The Earth, the third planet in line from the Sun, is a unique member of the Solar System. It is 92,955,820 miles or 149,597,891 kilometers away from the sun and its diameter is just a few hundred kilometers larger than that of Venus. It is the only planet that has an atmosphere, stretching for about 1600 kilometers in all directions. The atmosphere protects the Earth from the harmful rays of the Sun and from dangerous meteors. It has the air, that we breathe, that also keeps plants and animals alive. The atmosphere is made up mostly of nitrogen (78%), oxygen (21%), carbon dioxide (0.03%) and a tiny amount of other gases.
Apart from air, another basic condition for life is availability of sufficient water. The Earth is the only planet that has liquid water on its surface. No other planet has such an abundant storage of water. This water evaporates and comes down as rain or snow and provides sweet water to all living things. The Earth stays in an advantageous position in the Solar System, between Mars and Venus. It is neither too far, nor too close from the Sun. Its mean approximate daytime temperature on the sunlit side is 17ºC, which is a favourable condition for the living beings.
The Earth is composed of four different layers. The extremely hot (5,400ºC) inner core is a solid sphere made of mainly iron and nickel metals. It is about 1,221 km in radius and is surrounded by a layer of about 2,300 km thick fluid, made of iron and nickel. In between the outer core and the crust lies the mantle, the hot sticky thickest layer, comprising a viscous mixture of molten iron and magnesium-rich silicate rocks. The Earth has two kinds of crust above the mantle. The dry continental land consists mainly of granite and other light silicate minerals, while the ocean floors are made up mostly of a dark, dense volcanic rock called basalt. Continental crust averages some 40 km thick, while the Oceanic crust is usually only about 8 km thick. The lower areas of the Oceanic crust of basalt are filled with water to form the oceans.
The atmosphere of the Earth has a series of layers. The lowest layer of the atmosphere is known as the Troposphere, which is constantly in motion, causing the weather. From the ground level, it extends upward to a height of 18 km at the Equator and 8 km at the Poles. This layer contains 99% of the water vapour and so most clouds appear here. Above the Tropopause, or the outer layer of the Troposphere, extends the Stratosphere
upto a height of about 50 km from the surface of the Earth. The still air of the Stratosphere contains the ozone layer, which protects the Earth from the harmful ultraviolet radiation of the Sun. The layer above the Stratosphere is the Mesosphere, extending upward to a height of about 85 km above our planet. Most meteors burn up in the mesosphere. The top of the Mesosphere is the coldest part of the atmosphere, where it can go down to - 90° C. The other layers above are called the Thermosphere and the Exosphere.
The Earth is the only planet in the Solar System that has a single satellite, which is very much familiar to us as The Moon.