THEORIES OF THE WORLD:  1543-1687

 

HELIOCENTRIC

GEOCENTRIC

Copernicus (1543)

 

1.        Natural explanation of retrograde motion.

2.        Natural explanation of why Mercury and Venus stay close to the Sun.

3.        Straightforward way to obtain relative size of the solar system.

4.        Predictions of planetary positions no better than Ptolemy, but are just as good and somewhat easier to calculate

Objections to Copernicus

 

1.        Evidence of our senses indicates that the Earth does not move.

2.        No parallax shift seen when Earth moves from one side of its orbit to the other

3.        Position predictions no better than Ptolemy.

4.        Heliocentric view implies that planets are “worlds”, and this cannot be verified.

5.        Does not fit current (Aristotle’s) physical theory:

a.        Objects fall to Earth because Earth is at the center of the universe.

b.        Planets move because they are connected to interlocked crystalline spheres

  1. What keeps the planets moving?

Kepler (1619)

 

1.               Use of elliptical orbits make predictions much more accurate than Ptolemy’s.

2.               Three laws of planetary motion agree with observation

Objections to Kepler

 

Okay, the predictions are much better but:

1.               Still no sensual evidence that the Earth is moving.

2.               Still no parallax shift

3.               Still no evidence that the planets are worlds

4.               Still contradicts known physics

 

Galileo (1632)

 

Observations with the telescope demonstrate:

1.               The Moon is a world

2.               Jupiter is a sphere and has satellites (at least some celestial objects are not moving around the Earth, a.k.a. the center of the universe)

3.               Phases of Venus show that at least Venus must be orbiting the Sun (only way to get a full Venus).

4.               The existence of stars not visible to the naked eye indicates the universe is very large, as required to explain lack of parallax.

 

Objections to Galileo

 

1.        Observations are not straight-forward – spherical and chromatic aberrations in telescope distort images, leaving them open to interpretation.

 

If we accept the observations, the planets (at least the Moon) are worlds, Jupiter is a miniature solar system, at least Venus is orbiting the Sun, but:

 

2.               Still no evidence that the Earth is moving

3.               Still contradicts “known” physics

 

Newton (1687)

1.               Proposed universal gravity, a property of all matter (predicts that objects will be attracted to all large masses like the Sun, Moon and planets, not just Earth).

2.               Showed that an inverse square law for gravity will automatically produce elliptical orbits.

3.               Gravity plus three laws of motion replaces physics of Aristotle (i.e., explained why planets keep moving)

 

Objections to Newton

 

Okay, everything seems to work, but:

1.               Physics is very strange - implies gravity is a mysterious “force field” acting across a void.

2.               Still no experimental evidence that the Earth is moving.