ASTRONOMY BEAT
Number 19 • March 23, 2009
Publisher: Astronomical Society of the Pacific
Editor: Andrew Fraknoi
Designer: Leslie Proudfit
www.astrosociety.org
© 2009, Astronomical Society of the Pacific
390 Ashton Avenue, San Francisco, CA 94112
Galileo: Myths Versus Facts
Jim Lattis
University of Wisconsin-Madison
Editor’s Introduction
ASTRONOMY BEAT
In 2009, we are celebrating the 400th anniversary
of Galileo turning the telescope to the heavens. The
International Year of Astronomy is in full swing, and
April 2–5 astronomers, educators, and amateurs will
be putting on 100 hours of astronomical webcasts,
open houses, and star parties. There are bound to be
many questions about Galileo, so we asked historian of
astronomy Jim Lattis to help sort out some of the key
issues about what Galileo did and did not do.
This February, by the way, the Royal Astronomical
Society in England released the results of a survey of
1002 British adults, who were queried about Galileo’s
work. In one question, they were asked whether they
related the name Galileo to “wine, fashion, astronomy,
a ship, or don’t know.” About 40% did not choose
astronomy. Then they were told “Galileo was an Italian
astronomer,” and were asked which of the following he
first observed with his telescope 400 years ago: “Neptune,
a black hole at the center of the Galaxy, 4 large satellites
of Jupiter, that the Sun will die out in less than 30,000
years, or don’t know?” Only 27% chose the satellites of
Jupiter. We clearly have our work cut out for us.
1. Did Galileo invent the telescope?
Painting of Galileo by Justus Sustermans from1636
the device in the summer of 1609 and was able to build
one. Galileo recognized that an improved telescope
could be more than a curiosity or toy and turned it
into an instrument of great military, commercial, and
scientific value. His improved telescopes led him to his
famous astronomical discoveries.
2. What were Galileo’s most important
astronomical discoveries?
Galileo began his astronomical observations in late
No. To the best of our knowledge, the telescope was
1609. He rushed the earlier ones into print by March
invented in the Netherlands by 1608 and was being sold
1610 in a little book written in Latin entitled Sidereus
across northern Europe by 1609. Although he had never
nuncius (i.e. Starry Messenger). His most important
actually seen one, Galileo read a rough description of
observations published there were:
Astronomy Beat No. 19 • March 23, 2009
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a) The Moon has a rugged landscape containing
mountains and valleys that look very much like
landscape features on Earth. This contradicted the
Aristotelian notion that all bodies beyond Earth are
made of a perfect celestial substance: The Moon
looked like it was made of the same stuff as Earth.
b) The light of the Milky Way comes from the
combined light of countless stars too dim to see with
the human eye, but revealed by the telescope. Other
parts of the sky also contain previously unknown stars
and nebulous objects that are revealed by the telescope.
c) Jupiter is orbited by four moons (Galileo called
them planets), previously unknown, which show
that Earth is not the sole center of motion in the
cosmos (as was maintained by the Aristotelians). This
made plausible the Copernican assertion that Earth
orbits the Sun, and is in turn orbited by the Moon.
After all, Jupiter’s moons faithfully continue to orbit
as their planet moves through space. In a political
masterstroke, Galileo named these new objects the
“Medicean planets” after the family of the Medici dukes
of Tuscany. This won him the position he coveted as
the duke’s mathematician and philosopher, free to
research and write without the burden of teaching.
d) Galileo did not mention sunspots in Starry
Messenger, but he must have found them soon after,
because by the following year he was demonstrating
them to others. He used the changing appearance of
sunspots to argue against the Aristotelian notion that
celestial bodies are perfect and changeless, and he
measured the Sun’s rate of rotation by tracking the
spots across the solar disk.
e) The phases of Venus also did not appear in Starry
Messenger. But by late 1610 Galileo began circulating
word of this discovery — that Venus changed its
appearance, going from more rounded to crescent
shape, in a way that could only be explained if it
was orbiting the Sun. This was the coup de grâce
for the Aristotelian-Ptolemaic cosmos because it
demonstrated irrefutably and “to the scandal of the
philosophers” (as Jesuit mathematician and Galileo
fan Gregory St. Vincent crowed) that Venus orbits the
Sun, not the Earth. While not proof of the Copernican
cosmos (because it also fit the intermediate theory
proposed by Tycho Brahe, for example), the phases
of Venus convinced even the most conservative
astronomers that a complete rethinking of planetary
Astronomy Beat No. 19 • March 23, 2009
Galileo’s copy of observations of the Jovian moons made by Roman Jesuit
astronomers in late 1610-early 1611. From Favaro’s Opere di Galileo.
theories was inevitable.
3. Is it true that philosophers refused to look
through Galileo’s telescope?
Yes. Among others, at least one of Galileo’s colleagues
at the University of Padua, the philosopher Cesare
Cremonini, refused to look through Galileo’s telescope
at the rugged lunar surface. But such cases were very
much the exception. Most people, and especially his
fellow astronomers, were actually eager to get a look
through a telescope, which was a rare opportunity in
those days. However, many observers were skeptical at
first that the sights revealed in the telescope were really
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5. Did Galileo blind himself looking at the Sun
through his telescope?
No. In fact, Galileo published his method for solar
observing, so we know how he did it, and it is a safe
method still used today. Rather than looking through the
eyepiece of his telescope, Galileo projected the sunlight
onto a screen placed a few feet behind his telescope’s
eyepiece and safely examined the projected image. Not
only is this safe (if done without putting the eye in the
path of the sunlight), but it allowed him to trace the
outlines of sunspots directly onto the paper. Galileo
would have been well aware of the perils of looking at
the Sun even without a telescope. Moreover, the power
of “burning” lenses to start fires was well known in his
time; so it is unlikely that he would have risked his vision
observing the Sun at the eyepiece. Like many people,
Galileo lost his vision in his later years, probably as a
direct result of aging, not foolhardy observing.
6. Did Galileo’s discovery of Jupiter’s moons/
sunspots/phases of Venus immediately convince
people about the truth of the heliocentric theory
of Copernicus?
Two telescopes assembled and used by Galileo
in the sky and not some kind of reflections, flaws, or
tricks in the instrument itself. As the knowledge of how
to make and use good telescopes spread, with Galileo’s
encouragement, this attitude quickly subsided.
4. Was Galileo the first to see Jupiter’s moons/
sunspots/lunar craters/phases of Venus?
Maybe not. As knowledge of the telescope spread,
Galileo was neither the first nor the only experimenter
who improved and used the new instrument. There
is some evidence that Thomas Harriot in England,
Simon Marius in Germany, and the Jesuit astronomers
working with Christoph Clavius in Rome, among
others, observed the Moon and Jupiter’s satellites
independently of and about the same time as Galileo’s
observations. Galileo, however, was the first into
print with these epochal discoveries and was the one
who realized and explained to the world what the
discoveries revealed about the cosmos.
Astronomy Beat No. 19 • March 23, 2009
No. Galileo tells us that even before his telescopic
discoveries he had already concluded that the
Copernican theory (of a heliocentric solar system in
which Earth is just one of the planets) was preferable
to the ancient Aristotelian-Ptolemaic system (with
the Earth stationary at the center of the cosmos
and everything else circling around it.) But in his
Copernican conviction, Galileo was very unusual. With
absolutely no evidence for its truth, the Copernican
theory was favored mostly by those who appreciated
its mathematical harmonies or its calculational
advantages. The vast majority of astronomers preferred
to stick with the ancient geocentric models, not only
owing to force of tradition, but also because they could
not imagine the Earth sailing through space with three
distinct motions, none of which we could feel at all, all
the while keeping the Moon in tow as well.
When Galileo showed that Jupiter has moons, he
showed that the Earth was not the only center of
motion. And the phases of Venus showed that without
any doubt Venus orbits not the Earth but the Sun.
These things were not evidence that the Copernican
system was true, because although consistent with the
Copernican theory, they could be accounted for with
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other theories (like that of Tycho Brahe). But they were
utterly irreconcilable with the Ptolemaic-Aristotelian
theory. Galileo’s discoveries thus overthrew what
had been the predominant cosmology since classical
antiquity, and forced consideration of the alternatives.
7. Did the Inquisition condemn Galileo because of
his telescope discoveries?
No. Galileo got into trouble with the Inquisition in
1633, decades after his telescopic discoveries. In fact,
his telescopic work had been confirmed and praised
by Church astronomers, among others, soon after
he published Starry Messenger in 1610. But in 1632,
Galileo published his Dialog on the Two Great World
Systems. In that book Galileo openly advocates the
truth of the Copernican system (or so the Inquisition
concluded), which he had earlier (in 1616) been
ordered to consider only hypothetically. Thus,
although there were many complicating factors, it was
Galileo’s defiance of the Church’s attempt to control
intellectual inquiry that began his troubles, not his
astronomical observations.
8. Did the Inquisition torture/burn/
excommunicate Galileo?
No. Torture, in those days, and for quite some time
to follow, was an accepted method for obtaining
confessions, not only by the Inquisition but by
Protestant thought police and secular authorities as
well. It would have come as no news to Galileo that the
Inquisition might use torture, but he was nevertheless
formally notified of this fact as part of his trial. In the
end, Galileo saw that he could not change the minds
of the pope and cardinals who judged him, so he
exercised the better part of valor: accused of heresy,
he accepted responsibility, acknowledged his errors,
and recanted his teachings. In return, the Inquisition
spared the aged scientist anything harsher than house
arrest. (This, however, was bad enough, because it was
a life sentence which forbade him to publish or teach.)
About the Author
Jim Lattis is Director of Space Place, which is
the outreach and public education facility of
the Astronomy Department of the University
of Wisconsin-Madison. Lattis is an experienced
Astronomy Beat No. 19 • March 23, 2009
astronomy educator, a Fellow of
the American Academy in Rome,
and a historian of astronomy
who specializes in the age of
Galileo. His published work
includes Between Copernicus and
Galileo (Chicago, 1994), which
examines the final stages of the
ancient cosmology shattered by Galileo’s discoveries.
Resources for Further Information:
You can read Galileo’s own words in an excellent
modern English translation, which includes a very
useful historical introduction: Galileo Galilei, Sidereus
Nuncius. Albert Van Helden, trans. 1989, University of
Chicago Press.
A very accessible introduction to Galileo, excellent for
all ages: Richard Panchyk, Galileo for Kids: His Life and
Ideas. 2005, Chicago Review Press.
On the cosmological significance of Galileo’s telescopic
discoveries, see James Lattis, Between Copernicus and
Galileo: Christoph Clavius and the Collapse of Ptolemaic
Cosmology. 1994, University of Chicago Press.
The ASP’s resource guide on Galileo’s life and work can
be found at:
http://www.astrosociety.org/education/resources/
galileo.html
For more about Space Place at the University of
Wisconsin, see:
http://spaceplace.wisc.edu F
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