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?

• 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 Jupiterphotojournal.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 onlyshow crescent and new phases as seen fromthe 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!

What else did Galileo contribute?

• Galileo also performed a series of experiments where he rolled balls of different masses down inclined planes.

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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.