Newton hypothesised that the earth is rotating about

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Newton hypothesised that the earth is rotating about the polar axis which lead to the oblate ellipsoidal shape of the earth. Fig.4: Newtonian Oblate Ellipsoidal Earth Sun Axis of rotation Permanent night zone Permanent day-light zone Fig.3: Cassini’s Prolate Ellipsoidal earth Polar axis

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- 25 - The geophysical evidence such as longer equatorial radius than the polar radius, centrifugal sorting of the earth into its density regimes in which the density increases from the surface towards the centre and occurrence of day and night support Newtonian hypothesis. 3.3 Self-Assessment Exercise1 (i) Draw a wobbling earth with its poles in a vertical position. (ii) What would be the length of days and nights when the earth wobbles to the position you draw above? (iii) List the practical experiences on the earth that negate Cassini’s hypothesis on the rotation of the earth (iv) Mention the practical experiences on the earth that supports Newtonian hypothesis about the rotation of the earth. 3.4 Effect of rotation on the internal structure Given the fact that the earth was initially melted, a state attained during Iron Catastrophe, as the earth rotates it behaves like a centrifuge, sending the lighter materials of crust to the flanks while the denser and homogeneous mantle and core materials remain closer to the centre of the earth. Geophysical studies have confirmed the inhomogeneous crust, denser and homogenous mantle (Fig.5)
- 26 - 3.5 Earth Shaping due to its Rotation Homogeneous layers Inhomogeneous layer A B C Fig.5: Centrifugal Sorting of the Earth Interior Equator R e R p C f C C Fig.6: Deviation of the Earth Shape from a Perfect Sphere

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