made by different cooling gases and dust particles coing together into the different planets over time..

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