EARTH'S ROTATION AND REVOLUTION Prepare Yourself For The Revolution Rotation Computing the speed of Earth's rotation (at the equator): • Circumference of Earth = 40,000 kilometers • Time for one rotation = 24 hours • Speed of rotation = Distance/Time = 40,000 km / 24 hr = 1670 km/hr Around and Around We Go • Every second, that you sit in this classroom, the Earth's orbital motion carries you 30 kilometers, or about 18 miles, through space. Revolting Developments!!! Computing the speed of Earth's revolution around the Sun: • Circumference of Earth's orbit = 940,000,000 kilometers • Time for one revolution = 365 1/4 days = 8766 hours • Speed of revolution = Distance/Time = 940,000,000 km / 8766 hr = 107,000 km/hr = 30 km/sec Evidence • Uniform motion is difficult to detect. Although it is possible to detect the Earth's circular motions, the effects are subtle, and were not detected until the 18th and 19th centuries, long after Copernicus proposed that the Earth was in motion. www.theozonehole.com First line of evidence! • The Coriolis effect was first described in 1835 by a French scientist by the name of Gustave Coriolis. If you are located at the equator, and fire a cannonball north or south, you find that the cannonball swerves to the east. Why is this so? • Demonstration please. First line of Evidence • The net result of the Coriolis Effect: In the Northern Hemisphere, projectiles swerve to the right. In addition, air rushing inward to a low pressure area will swerve to the right, and set up a COUNTERCLOCKWISE hurricane. • In the Southern Hemisphere, projectiles swerve to the left, and air rushing inward to a low pressure area will set up a CLOCKWISE hurricane. Second line of Evidence • The Foucault pendulum was first demonstrated in 1851 by yet another French scientist; his name was Jean Foucault. The Foucault pendulum is nothing more than a very long pendulum suspended from a well-oiled ball-andsocket joint overhead, so it is free to swing in any direction. Foucault Pendulum http://mxp.physics.umn.edu/s05/Projects/S05SphericalPendulum/foucault-pendulum.jpg Foucault Pendulum • Foucault set up such a pendulum in the Pantheon in Paris, and set it swinging north to south. As hours passed, however, the direction in which the pendulum was swinging moved around in a clockwise direction. After a while, the pendulum was swinging northeast-southwest; after a while longer, it was swinging east-west, then southeast-northwest, then northsouth again Foucault Pendulum • What causes this change in the pendulum's direction of swing? The rotation of the Earth, of course. • The important fact (independent of where you're standing) is that the Earth and the pendulum's swing are rotating relative to each other. If the Earth did not rotate on its axis, the direction of swing of a Foucault pendulum would remain fixed relative to the surface of the Earth. Copernicus • When Copernicus proposed his heliocentric theory, his critics pointed out that if the Earth orbits the Sun once per year, then the Earth's location in October (for instance) should be 2 astronomical units away from its location in April, half a year later. This change in the Earth's location must cause the nearby stars to shift in apparent location relative to more distant stars. Could that evidence be seen? Third line of Evidence. • Stellar parallax was searched for by astronomers from antiquity onward. However, prior to the invention of the telescope, stellar parallax was not observed. • Why not? Stellar Parallax There are two possible hypotheses: • (1) There is no stellar parallax because the Earth is stationary. This is the hypothesis put forward by the supporters of the geocentric universe. • (2) Stellar parallax exists, but it is too small to be detected, because the stars are too far away. This is the hypothesis put forward by Copernicus and other supporters of the heliocentric universe. Stellar Parallax • In fact, stellar parallax was first detected by Bessel (using a telescope) in the year 1837, nearly three centuries after the death of Copernicus. • In general terms, parallax can be defined as the shift in the observed position of an object, resulting from a change in the observer's location. http://images.encarta.msn.com/xrefmedia/aencmed/targets/illus/ilt/T041360A.gif Sorry about that. So, Copernicus was right: • The Earth does rotate about its axis. • The Earth does revolve around the Sun. • The stars are very distant from the Sun. However, Copernicus wasn't vindicated by direct observational evidence until centuries after his death References • http://www-astronomy.mps.ohiostate.edu/~ryden/ast161_4/notes13.html • http://images.encarta.msn.com/xrefmed ia/aencmed/targets/illus/ilt/T041360A.gif • http://mxp.physics.umn.edu/s05/Project s/S05SphericalPendulum/foucaultpendulum.jpg • www.theozonehole.com
