How do we describe the 'big ball' on which we live? Seen from space, it appears as a bluish-white sphere. Its wealth of plant and animal life, snow covered peaks, blue oceans and white clouds make the Earth a beautiful planet. Human beings have explored it extensively. Yet, the first hand knowledge of the Earth is limited to a thin shell of rock and water extending to a few kilometres below the surface, and to the atmosphere above. However, using indirect methods, such as the study of waves generated during earthquakes, scientists have been able to picture the Earth's interior. without ever seeing or sampling it. We will now briefly describe the current scientific knowledge about the Earth.
The Earth's atmosphere has been studied extensively with the help of ground based experiments, aircraft, rockets and balloons. Though it is not divided into distinct layers, it-is helpful to think of the atmosphere in this way. The troposphere, nearest to the surface, is made up of 78 per cent nitrogen and 21 per cent oxygen, with water vapour, carbon dioxide, neon and argon making up most of the remaining one per cent. It has an average temperature of about 16°C at sea level and- 16OC near its top. The stratosphere, coming next contains ozone and has a temperature ranging from - 16OC to -4OC. This ozone layer absorbs the harmful UV radiations fron, the Sun, thus protecting us from them. The carbon dioxide in the Earth's atmosphere traps heat and makes it warmer through the greenhouse effect. Were it not for this, the Earth's surface temperature would be much lower and it would always be covered with ice.