# Occurrence of day and night seasonal variation as

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occurrence of day and night, seasonal variation as well as annual irregular length of days and night are some of the evens that exemplified the movement of the earth. This unit examines the various modes of motion like rotation, revolution, wobbling and their effect such as centrifugal sorting and oblate nature of the earth. 2.0 Objectives By the end of this unit you should be able to: (a) Describe the three main types of motion of the earth (b) State the effect of each of the three motions. (c) Show diagrammatically the two hypotheses of earth’s rotation (d) Explain the cause of the density variation of the solid earth (e) Relate the centrifugal sorting to the density variation of the solid earth 3.1 Types of motion of the earth The earth, in order to maintain celestial dynamic equilibrium with other planets and other heavenly bodies, is under going series of complex motions. The three major types of the motion whose effects are observable in our daily life are discussed ahead.

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- 22 - 3.1.1 Revolution The earth moves along an elliptical orbit whose plane is fairly perpendicular to the polar axis of the earth. F 1 and F 2 are the foci of the orbit with sun at one of the foci. This motion has a periodicity of 31557600 seconds (i.e. a year) and determines the global season. The seasonal change is brought about as a result of earth distance from the sun at each location on its orbit. For instance, at location B the earth is far from the sun, so the sun’s heat may not be much to cause enough evaporation for much rain. At location A the earth experiences much heat of the sun rays. This causes intense evaporation, leading to rain possibility. 3.1.2 Wobble Earth Su A F 2 B Polar Axis Elliptical Orbit Fig.1: Revolutionary motion of the earth F 1 Left flip A Right flip B Sun Fig. 2: Flip-flop Motion of the Earth
- 23 - The earth exhibits Flip-flop (wobble) motion about its equator as it is revolving round the sun. It should be noted that northern hemisphere is closer to the sun than southern hemisphere. Thus, northern hemisphere experiences longer day and shorter night at this time; But reverse is the case at the location B. 3.1.3 Rotation This is a rotation about polar axis from west to east in an anticlockwise direction, viewing from the tip of the northern pole. The average period of earth’s rotation is 86400 seconds. (i.e. a day). Hence, rotation of the earth: - determines the length of the day and night - causes its ellipsoidal/oblate shape i.e. equatorial radius being longer than the Polar radius -results in density variation from the surface to the centre of the earth 3.2 Hypotheses about the Rotation of the Earth 3.2.1 Cassini’s hypothesis He proposed that the earth is rotating about the equator with the polar axis perpendicular to the axis of rotation. If this is true then the earth will assume prolate ellipsoid shape and one side of it would be experiencing permanent day light while the other side would be in darkness permanently.

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- 24 - 3.2.2 Newtonian Hypothesis
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