Earth The Earth revolves around the sun is an elliptical orbit with average radius 149 598 000 km. The orbit is approximately a circle . The ratio of the maximum value of radius to the minimum value of radius is 0.97. The Earth also rotates on its axis which is counterclockwise as viewed from above the North Pole. The axis of the Earth is tilted at an angle of 23· 26'59", to the plane of its orbit around the sun. This tilt causes the seasonal variations.
Table EI, Physical data of the Earth
Size of the Earth
Equatorial diameter 12 757 km
Polar diameter 12 710 km
Equatorial circumference 40 076 km
Polar circumference 40 000 km
Shape pear shaped or triaxial ellipsoid
Mass 6 ´ 1024 kg
Rotation
axial 23 hrs, 56 min., 4.09 sec.
revolution 365.244 days
Age 4.6 ´ 109 years
The Earth is divided into three basic layers - gaseous atmosphere, liquid hydrosphere and solid lithosphere.
Atmosphere : The atmosphere is a mixture of gases. The volume percentages of various gases are nitrogen - 78.09, oxygen - 20.95, argon - 0.09, carbon dioxide - 0.03, other gases (neon, helium, krypton, xenon) - 0.01. There are five concentric layers in the Earth's atmosphere. Descriptions of these layers are given below.
(i)Troposphere: This layer extends up to 10 km above the sea level. It is the part of atmosphere where weather takes place. Temperature range in troposphere is 15oC on the surface, to 0.6oC at the top of the layer.
(ii)Stratosphere: (10 to 50 km above sea level). In this layer temperature is fairly constant (~ 50oC).
(iii)Mesosphere: (50 to 80 km above sea level) In mesosphere and stratosphere UV radiations from the sun interact with oxygen to produce ozone and also oxygen and nitrogen ions. The ozone in the mesosphere, the water vapor and carbon dioxide in the troposphere, control the temperature of the Earth by green house effect*. In the middle part of the mesosphere there is a high concentration of free electrons and ions. This part is called ionosphere*.
(iv)Thermosphere: (80 to 640 km) Temperature again rises in this layer, which varies between 300oC to 2000oC, depending upon the solar activity.
(v)Exosphere: (beyond 640 km). The gases are extremely rarefied. The main components are light gases, hydrogen and helium, a large part of which diffuse out into the space.
Hydrosphere : The oceans and seas cover 71% of the Earth's surface. The rest of the hydrosphere is composed of the continental ice, lakes, rivers and ground water. It constitutes only 0.03% of the mass of the Earth. The cycle of evaporation of water from the tropics and subtropics and its eventual precipitation in the form of rain or snow is called hydrological cycle. Hydrosphere is unevenly distributed. The Northern Hemisphere contains most of the land area. It is because of this uneven distribution, climate in the Northern Hemisphere has such wide variations of temperature, as compared to that in Southern Hemisphere.
Lithosphere : It has three concentric layers. The upper most layer is called the crust which has an average thickness of ~32 km. The crust is thin under the oceans (4 to 8 km) and thick under the continents (19 to 90 km). The second layer is called the mantle, which extends to 2900 km below the crust. It is mainly composed of dense greenish black masses of magnesium iron silicates, with average relative density 3. The innermost part of the Earth is called the core which contains dense metals iron and nickel. The outer core is liquid while inner core (below 5000 km) is solid. The maximum temperature at the core is 6400 K. Its relative density is between 8 to 10.