72 Chapter 1 Is Earth the Center ofthe Universe?
Kepler Completed the Heliocentric Theory
The person responsible for a mainstream cosmological model that brought about the
Copernican Revolution in its true sense was the German astronomer Johannes Kepler
(1571–1630). Kepler showed that the motion of the planets followed a distorted circle
(or ellipse) and ultimately established three laws that came to be known as Kepler’s Laws.
These laws showed for the first time that the heliocentric theory was more logical, accurate,
clear-cut, and easy to understand than any of the previous theories.
First Law: The orbit of every planet is an ellipse with the Sun at one of its foci.
Second Law: A line joining a planet and the Sun sweeps out equal areas during equal
intervals of time.
Third Law: The square of the orbital period of a planet is directly proportional to the
cube of the semimajor axis of its orbit.
Kepler’s Laws are described in more detail starting on the next page.
Kepler was able to capture the motion of the planets, which was thought to be very
complex, in these simple laws because he had been an assistant to Tycho Brahe and was
able to use the massive amount of observational data left by his teacher. Tycho was an
extremely diligent scientist, and his observational records are said to be the most accurate
and precise of any collected before the telescope was invented. The refracting telescope
known as a Galilean telescope was invented just after Tycho died. If it had been invented
before his death, Tycho might have been one of the pioneers of the heliocentric theory.
What Did Galileo Do?
Generally, when it comes to the heliocentric theory, Galileo is more famous than Kepler.
But first, a quick note on Italian language and culture. Since Galileo’s full name is Gali-
leo Galilei, his surname is Galilei. Therefore, by all rights, we should call him Galilei. Why
then, do we generally call him Galileo? Also, why are his last name and first name so similar?
In the Tuscany region of Italy, where Galileo has born, the first name of the eldest son
was often a singularized form of his last name. So this naming convention carries a con-
notation like, “Mr. Sato, who represents the Satos.” Therefore, for people in Galileo’s time,
Galileo was equivalent to oldest son of the Galilei family.
Now, although Galileo was treated by the Inquisition as a representative of the helio-
centric theory faction, he made many blunders—such as believing that the planetary orbits
were circles, as Copernicus did. In any case, Galileo was a rather stubborn man who contin-
ued to insist that the planets did not move in elliptical orbits, even after Kepler’s Laws were
published (both men lived at almost the same time).
Nevertheless, there is no doubt that Galileo, who did research in the various fields of
medicine, mathematics, astronomy, and physics and who created the refracting telescope