The Solar System

CHAPTER 4 | THE ORIGIN OF MODERN ASTRONOMY 67

Much later, Kepler derived his three laws of motion from the

observations made by Tycho Brahe during 20 years on Hveen.

The observations were the evidence, and they gave Kepler a real-

ity check each time he tried a new calculation. He chose ellipses

because they fi t the data and not because he thought ellipses had

any special signifi cance.

The Copernican model was a poor predictor of planetary motion,

but The Rudolphine Tables was much more accurate. What first

principle did Copernicus follow that was abandoned when

Kepler looked at the evidence?

Galileo Galilei

Most people think they know two facts about Galileo, but
both facts are wrong; they are Common Misconceptions,
so you have probably heard them. Galileo did not invent the

4-4

telescope, and he was not condemned by the Inquisition for

believing that Earth moved around the sun. Th en why is

Galileo so famous? Why did the Vatican reopen his case in

1979, almost 400 years after his trial? As you learn about

Galileo, you will discover that his trial concerned not just the

place of Earth and the motion of the planets but also a new

and powerful method of understanding nature, a method

called science.

Telescopic Observations

Galileo Galilei (■ Figure 4-16) was born in 1564 in Pisa, a city

in what is now Italy, and he studied medicine at the university

there. His true love, however, was mathematics, and, although he

had to leave school early for fi nancial reasons, he returned only

four years later as a professor of mathematics. Th ree years after

that he became professor of mathematics at the university at

Padua, where he remained for 18 years.

■ Figure 4-15

Kepler’s three laws: The fi rst law says the orbits of the planets are ellipses.
The orbits, however, are nearly circular. In this scale drawing of the orbit of
Mercury, it looks nearly circular. The second law is demonstrated by a planet
that moves from A to B in 1 month and from A’ to B’ in the same amount
of time. The two blue segments have the same area. The third law shows
that the orbital periods of the planets are related to their distance from
the sun.

A

B

Sun

Sun

Orbit of

Mercury

Law I Circle

Law III

B′

A′

100

0

200

P (yr)

Law II

020

a (Au)