Observation, Theories, and the Planets

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Human have always been fascinated by the heavens, by the behaviour of the sun by day and the stars by night. Although more accurately measured now becasue of precise instruments the basic observations of these events have remained the same over the past 4000 years. However, our interpretation of the events have changed dramatically. For example, about 2000 в.с. the Egyptian postulated that the sun was a boat inhabitited by the god Ra, who daily sailed across the sky.

Over the years, patterns in the changes in the heavens were recognized and, through marvelous devices such as Stonehege in England, were connected to the season of the years. People also noted that seven objects seemed to move against the background of "fixed stars". These objects, actually the sun, the moon, and the planets Mercury, Venus, Mars, Jupiter, and Saturn, were called the "wanderers". The planets appeared to move from the west to east, except Mars, which seemsed to slow down and even move backwards for a few weeks. One of the first explanations for these observations came from Eudoxus, born in 400 в.с. He imagined the earth as fixed, with the planets attached to a nested set of transparent spheres that moved at different rates around the earth. The stars were attached to the outermost sphere. This model, although clever, still did not account for the strange behaviour of Mars. Five hundred years later, Ptolemy, a Greek scholar, worked our a plan more complex than that of Eudoxus, in which the planets were attached to the edges of spheres that "rolled around" the spheres of Eudoxus. This model accounted for the behaviour of all the planets, including the apparent reversals in the motion of the Mars. Because of human prejudice that the earth should be the center of universe, Ptolemy's model was assumed to be correct for more that a thousand years, and its wide acceptance actually inhibited the advancement of astronomy. Finally, in 1543, a Polish cleric, Nicholas Copernicus, postulated that the earth was only one of the planets, all of which revolved around the sun. This "demotion" of the earth's status produced violent opposition to the new model, and in fact, Copernicus's writings were "corrected" by religious officials before scholars were allowed to them.

The Copernican theory persisted and was finally given a solid mathematical base by Johannes Kepler. Kepler postulated elliptical rather then circular orbits for the planets in order to account more completely for their observed motions. Kepler's hypotheses were in turn further refined 36 years after his death by Isaac Newton, who recognized that the concept of gravitation could account for the positions and motions of the planets. However, even the brilliant models of Newton were found incomplete by Albert Einstein, who showed that Newton's mechanics was a special case of a much more general model. Thus the same basic observations were made for severl thousand years, but the explanations - the models - have changed remarkably from the Egyptians' boat of Ra to Einstain's relativity.

Our models will inevitably change, and we should expect them to do so. They can help us make scientific progress, or they can inhibit progress if we become too attached to them. Although the fundamental facts of chemistry will remain the same.