The rotation of the earth on its axis causes the sun and the stars to appear to revolve about the earth, the sun appearing to rise in the east and set in the west, producing the successive changes of day and night and thus giving the measure of time, the day.
The rotation of the earth is one of the factors along with others in producing the tides, the belts of planetary winds and calms; and it affects the direction of the ocean currents.
The rotation also produces the bulging at the equator and the flattening at the poles. It causes a deflection of falling bodies. A ball dropped from the top of a tower would be deflected to the east of the base of the tower, instead of falling directly vertical. The deviation is greatest at the equator and zero at the poles.
Foucault's Pendulum - In the middle of the last century Foucault demonstrated the rotation of the earth by means of a pendulum consisting of a heavy weight suspended on a long, slender cord which is started to swing due north and south across a plane surface covered with fine sand. Attached to the bottom of the pendulum is a sharp point which traces a mark in the sand as it swings.
If the earth were still, the pendulum would continue to swing on this line but the rotation causes this plane under the pendulum to rotate to an extent varying with the latitude, from zero at the equator to a complete revolution at the poles. This pendulum is still in use at the Pantheon in Paris where visitors may see the rotation taking place as Foucault did in 1851.
Directions - The terms north, south, east and west are used to signify directions on the surface of the earth and also in space. North with reference to the earth really means the direction of the north pole, one end of the axis of the earth, and would be a curved line corresponding to the meridian at the point where the direction is taken.
What we really think of, however, is the line on the plane of the horizon which marks its intersection with the plane of the meridian. North in the heavens refers to the direction of the axis of the earth prolonged to infinity passing nearly through the north star.
At a point on the equator this direction would be identical with north on the earth, but as one approaches the north pole the two lines diverge until near the pole they are at nearly right angles to each other. Represent this by a diagram for (1) your latitude, (2) the equator and (3) the north pole.
At the north pole all directions on the horizon are south and the line to the north star is perpendicular to the horizon. At all other points on the earth, south is the opposite direction from north until one arrives at the south pole where there is no south but all directions are north.
East refers to the direction on the horizon at right angles to the north and south line but which if followed proves to be a curved line. West is the opposite of east. The equator and the parallels of latitude are east and west lines yet they are circles on the globe.
The terms east and west are used for directions of rotation and revolution thus the earth rotates toward the east because any one point on the earth at any instant is moving east in the plane of the horizon.
The plane of the horizon is the plane perpendicular to the plumb line. The point where the extension of the plumb line pierces the heavens is called the zenith and the direction is up. The point opposite the zenith is the nadir and the direction is down.
Source : Elements of Physical Geography
BY : THOMAS CRAMER HOPKINS, Ph. D
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