(1) The best known experiment for showing that the Earth really turns on its axis was made by Foucault (pronounced Foo-ko´), a French philosopher, in 1851, who used a pendulum for the purpose.
Fig. 72.—Sailing Round the Earth.
In the top of a great dome in a building in Paris, called the Panthéon, Foucault hung a large metal ball by means of a wire about 150 feet long. On the floor he made a mark, exactly under the ball, running due north and south. Then drawing back the ball, he let it go, when it swung directly over the line.
The heavy pendulum, which after being started swung for hours, seemed to move away from the line in the direction of the hands of a watch. This showed that the floor of the Panthéon was really skewing around under the swinging pendulum. This is easily explained as due to the rotation of the earth because the northern edge of the floor was nearer the axis of the earth than the southern edge and therefore was carried more slowly eastward.
(2) A much simpler way to show the turning of the Earth on its axis, though this experiment only proves that either the Earth or the whole sky turns, is to make a photograph of the North Star and the stars in its neighborhood, as explained in [Chapter XII].
During the time the sensitive plate is being exposed the camera will be carried round by the Earth turning on its axis and the fixed stars will leave bright trails on the plate in arcs of circles.
The Earth Turning on Its Axis Makes Day and Night.—The Earth, in turning round on its axis once every 24 hours, receives the light of the Sun on half of its surface at a time, making the day, while the other half is in the shadow, which makes the night.
Fig. 73.—The Earth Moves Away From the Swinging Pendulum.