Kepler’s Laws
The stage was well set for the appearance of Kepler (15711630) who had a life-long struggle against poverty and ill health. Although he did not inherit the silks and ruffles that fell to Tycho Brahe, he did inherit great riches in the astronomical observations of his more prosperous predecessor. Kepler took up the Copernican theory of circular planetary motions, but was unable to reconcile the assumptions of uniform circular motion with the observational data available. He gave up the idea of uniform motion and also began to experiment with other closed curves. Finally he discovered that Mars describes an ellipse around the Sun and that the Sun was in one focus of the ellipse. This led to other conclusions which are known as Kepler’s three laws pertaining to the motions of the bodies of our solar system. They are,
- The orbit of each planet is an ellipse, having the Sun in one of the foci.
- The straight line joining a planet and the Sun describes equal areas in equal times.
- The squares of the times of revolution of any two planets about the Sun are proportional to the cubes of their mean distances from the Sun.
It will be noted that the first two laws concern the motion of anyone planet and the third gives a relation between the periods and distances of the various planets. The determination of anyone distance in the solar system, therefore, makes it possible to determine all other distances, because the periods of revolution can easily be determined by observation. It has been stated by some authorities that in the entire history of astronomy only two other works, those of Copernicus and Newton, are of equal importance.
It is seen that in the earlier period of scientific development, astronomical observations were contributing much toward an understanding of the physical world. This was inevitable because these data were obtainable by direct observation. Much of our present knowledge of the electron, of radiation, and of atomic structure is obtained by indirect methods which naturally are the result of this period of greater knowledge. However, even now, observations of the stellar universe and of the phenomena taking place in stellar crucibles are contributing much toward the development of physical science.