Copernicus (1473-1543)

1. Not an observational but a theoretical astronomer. He accepts Ptolemy’s data.

2. Copernicus wants to:

• systematize astronomy by using only (linear) uniform circular motion, thus rejecting equants
• simplify astronomy by reducing retrograde motions and brightness variations to the motion of the earth.

Note that one of the reasons for doing this is his belief in the mathematical wisdom of the Author of things.

3. Main features of the system, which had more than one version, however:

• The system uses deferents and epicycles. It is quite complex for inferior planets (especially mercury), but simpler for superior planets (basically, one deferent eccentric and one epicycle).
• there is no common center to the deferent circles. For example, the center of the deferent of the Earth and those of other planets do not coincide; moreover, the deferent of the Moon has its center on the Earth.
• the Earth is not at the center of the universe; the Sun is.
• The centers of the planets' deferents is not the Sun. They are off by about 1/25 of the radius of the Earth’s revolution about the sun. So, since the Sun is at the center of the universe, the center of the deferents carrying the planets’ epicycles is not. Hence, although the universe is heliocentric, C.’s planetary model is only heliostatic, as the orbits of the planets do not have the Sun at their centers. To have our solar system, one has to wait for Kepler. All in all, C. was not a Copernican.
• the Earth-Sun distance is insignificant with respect to that between the Sun and the fixed stars.
• the motions of stars, Sun, and planets arise, at least in part, from the motion of the Earth. In particular, retardations, stations, regressions are optical illusions due to the relative motions of the planets and the Earth.

4. Evidence for the theory:

• there’s theoretical simplification because:
• the motions of the orbs is circular uniform (no equants). However, Ptolemaic theory can be done without equants.
• the distance of a planet from the Sun is proportional to the planet’s period of revolution, and this explains retrograde motion uniformly.
• there are some computational simplifications in lunar theory.

Problem: However, while in Ptolemy the apparent geocentric position of the planets is built into the theory, in Copernicus it has to be inferred by considering the position of both planets and Earth with respect to the Sun. Hence, at times the system is at least computationally as complex as Ptolemy's. Famously, Galileo himself noted the difficulty of understanding Copernicus.

• the idea of the sphere of the stars spinning once a day seems preposterous if the same data can be more easily explained by the rotation of the earth.

Problem: the Earth is heavy; so, what makes it rotate? By contrast, the sphere of the stars is made of ether, a weightless substance

• the phases of Venus (observable, however, only by telescope, which comes about only 60 years later). Note that they can be accounted for by Brahe's system as well.
• The paths of the planets are smooth, without nodes or cusps (good for mathematics)

5. Problems for the theory:

• absence of stellar parallax.

Answer: the universe is immense, although finite.

• since the earth is heavy, what prevents it from returning to its natural place, the center of the universe?

Answer: heaviness is just the tendency of every celestial body to form a whole with things of the same nature, e.g., Earth objects with other Earth objects. There is no cosmic gravity.

• why aren’t clouds, birds, arrows, etc. left behind by the earth’s rotation?

Answer: all terrestrial bodies share a natural rotational and orbital motion because, being terrestrial, they have the same nature as the Earth. Note that this is not a physical account. Copernicus lacks a proper mechanics.

· Inconsistent use of eccentricity, sometimes measured from the Sun, sometimes from the earth

• what makes the Earth rotate?

Answer: round objects placed in space rotate just because they are round.

6. The Sun is at the center of the universe because it is the giver of light, the visible symbol of God. Around it (more or less) are the deferents of the planets. The periods of revolution increase with the distance from the Sun,

7. The instrumentalism of Osiander's preface.