Copernicus, Kepler, Galileo, Newton

Steven Dutch, Natural and Applied Sciences, University of Wisconsin - Green Bay

Historical Setting

Precursory events

• Thomas Aquinas reconciles Aristotelian Philosophy with Christianity.
• King Alfonso the Wise of Castile publishes Alfonsine tables, based on Ptolemaic System, late 1200's.
• Concept of "Law of Nature" arises from medieval theology and philosophy.
• Motion thought to be perfectly circular in heavens, rectilinear on earth.

Immediate Setting

• Printing invented 1457.
• Michelangelo, 1475-1564.
• Luther, 1483-1546.
• Protestant Reformation begins ca. 1520.
• Shakespeare 1564-1616.
• Age of Exploration, Colonization.
• Supernovae, 1572, 1604 shook idea of heavens as unchanging.

Copernicus (1473-1543)

Ptolemaic System

• Planets appear to reverse motions at times.
• Ptolemy explained motions in terms of orbits (epicycles) carried on a larger orbit (deferent).
• Epicycle deferent ratios were very close to modern values of planet/earth orbit ratios. System worked very well.
• Contrary to popular myths, Ptolemy's system was not overly cumbersome, and it accounted for subtleties like the uneven motion of the Sun
• It is not Ptolemy's fault he did such a good job that it took 1500 years to improve on him!

Emerging Problems

• System began to seem cumbersome and inelegant.
• System inaccurate. Alfonsine tables out of date by 1500.

Possible clues to Copernican idea

• Epicycle motions for Venus and Mercury opposite other planets.
• Epicycle for Sun's motion appeared in schemes for all other planets.
• References to now-lost ideas of Aristarchus of Samos

Heliocentric Idea

• Copernicus replaced epicycles with orbital motion of Earth.
• Less accurate than Ptolemaic system but conceptually simpler.
• Published as De Revolutionibus Orbium Coelestium (on the Revolutions of the Heavenly Spheres) 1542.
• Little immediate hostility. Earlier speculations on moving earth had been decreed heretical, hence moving earth idea could be lightly dismissed.
• One of most vigorous critics was Martin Luther.

Galileo (1564-1642)

Galileo's most original contributions to science were in mechanics: he helped clarify concepts of acceleration, velocity, and instantaneous motion.

Galileo's effect on Astronomy

• Galileo did not invent the telescope (known since at least 1590).
• One of the first to use a telescope on the heavens. Found observational evidence against traditional views.
  • Craters on moon
  • Phases of Venus
  • Satellites of Jupiter
  • Others independently used telescopes on celestial objects at nearly the same time. Galileo had the best publicity.
• Main impact: An aggressive popularizer of Copernican viewpoint and satirist of Aristotelian physics.

Kepler (1572-1630), Brahe (1546-1601)

Kepler was a medieval mystic.

• One of the last of the "scientific astrologers."
• Attempted to explain spacing of planet orbits by reference to Platonic solids.
• Kepler was reluctant to abandon perfectly circular motion (but despite his mystic tendencies, he did when the evidence required it).
• Kepler found many numerological relationships among the planets, of which "Kepler's Laws" are the three that have proven to have a physical basis.
• Kepler's concept of the Sun as center of solar system may have had a mystical basis.

Need for observational data.

• Copernican theory based on same observational data as Ptolemaic, hence no more accurate.
• Kepler turned to Brahe, who had the most advanced observational data.

Kepler's Laws

• Orbits are elliptical, Sun at one focus, nothing at other.
• Radius vector sweeps out equal areas in equal times.
• Square of period proportional to cube of distance.

Kepler and Galileo

• Galileo and Kepler corresponded.
• Galileo defended Copernican astronomy but never wrote about Kepler's model.
• Galileo may have been repelled by Kepler's mysticism.
• Moral: even the best and most innovative workers can sometimes fail to recognize a major advance.

Newton (1642-1727)

Experiments on dispersion, nature of color, wave nature of light (Opticks, 1704)

Development of Calculus, 1665-1666

• Built on Galileo and others' concepts of instantaneous motion.
• Built on method of infinitesimals of Kepler (1616) and Cavalieri (1635).
• Priority conflict with Liebniz.

Gravitation 1665-1687

• Built in part on Kepler's concept of Sun as center of solar system,
• planets move faster near Sun.
• Inverse-square law.
• Once law known, can use calculus to drive Kepler's Laws.
• Unification Kepler's Laws; showed their common basis.
• Priority conflict with Hooke.

Priority conflicts (Hooke, Liebniz)

• Have occurred since earliest days of science even when workers were few.
• Major problems attract many workers, most attention: challenge, prestige
• Solutions often emerge simultaneously from many sources.

Why was the Copernican Revolution so pivotal?

• Chance (science had to start somehow)
• Intellectually respectable pursuit, suitable for elite
• Don't get hands dirty
• Problem literally of cosmic significance
• Big problems tend to attract the best minds
• Opportunity to observe laws of nature in "pure" form
  • Gravity and momentum are the only laws at work
  • Uncomplicated by friction, air resistance, etc.
  • In a sense, the ancients were right; the heavens are more harmonious than Earth.

The Scientific Establishment

• Earliest means of communication, privately published books, pamphlets,
• letters.
• Often vigorous counter-responses published by others.
• Martin Mersenne (1588-1648), French mathematician, copied and
• distributed letters, acted as clearing-house. Nicknamed "Post-Box of Europe." Good analogy to informal Internet discussion networks today.

Scientific Societies arise in mid-1600's

Journals first published late 1600's (about 100 by 1800, nearly 50,000 now).

• By ca. 1700 books had become so specialized that publishers often
• refused to print them.
• Abstract journals (summaries) appear about 1840 (about 3000 now).
• About 6,000,000 scientific articles published annually now, worldwide.
• Information content (no. of journals, etc.) has doubled about every 15 years since 1700.
• Most of history's scientists are alive now, most knowledge gained in
• living memory. Same has been true since about 1700.
• Approaching saturation of resources?

Scientific Support

Many early scientists were amateurs. Every science was founded by somebody not formally trained in it.

• Few opportunities for scientific employment
  • physicians
  • engineers
• Some scientists supported by occupations that allowed leisure.
  • clergy
  • government sinecures (jobs with no responsibilities)
  • independent wealth
  • royal patronage
• Present forms of support very recent in origin
  • corporate research
  • government grant

Return to Outline Index
Return to Exploration of the Universe Index
Return to Professor Dutch's Home Page

Created 20 May 1997
Last Update 24 January 1998

Not an official UW Green Bay site