What Is Called Our Earth?

The most important planet to us is the Earth that distinguishes itself from other heavenly bodies of the solar system in respect of the following facts. (i) It is the largest of the minor or inner group of planets.

(ii) The earth, with a radius of about 6400 km and large circumstance of about 40,000 km, might seem to be large enough but in fact, this whirling planet is but a tiny speck in the astoundingly vast Universe.

(iii) It is the only planet of the solar system and the only member of the myriads of systems in the universe, which presents an intermingling condition of air, water and land. Thus, it is unique among the planets in having abundant water, an atmosphere and surface temperature conditions that has supported life.

(iv) It is the only planet known till date, where living being including intelligent creatures like man exists. The other planet, where probability of life is surmised is the Mars.

(v) Unlike other planets, the Earth has a strong magnetic field of its own.

(vi) It is the densest planet. It shows evidences of erosion of its surface (change of landforms by actions of air, water etc.), which is completely lacking in other

(vii) While in other planets and satellites there are innumerable meteoritic craters (depressions made by impact of meteorites), the Earth’s surface shows much less evidence of such craters, which are very much modified by erosion.

Equatorial km Equatorial diameter 12755 kn km Polar Pola@ diameter 1 circumference F I 40075 km

Physical Aspects of Earth: The following physical aspects of the Earth are noteworthy.

(i) Shape: The figure of the earth corresponds to an imperfect sphere which is slightly flattened (oblate spheroid).

The idea of earth’s shape as an oblate sphere with slight flattening at the poles and bulging at the equator was propounded by Newton in 17th century. According to Newton; the earth being a rotating sphere would be subjected to centrifugal flattening in the Polar Regions. Thus, the earth departed from perfect sphere and would be roughly orange-shaped. This is attested by the fact that the

equatorial diameter of the earth is about 42 km greater than the polar diameter (Fig. 1.3).

The idea of Pear-shaped earth was first proclaimed by Sir James Jeans by the beginning of 20th century. The latest measurements by artificial satellites confirm the idea of Jeans and indicate the shape to be a slightly flattened sphere with some irregularities at the north and south poles. Thus, the shape of the earth as per the latest computations is more nearly PEAR – shaped than orange-shaped.

(ii) Size: Earth is an average-sized planet in the solar system. Latest data based on geodesic survey (Geodesy deals with the study of measurement of dimensions of the Earth) and satellite observation, speak of the following measurements relating to the dimension (size) of the Earth.

The difference between equatorial and polar diameters of the earth (12,755 – 12713 = 42 km) indicates the bulging and flattering of the earth at the equator and poles respectively (Fig. 1.3).

Internal Temperature:

The following estimates of temperature within the earth’s various zones are made,

(i) Crust: From near surface temperature to about 1000° C at the base

(ii) Mantle: From 1000° C to 3500° C at the base of lower mantle. 

(iii) Core: 3500° C to nearly 6000° C at the centre of the earth. 

Along with temperature, pressure also increases with depth and at the centre, the 

prevailing pressure would be 3.75 million times as high as the pressure at the 

earth’s surface (i.e. 3.75 million atmospheric pressure)

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