Johannes Kepler

Abstract

The mathematician and astronomer Johannes Kepler (b. 1571–d. 1630) was a leading figure in what is commonly regarded as the scientific revolution. He is best known for his three laws of planetary motion (the planets move round the sun in an ellipse, and the sun is one of the foci; as they revolve, a radius vector drawn from the sun shows that the planets sweep out equal areas in equal times; the squares of the periodic times of any two planets is proportional to the cubes of their mean distances from the sun), which are still valid today, but at the time they were formulated they battled the prevailing belief in uniform circular planetary motion. At the same time he developed the idea of a celestial physics: that is, he maintained that the celestial bodies are physical, such as the earth, and moved by physical forces. His studies in optics were also pathbreaking: he showed that the eye was an optical instrument and worked according to natural laws, and he intuited the workings of the retina and the brain. He was also able to describe how lenses worked. He realized light propagated in spheres and formulated his intensity law—that the intensity of light is inversely proportional to the square of the distance from the source. Like most astronomers of his day, Kepler was an astrologer, but he tried to reform the way astrology was done and maintained it had limited validity. Kepler had originally studied for the ministry and wrote a number of theological works. His scientific writing is suffused with his piety. Because of his writings on astrology and the religious tenor of his nontheological writings scholars debate whether Kepler was a number mystic and whether mysticism influenced his scientific discoveries. This is perhaps a factor in the lack of studies about Kepler in English relative to other major players in the scientific revolution until the last several decades.