Johannes Kepler

by

Sheila J. Rabin

• LAST REVIEWED: 13 November 2023
• LAST MODIFIED: 21 June 2024
• DOI: 10.1093/obo/9780195399301-0047

Introduction

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.

General Overviews

These studies about the scientific discoveries in the period known as the scientific revolution reflect changes in attitudes toward the period in general and toward Kepler in particular. Earlier treatments of the scientific revolution, such as Cohen 1960 and Hall 1994, tend to view the scientific revolution as the work of great men and thus devote a full chapter to Kepler; however, they are uncomfortable with Kepler’s open espousal of astrology and tend to portray him as a religious mystic. More recent treatments, such as Dear 2001, emphasize trends and subjects over individuals and tend not to see Kepler’s astrology as problematical given the period; Dear practically ignores it in his section on Kepler.

• Cohen, I. Bernard. The Birth of a New Physics. Garden City, NY: Doubleday, 1960.

  For general audiences. Account of the discovery of the physics for a moving earth from Copernicus to Newton. Chapter 6 deals with Kepler but suggests that his concept of physics was Aristotelian and his style of writing and beliefs arcane: as a result, his ideas had little effect on his contemporaries.

• Dear, Peter. Revolutionizing the Sciences: European Knowledge and Its Ambitions, 1500–1700. Princeton, NJ: Princeton University Press, 2001.

  For general audiences. Covers material chronologically from 1500 to 1800. Chapter 4 has a section on Kepler. Focuses on astronomy, celestial physics, and optics.

• Hall, Marie Boas. The Scientific Renaissance, 1450–1630. Mineola, NY: Dover, 1994.

  For general audiences. Sees the development of science as an intellectual process starting with Renaissance humanism and culminating in Galileo. Chapter 10 on Kepler sees him as torn between mystical Neoplatonism and rationalism. Originally published 1962.