(Dopplershift).EARTH AND SPACE SCIENCEEarth is a fairly small planet. The distance of the earth from the sun ensures that energyreaches the planet at a rate sufficient to sustain life. The earth is mostly rock, with three-fourths of its surface covered by a relatively thin layer of water and the entire planetenveloped by a thin blanket of air. Bulges in the water layer are raised on both sides of theplanet by the gravitational pull of the moon and sun, producing high tides about twice a dayalong ocean shores. Similar bulges are produced in the blanket of air as well.The earth is approximately 1.28 x 104km in diameter at the equator. The diameter at thepoles is approximately 70 km less than at the equator because of its rapid rotation on its axis(equatorial bulge).The motion of the earth and its position with regard to the sun and the moon has noticeableeffects. The earth's one-year revolution around the sun, because of the tilt of the earth's axis,changes how directly sunlight falls on one part or another of the earth.This difference in heating different parts of the earth's surface produces seasonal variations inclimate. The rotation of the planet on its axis every 24 hours produces the planet's night-and-day cycle. The combination of the earth's motion and the moon's own orbit around the earth,once in about 271/3, results in the phases of the moon (new moon, 1stquarter, full moon, lastquarter.Transfer of heat energy at the interfaces of the atmosphere with the land and oceansproduces layers at different temperatures in both the air and the oceans. These layers rise orsink or mix, giving rise to winds and ocean currents that carry heat energy between warm andcool regions. The earth's rotation curves the flow of winds and ocean currents, which arefurther deflected by the shape of the land.The water cycle plays an important part in determining climatic patterns—evaporating fromthe surface, rising and cooling, condensing into clouds and then into snow or rain, and fallingagain to the surface, where it collects in rivers, lakes, and porous layers of rock. There arealso large areas on the earth's surface covered by thick ice (such as Antarctica), whichinteracts with the atmosphere and oceans in affecting worldwide variations in climate.Fresh water is an essential resource for daily life and industrial processes, obtained fromrivers and lakes and from water that moves below the earth's surface (groundwater). Manysources of fresh water cannot be used because they have been polluted. It can be veryexpensive to clean up polluted air and water, restore destroyed forests and fishing grounds,or restore or preserve eroded soils of poorly managed agricultural areas.Although the oceans and atmosphere are very large and have a great capacity to absorb andrecycle materials naturally, they do have their limits. They have only a finite capacity towithstand change without generating major ecological alterations that may also have adverseeffects on human activities. Wind, tides, and solar radiation can also be harnessed to providesources of energy.
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