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PLANET EARTH
The Earth is one among the nine Planets of the Solar System. It is the only planet where life exists owing to the presence of water and air.
Physical Features
The Earth is not a perfect sphere. It is an oblate spheroid. It is bulged along the equatorial region and flat along the Polar region. The Equatorial radius is about 3963 miles (6378 KMs) and the Polar radius is about 3950 miles (6357KMs). The Earth rotates about a diameter from west to east in 23 hours 56 minutes 4 seconds of apparent solar time. Due to this rotation, the inhabitant of Earth is carried eastward constantly without any awareness of this motion in space.
Spheres of Earth
There are three spheres of Earth namely Atmosphere (air), Hydrosphere (water) and Lithosphere (land).
Atmosphere (air)
The atmosphere is made up of layers of gas and water vapour. It is like an insulating blanket that protects the earth. It absorbs the harmful ultra-violet rays of the Sun and protects the living beings from million of Meteor particles that fall on earth. The Meteors fall through the atmosphere and in the process get friction and burn themselves. The atmosphere recycles water and other chemicals. More than 3/4th of the gases in the atmosphere are found in the lowest 16KMs.
The main elements in the lower level are Nitrogen (78%), Oxygen (21%), Carbon-di-Oxide, Argon, Ozone and other gases in minute quantities. All animals need Oxygen to live. Nitrogen is necessary for the growth of plants. Carbon-di-Oxide is necessary for Photosynthesis. Plants breathe in Carbon-di-Oxide and breathe out Oxygen. Carbon-di-Oxide maintains the earth's temperature through the green house effect and thus traps the solar heat. Without Carbon-di-Oxide, the Ocean would freeze.
Air contains water vapour (from 0.01% to 1%) and dust particles. Which help in the formation of clouds, fogs and mists.
Air exerts pressure. The atmosphere is pressing down on all the living beings all the time. We do not feel it because, it is pressing over all parts of our body at the same rate. At the earth's surface, it is 1.03 Kg per sq.cm.if you trace out 1 sq.cm on the back of your hand; there is 1 Kg of air pressing on it.
There are four distinct layers in the atmosphere. The troposphere is the lowest and closest to the earth, which contains 80% to 90% of the atmosphere's total mass. It extends to about 16 KMs over the poles and to about 16 KMs over the equator. All weather phenomenons like storm, wind, rain and clouds take place here. Temperature drops with increasing altitude or height. Stratasphere, which extends up to 80 KM over the earth follows the troposphere..Stratasphere has less oxygen and absorbs the harmful ultra-violet rays of the Sun. The temperature is below freezing and more or less steady here, Jet planes fly in this layer. The Stratasphere is followed by the Ionosphere, which extends up to 400 KMs to 480 KMs above the earth. Ionosphere consists of electrically charged icons which makes telecom and radio services possible. Radio waves used in long distance communication are reflected back to the earth by this layer. It absorbs the Sun's deadly X-rays. Beyond the Ionosphere is the Exosphere, which extends up to 2,400 KMs over the earth. Studies are being conducted through space probes. The atmosphere then gets thinner and thinner and fades into empty space.
Sound travels only when there is air. Without it, the earth would be a silent place like the Moon. Astronauts on the Moon can only communicate through walkie-talkies. If a Meteor crashed on the Moon, the astronaughts would not hear it.
Hydrosphere (water)
Oceans cover the major part of the earth's surface. They completely encircle the continents and the Islands; Each Ocean has many inlets, bays, gulfs and seas at its edge. Oceans contain 97% of the water on earth and this water is salty. The fresh water in the polar ice caps, rivers, streams and other sources adds up to only 3% of water resources.
At the edges of the continents, the Oceans are shallow, then the floor slopes down to Ocean plains and often dips down to Ocean deeps The Pacific ocean is the longest and deepest, over 10,000 m deep and Mt.Everest can get totally submerged.
Sea water never standstill. Winds blowing over the surface of Ocean causes the up and down movement of water which is called "waves". The periodic changes in the water level are called "tides". Currents are regular movements of water from one part of the ocean to another.
The gravitational pull of the Sun and the Moon causes the sea level rise and fall regularly during the course of the day. During the Full Moon and the New Moon, the Earth, Sun and the Moon are in a line. Because of this combined gravitational effect of the Sun and the Moon, tides are height. During the First and Third Quarters of the Moon, when the Sun and the Moon are at right angles, the gravitational pull of the Sun opposes that of the Moon and so, tides are lowest.
Lithosphere (land)
Four billion years back, earth had no Plants, no animals, no bacteria, no viruses, no life at all.
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