Galileo Galilei
Astronomy enters the
modern age
The State of Things
• At the time of Copernicus’ death (21 years
before Galileo’s birth), the heliocentric
model was gaining some limited
acceptance among scholars for its
simplicity and elegance, but not with the
Church.
• The heliocentric model was no better than
the Ptolemaic geocentric model at
predicting the positions of the planets.
A Change in Thinking
• Within 100 years of Copernicus’ death, the
heliocentric model would be widely
accepted by both scholars and lay people
alike.
• Like many advancements in science, an
advancement in technology changed the
conventional wisdom.
Galileo’s Life
• Born 1564 in
Pisa, Italy
• Galileo goes to
Univ. of Pisa, but
doesn’t graduate
• 1597 he invents
a calculator!
Galileo’s Life
• 1589, conducts
Leaning Tower of
Pisa experiment.
• 1609, makes his
own telescope
• 1642, Galileo
dies.
What did he do for astronomy?
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Built his own 20 power telescope
Observed the moon
Saw that the sun has sunspots
Observed the stars
Discovered 4 largest moons of Jupiter
Observed that Venus has phases like
the moon
Yeah? But so what?
The Telescope
• Galileo’s
refracting
telescope (using
lenses) is still a
standard design
in use today.
• Galileo didn’t invent
the design though.
Moon Observations
• To Galileo,
the moon
looked like
this.
• These are
his
drawings.
Moon observations
• The Greeks
thought that the
heavenly bodies
were perfect
spheres.
• Galileo proved
they weren’t
perfect.
http://www.russellsastronomy.com/sky/moon.htm
Sunspots
• Galileo also
observed
sunspots.
http://solar.physics.montana.edu/canfield/ssu_images/sunspots.gif
Sunspots
• Many people think
Galileo went blind
from looking at the
sun. Really it was
just old age &
disease.
• Galileo’s sunspot
drawings at right.
http://physics.ship.edu/~mrc/pfs/110/inside_out/vu1/Galileo/Images/Astro/Sunspots/harriot_ss1.gif
How did Galileo find sunspots?
• He used a camera
obscura – just a
pinhole in a wall,
with the sunlight
shining on an
opposite wall.
• You can make one
from a piece of
cardboard.
How did that hurt the Greeks?
• The sun was the second most
important heavenly body, after the
earth.
• To find imperfections even on the sun
proved that their philosophy was
wrong. If that was wrong, what else
were they wrong about?
Galileo observed the stars
• Before the
telescope, a field of
stars looked like
this.
Galileo observed the stars
• But when he
viewed the stars
through his
telescope, the star
field looked more
like this.
• There were many
more stars!
Conclusion
• Because he could see many more stars
with his telescope, Galileo realized that
some of them must be too faint to see with
the naked eye because they were farther
away.
• The whole idea of the eggshell-thin
crystalline sphere of stars was worthless!
Jupiter has moons!
• Galileo discovered the 4 largest moons of
Jupiter. They’re named after him: the
Galilean moons.
Jupiter has moons!
• As Galileo watched over several nights, he
made drawings of the moons revolving
around Jupiter.
• If you watch at just the right time, you
might see the fastest-moving, Io, actually
crossing the face of Jupiter.
• But how does this shoot down the ancient
Greeks?
Io crossing the face of Jupiter
photojournal.jpl.nasa.gov
Next Conclusion
• The ancients insisted that everything
revolved around the earth, since the earth
was the most important celestial body.
• The Galilean moons proved that revolution
could be centered on another celestial
body.
• Earth’s privileged place in the cosmos was
gone!
The Phases of Venus
• Galileo observed that Venus showed
phases, just like the moon.
• These are his drawings.
Phases of Venus
• Aristotle’s model
predicted crescent
phases for Venus,
but since Venus
never went behind
the sun, it couldn’t
have a full phase!
• Galileo’s observation of a full phase
for Venus proved that Venus revolved
around the sun - it wasn’t pinned to
a line between the sun & earth.
Epicycle of Venus
(In the geocentric model, Venus could only
show crescent and new phases as seen from
the earth. In the heliocentric model, Venus
could also show a nearly-full phase.)
Let’s summarize
• The moon & sun proved that the heavenly
bodies weren’t perfect spheres.
• The stars proved that there weren’t thin
crystalline spheres.
• Jupiter’s moons proved that not all things
revolved around the earth.
• The phases of Venus proved that Venus
revolved around the sun, not earth.
Galileo didn’t stop here
• Galileo published
his findings in a
book called The
Starry Messenger.
• Most books at that
time were
published in Latin –
ordinary people
couldn’t read them.
Starry Messenger
• Galileo was smart enough to publish
his book in Italian - everyone could
read it!
• Galileo went into business publishing
his book & building telescopes for
people. He became fairly wealthy
from his business.
Galileo gets into trouble!
• Several years
later, Galileo
published a play
called “A
Dialogue on the
Two Chief World
Systems”
The Pope gets mad!
• Galileo’s “Dialogue on the 2 Chief
World Systems” contrasted the
geocentric model with the heliocentric
model.
• Galileo called the character who
upheld the geocentric model
“Simplicio” or “fool”
The Pope gets mad!
• Simplicio used goofy arguments in support
of the geocentric model. The character
was really an idiot.
• Problem was, the Pope was very much in
favor of the geocentric model. He felt like
Galileo was personally making fun of him!
• If Galileo was making fun of the Pope, he
was making fun of the whole Church!
• The Pope ordered Galileo to be tried
before the Inquisition.
• Galileo was convicted, labeled a heretic,
forced to recant his book, and placed
under house arrest until he died 25 years
later.
• The Catholic Church didn’t officially
overturn his conviction until 1992!
• Galileo also performed a series of
experiments where he rolled balls of
different masses down inclined planes.
http://ircamera.as.arizona.edu/NatSci102/NatSci102/lectures/galileo.htm
What else did Galileo contribute?
Inertia
• Galileo recognized that when the ball
reached the bottom of the plane, it
continued to move forward on the level
surface, even though no force was acting
on it.
• Galileo had recognized that the rolling
object possessed inertia.
Natural Motion Reversed
• Galileo reasoned that a force of attraction
had been applied to the rolling ball to
begin its movement. That the ball
continued to move on the level surface
was a natural or inertial motion.
• With one experiment, Galileo had reversed
the ancient notions of natural and forced
motion, and “discovered” inertia!
Inertia
• Today, we recognize inertia as the
tendency for a mass to continue doing
what it is doing:
– A moving mass will continue moving at the
same speed in the same direction (unless an
external force changes the mass’s motion.)
– A stationary mass will remain stationary.