The age of the earth has been estimated as 4.6 billion years. Many methods like radioactive dating, fossil records etc. have been used to determine the age of the earth. Scientists have also discovered that life on earth started some 570 million years ago. The first 345 millions years saw the development of marine life. For the next 160 million years earth was ruled by reptiles. The next 65 million years it was the development of mammals. The development of humans is only one million years old.
Shape of the Earth : Shape of the earth has been a big puzzle to the people. Pythagoras (570-500 B.C.) a Greek mathematician suggested that the earth was shaped like a globe.
Ferdinand Magallan made a sea journey in 1559 from Spain and after travelling for 3 years he came back to the same point proving that the earth was not flat but like a ball.
When space flights began earth appears spherical from space.
Following facts shows that earth is not flat but spherial in shape—
1. When a ship, approaches land its mast is seen first and then its hull. If the earth was flat, the whole ship would have been seen at one time.
2. During a lunar eclipse the shadow of the earth as an arc of the circle is casted on the surface of moon. For any lunar eclipse the shadow is always circular in shape. It proves that earth is spherical in shape.
3. The Pole star can be seen in sky at 90o at the North Pole because it lies in line with the axis of the earth. As we move sourthwards the angle of Pole star decreases and at the equator it becomes zero degrees.
These observations shows that the path of travel is an arc of the circle.
4. All heavenly bodies like sun, moon etc. are spherical, so earth should also spherical in shape.
5. The earth as observed from outer space is spherical beyond doubt.
Accurate measurements shows that earth is an oblate spheroid i.e. it is spherical but compressed at poles and bulged at the equator.
Facts about Earth
It is the fifth largest planet in the solar system.
1. Diameter
(Equatorial diameter) 12,757 km.
(Polar diameter) 12,714 km
2. Circumfrence
(Equatorial circumference)40,077 km
(Polar circumference) 40,009 km
3. Water (Oceans) 71% of earth’s surface
4. Land (Continents) 29%
5. Volume 1,000,000 million cu. km.
6. Mass 5.98 × 1021 metric ton
Density 5.52 gm/cm3
Great and Small Circle
Great Circle : When a sphere is divided exactly in half through its centre, the circumference represents the largest circle that can be drawn on the sphere is known as great circle.
Small Circle : When a sphere is divided not exactly in half, circles produced are always smaller than the great circle, it is called small circle.
Characteristics of a Great Circle
1. It is the largest possible circle.
2. This circle is produced when sphere is divided exactly in half.
3. Infinite number of great circles can be drawn on a sphere.
4. Only one great circle that will pass through two given points on a sphere can be drawn.
5. An arc of a great circle is the shortest distance between two points on a sphere.
6. Great circles when intersect always bisect each other.
The earth is the only planet where life exists because it has water and air which are essential for life. There is no other planet known so far in the universe, where life exists.
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