Nicolaus Copernicus (1473-1543)

Topic 3: Development of Modern Astrophysics Reading: The Science of Astronomy, Chapter 3, Making Sense of the Universe: Understanding Motion, Energy, and Gravity, Chapter 4.2-4.4

I. Helio-centric vs. Geo-Centric Models?

The earliest attempts to construct scientific models were made by the Greeks. The models were usually Earth-centered, geo-centric. The Greeks attached physical significance to the Celstial Sphere construction. The Greeks not only used the notion that the Earth was sitting in the center of a large sphere to build a toy model with which to visualize the motions of objects in the Solar System, the Greeks also believed that this was how the Solar System (Universe) worked. The Greeks had scientific reasons for believing this idea, e.g., their inability to detect annual trigonometric parallax . The Greeks were, however, quite clever. They had many triumphs.

• Ptolemy (~ 140 A.D.) was not the first person to propose the geo-centric model, but the model did reach its pinnacle in his work which he described in his book, the Almagest. The geo-centric model worked well enough because the observational data was not accurate enough to show that the model was wrong!!

• Copernicus (1473-1543) championed the idea of a Sun-centered helio-centric Universe. He published his work on his deathbed in the book, de Revolutionibus Orbium Celestium. The model was not immediately embraced because it did not work any better than the geo-centric model and it had no basis in physics. (For much the same reasons, the similar model proposed by Aristarchus of Samos [310-230 B.C] was rejected by the Greeks).

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II. Resolution of Issue: Development of Modern Physics

After Copernicus, in terms of scientific models, the pace of development of Modern Astrophysics picks up. Tycho Brahe develops a large and accurate database (constrains the observable properties of the REAL WORLD), Johannes Kepler develops a geometric model which explains the data, and Galileo and Newton develop a theory for the physical foundations of the model of Kepler.

Data: Tycho Brahe (1546-1601)

The first great optical observer. His primary contributions to astronomy were the design and development of observing instruments and the precise data he collected on the motions of the celestial bodies. His data were usually accurate to within an arc minute or so (there are 60 arc minutes in 1^o). The data were, in fact, accurate enough to show that neither the Ptolemaic nor Copernican models were acceptable. Both models were not consistent with Tycho's data!!!

Geometric Model: Johannes Kepler (1571-1630)

Kepler worked with Tycho and with Tycho's data (after the death of Tycho). He developed the first accurate model for the motions of the planets, Kepler's Three Laws of Planetary Motion. As we discuss below, Kepler's laws are based on Newtonian physics and in fact are easily generalized so that, in suitable form, Kepler's laws of planetary motion apply to any system of objects bound together by their mutual gravitational attraction. For example, Kepler's laws apply to binary star systems, to the orbit of the Moon about the Earth, to the orbits of the moons of Jupiter about Jupiter, as well as to the newly discovered extra-Solar planetary systems.

Physical Foundation: Issac Newton (1643-1727)

Newton developed the physical theory of motion and postulated the existence and form for the Gravitational Force that laid the foundations for Kepler's Laws of Planetary Motion. Galileo began the studies which culminated in Newton's work, however, here I concentrate on Newton because he tied things together and essentially started modern physics. Newton published his work on mechanics and optics in the book, Philosophiae Naturalis Principia Mathematica . In the Principia, Newton states his three laws of motion and his Universal theory of Gravitation. Armed with these laws, Newton demonstrated why Kepler's laws governed the orbits of the Celestial bodies in the Solar System.