LECTURE 7: GALILEO
- What did Galileo discover by using a telescope to observe the
- How did these discoveries undermine the geocentric model of the
universe and support the heliocentric model?
- What did Galileo discover about inertia and about the relation
between force, motion, and acceleration?
- How did these discoveries in physics support the Copernican
model and pave the way for Newton's theory of motion and gravity?
GALILEO GALILEI (1564-1642)
- Professor at Padua (Venice), then Florence
- Early work on mechanics (physics of motion) and mathematics
- 1609: Builds telescope, uses to study sky.
- 1610: Publishes The Starry Messenger,
astronomical discoveries with telescope
- 1616: Catholic Church bans Copernicus, declares heretical.
- 1623: Publishes The Assayer,
more astronomical discoveries with telescope.
- 1624: Publishes
A Dialogue on the Two Chief World Systems, Copernican and
Ptolemaic, a popularization and defense of Copernican theory
- 1633: Tried by Inquisition, threatened with torture, recants.
Under house arrest for rest of his life.
- 1642: Publishes Two New Sciences, presenting his
theories and experiments in mechanics.
A telescope does two things:
- Invented by Dutch spectacle maker.
- Galileo hears description, makes his own.
- First to use it for astronomy and publish results.
Modern astronomical applications emphasize first capability.
- Collects more light than the eye, since area of lens is
bigger than area of pupil. Allows one to see fainter objects.
- Magnifies images increasing the apparent size of objects
or angular separations between objects. Allows one to resolve objects that
can't be resolved with naked eye.
Both capabilities important to Galileo, especially second.
GALILEO'S DISCOVERIES WITH TELESCOPE
- Moon has craters and mountains!
Height of mountains (measured from
shadows) similar to Earth.
- Sun has spots! Rotates!
- Venus has phases, like the Moon!
Phases correspond to predictions of
Copernican model, inconsistent with Ptolemaic model.
- Jupiter has its own moons, which orbit around it!
- Milky Way is composed of many faint stars!
IMPLICATIONS OF GALILEO'S DISCOVERIES
- Sunspots, lunar mountains and craters: Celestial bodies not perfect,
more like Earth.
- Sun's rotation: If Sun can rotate on axis, why not the Earth?
- Phases of Venus: Sun must be center of Venus's orbit. Why not Earth's?
- Moons of Jupiter: Another center of rotation in solar system.
Jupiter has something in common with Earth.
- Jupiter moon system like miniature Copernican solar system.
Kepler shows it obeys P2 ~ a 3 law.
- Milky Way stars: enormous range of brightness suggests stars at
Universe probably big.
What do these results imply about validity of Copernican and Ptolemaic systems?
- Only Venus phases directly contradict Ptolemaic model predictions.
- Only Venus phases confirm a prediction of Copernican model.
- But discoveries completely undermine the philosophical basis of the
- All seem reasonable in Copernican system.
- Result: strongly favor Copernican model over Ptolemaic.
- Direct, simple, accessible arguments, no complex math like Copernicus
- Opponents mostly say telescope is deceptive and/or Galileo is lying.
- Church bans Copernicus only after he is convincingly proved correct.
GALILEO'S CONTRIBUTIONS TO PHYSICS
Scorns authority (e.g., Aristotle).
Bases conclusions on thought experiments and real experiments.
Relativity and inertia:
- Motions are relative, not absolute.
- Object on the mast of a moving ship is not moving relative to ship.
- If object is dropped, it will hit at bottom of mast, not further
back on deck.
- Principle of inertia: "[For some forms of motion, a body]
will maintain itself in that state in which it has once been placed:
that is, if placed in a state of rest, it will conserve that; and if placed
in movement toward the west (for example) it will maintain that movement."
Force and acceleration:
- Force not required to keep an object moving at constant velocity.
- Instead, force produces acceleration, a change in
- Dropped objects accelerate towards Earth.
- Dropped objects of different mass accelerate at the same rate.
- Calculates distance traveled at constant acceleration:
distance = (1/2) x (acceleration) x (time)2
- Inertia/relativity answers the Aristotelian objection to Earth's
- Major steps towards Newton's laws of motion.
- Minor step towards Newton's law of gravity.
- Other ideas still Aristotelian/medieval.
- In particular, still thinks objects in heavens move in circles.
- Unlike Kepler, doesn't apply ideas about terrestrial physics to
explaining planetary motions.
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Updated: 2005 April 9[dhw]