History of Modern Astronomy

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Topics

Major characters in the development of our understanding of the motions of planets.

Kepler’s three laws of planetary motion

Newton’s three laws of motion and the law of gravitation

Slide 3

A little drama

Characters in the great drama

Claudius Ptolemy (140)

Nicolaus Copernicus (1473-1543)

Tycho Brahe (1546-1601)

Johannes Kepler (1571-1630)

Galileo Galilei (1564-1642)

Isaac Newton (1642-1727)

Slide 4

Aristotle’s shoes

It seems natural that our first hypothesis regarding the “structure” of the Solar System would be geocentric.

Being more philosophical and less empirical, we would hope to see harmony and perfection in the heavens that fit our philosophy--thus, the motions of bodies are perfect circles.

Slide 5

Then came Ptolemy

A good theory should explain what is observed and be able to make predictions.

Planets move in circles called epicycles.

The center of the epicycle moves in a circle called a deferent.

To make theory match prediction, Earth isn’t exactly at the center of the deferent.

The test of all knowledge is measurement

Ptolemy’s theory explained the retrograde motion of the planets

Predicted future locations of the planets

Slide 6

1400 years later - heliocentric idea

Copernicus, for philosophical reasons, sought to explain the retrograde motion of planets using a heliocentric solar system. (animation)

He still assumed perfect circles for the orbits of planets (with the Sun at the center of the orbits).

He could calculate

the relative distances to the planets

the orbital periods of the planets

Predictions of the future positions of planets were not much better than those from the Ptolemaic model

Slide 7

We need better data!

Tycho Brahe had ideas for a new model but recognized the need for more precise measurements.

He devoted his life to making more precise measurements of the positions of stars and planets.

He built the first modern observatory

He amassed records of planetary positions from 1576 to 1591

His observations were 2.5 times more accurate than any previous records

Slide 8

Finding a needle in a haystack