Lesson 3: Astronomy after Copernicus
Johannes Kepler and the new Copernicanism
Tycho’s greatest disciple was Johannes Kepler. Kepler created the most mathematically powerful and physically accurate system of planetary astronomy to date, the necessary precondition for the achievement of Isaac Newton.
The last major astronomer to be a practicing astrologer, Kepler was deeply influenced by magical and mystical traditions of cosmic harmony as well as quantitative astronomy. He viewed science as an attempt to understand the mind of God. As God’s creation, the universe had to be rationally structured and ordered, rather than created by chance. This rational structure and order was mathematical and geometrical.
As God’s creation, the universe was imbued with religious meaning. For example, the universe was fundamentally organized in threes, reflecting the Christian Trinity. The relation between the Father, Son, and Holy Spirit was analogous to the relation of the sun, the outer heaven of the stars, and the space between them. But unlike many who approached the universe in a mystical or magical way, Kepler was obsessed with numerical precision, and even a slight deviation of the position of Mars from that he theoretically predicted led him to change his entire theory.
He was obsessed with showing that his astronomical theories did not merely accurately predict planetary motions, "saving the phenomena" but that they were physically true. He abandoned Copernicus’s use of the center of the earth’s orbit, the “mean sun,” as the center of the solar system, in favor of the actual sun, from which a force which governed the motions of the planets emanated.
In that sense his theory was truly heliocentric as Copernicus’s was not. Influenced by the English physician William Gilbert’s recent book on magnetism, Kepler identified the force governing planetary motion as magnetic.
Kepler’s astronomical theory was built on Tycho’s observational data (Kepler suffered from poor vision, and was not a particularly good observer himself), but differed enormously from Tycho’s theory. Kepler was a Copernican from very early in his career, but his view of the sun-centered universe greatly differed from Copernicus’s.
Kepler broke the longstanding tyranny of the idea that heavenly motion was perfect, and therefore circular. He reduced planetary motion to three laws, although he did not state them in that form, and subsequent generations of astronomers had to extract them from his works.
The first two laws appear in The New Astronomy in 1609; the third in the 1619 Harmonies of the World, a work explaining the universe as a structure organized by geometrical and musical harmonies. Oster includes excerpts from these books as documents 3.4 and 3.5.
The three laws are:
- Planets move around the sun not in circles but in ellipses with the sun at one focus.
- A line drawn between the planet and the sun will always sweep the same area in the same amount of time. Therefore, planets accelerate as they approach the sun, decelerate as they move away from it.
- The squares of the time planets take to go around the sun are proportional to the cubes of their average distances from the sun.
