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Shape and size of the Earth
It once was believed that the Earth was flat and that ships could sail over the edge. This view persisted even in the middle ages and was an issue in recruitment of Columbus.
Early Greek view was that the world was surrounded by the ocean (Oceanus), origin of all rivers. Anaximander (600 B.C) proposed that cylindrical earth was surrounded by celestial sphere. Pythagoras (582-507 B.C.) believed that the Earth was a sphere, which was considered the most harmonious geometric shape. Aristotle (384-322 B.C.) described observations that supported the theory that the Earth was a sphere. These included the fact that the shadow of the moon is circular in lunar eclipses and constellations were higher in the sky as one traveled south.
Eratosthenes (275-195 BCE) estimated size of earth from observations that the elevation of the sun varied with position on the Earth’s surface in Egypt. Observations of the following suggested that the Earth is a sphere.
1. Mountain peaks lit by the Sun after sunset.
2. Ships disappear below the horizon as they sail across ocean.
3. The moon looks like a disc.
4. The Earth casts a circular shadow during lunar eclipses.
The Earth is an oblate spheroid, bulged at the equator and flattened at the poles. It is called ‘Geoid’ (Figure 2.9) meaning the earth is earth-shaped. The bulge at the equator is caused by the centrifugal force of the Earth’s rotation. The gravitational pull of the earth is the strongest at the flattened poles and it is weaker towards the equator.
Chimborazo in Ecuador is higher than Mount Everest, if measured from the centre of the Earth. Why?
The Sun’s gravitational pull differs in force at the poles. The North Pole points in the same direction to the North Star when it revolves about the Sun. If the Earth would not have been tilted on its axis, the days and nights would have been of same duration always.
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