The Solar System

68 PART 1^ |^ EXPLORING THE SKY

showed that Jupiter, which everyone agreed

was moving, was able to keep its satellites.

Th at suggested that Earth, too, could move

and keep its moon. Aristotle’s philosophy

also included the belief that all heavenly

motion was centered on Earth. Galileo’s

observations showed that Jupiter’s moons

revolve around Jupiter, suggesting that

there could be other centers of motion

besides Earth.

Some time after Sidereus Nuncius was

published, Galileo noticed something else

that made Jupiter’s moons even stronger

evidence for the Copernican model. When

he measured the orbital periods of the four

moons, he found that the innermost moon

had the shortest period and that the moons farther from Jupiter

had proportionally longer periods. Jupiter’s moons made up a

harmonious system ruled by Jupiter, just as the planets in the

Copernican universe were a harmonious system ruled by the sun.

During this time, Galileo seems to have adopted the
Copernican model, although he admitted in a 1597 letter to
Kepler that he did not support Copernicanism publicly. At that
time, the Copernican hypothesis was not offi cially considered
heretical, but it was hotly debated among astronomers, and
Galileo, living in a region controlled by the Church, cautiously
avoided trouble. It was the telescope that fi nally drove Galileo to
publicly defend the heliocentric model.
Th e telescope was apparently invented around 1608 by lens
makers in Holland. Galileo, hearing descriptions in the fall of
1609, was able to build telescopes in his workshop. In fact,
Galileo was not the fi rst person to look at the sky through a
telescope, but he was the fi rst person to apply telescopic observa-
tions to the theoretical problem of the day—the place of Earth.
What Galileo saw through his telescopes was so amazing
that he rushed a small book into print. Sidereus Nuncius (Th e
Sidereal Messenger) reported three major discoveries. First, the
moon was not perfect. It had mountains and valleys on its sur-
face, and Galileo even used some of the mountains’ shadows to
calculate their height. Aristotle’s philosophy held that the moon
was perfect, but Galileo showed that it was not only imperfect
but was a world with features like Earth’s.
Th e second discovery reported in the book was that the
Milky Way was made up of myriad stars too faint to see with the
unaided eye. While intriguing, this could not match Galileo’s
third discovery. Galileo’s telescope revealed four new “planets”
circling Jupiter, objects known today as the Galilean moons of
Jupiter (■ Figure 4-17).
Th e moons of Jupiter were strong evidence for the Copernican
model. Critics of Copernicus had said Earth could not move
because the moon would be left behind, but Galileo’s discovery

■ Figure 4-16

Galileo, remembered as the great defender of Copernicanism, also made
important discoveries in the physics of motion. He was honored here on an
Italian 2000-lira note.

■ Figure 4-1 7

(a) On the night of January 7, 1610, Galileo saw three small “stars” near the

bright disk of Jupiter and sketched them in his notebook. On subsequent

nights (excepting January 9, which was cloudy), he saw that the stars were

actually four moons orbiting Jupiter. (b) This photo taken through a mod-

ern telescope shows the overexposed disk of Jupiter and three of the four

Galilean moons. (Grundy Observatory)

a

b

Jan. 7, 1610

Jan. 8, 1610

Jan. 9, 1610

Jan. 10, 1610

Jan. 11, 1610

Jan. 12, 1610

Jan. 13, 1610