CHAPTER 4 | THE ORIGIN OF MODERN ASTRONOMY 69
and others looked and claimed to see nothing (hardly surprising,
given the awkwardness of those fi rst telescopes) (■ Figure 4-19).
Pope Paul V decided to end the disruption, so when Galileo
visited Rome in 1616 Cardinal Bellarmine interviewed him pri-
vately and ordered him to cease debate. Th ere is some contro-
versy today about the nature of Galileo’s instructions, but he did
not pursue astronomy for some years after the interview. Books(See Figure 4-10b.) Th e similarity isn’t proof, but Galileo saw it
as an argument that the solar system could be sun centered rather
than Earth centered.
In the years following publication of Sidereus Nuncius,
Galileo made two additional discoveries. When he observed the
sun, he discovered sunspots, raising the suspicion that the sun
was less than perfect. Further, by noting the movement of the
spots, he concluded that the sun was a sphere and that it rotated
on its axis.
His most dramatic discovery came when he observed Venus.
Galileo saw that it was going through phases like those of the
moon. In the Ptolemaic model, Venus moves around an epicycle
centered on a line between Earth and the sun. Th at means it
would always be seen as a crescent (■ Figure 4-18a). But Galileo
saw Venus go through a complete set of phases, which proved
that it did indeed revolve around the sun (Figure 4-18b). Th ere
is no way the Ptolemaic model could produce those phases. Th is
was the strongest evidence that came from Galileo’s telescope;
but, when controversy erupted, it focused more on the perfection
of the sun and moon and the motion of the satellites of Jupiter.
Sidereus Nuncius was very popular and made Galileo famous.
He became chief mathematician and philosopher to the Grand
Duke of Tuscany in Florence. In 1611, Galileo visited Rome and
was treated with great respect. He had long, friendly discussions
with the powerful Cardinal Barberini, but he also made enemies.
Personally, Galileo was outspoken, forceful, and sometimes tact-
less. He enjoyed debate, but most of all he enjoyed being right.
In lectures, debates, and letters he off ended important people
who questioned his telescopic discoveries.
By 1616, Galileo was the center of a storm of controversy.
Some critics said he was wrong, and others said he was lying.
Some refused to look through a telescope lest it mislead them,
■ Figure 4-18
(a) If Venus moved in an epicycle centered on the Earth–sun line (see page 57), it would always appear as a crescent.
(b) Galileo’s telescope showed that Venus goes through a full set of phases, proving that it must orbit the sun.abEarthSunVenusPtolemaic universeCenter of
epicycleEarthSunVenusCopernican universe■ Figure 4-19
Galileo’s telescope made him famous, and he demonstrated his telescope and
discussed his observations with powerful people. Some thought the tele-
scope was the work of the devil and would deceive anyone who looked. In
any case, Galileo’s discoveries produced intense and, in some cases, angry
debate. (Yerkes Observatory)