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End Lecture 4

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Begin Lecture 5 (Jan 30. 2002) Galileo

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Galileo (Starry Messenger. Mar 12 1610)

• mountains on the moon • milky way consisted of stars • phases of venus • change in size of venus • 4 moons of Jupiter

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Galileo’s notebook showing motion of the 4 moons

A miniature solar system. A second center of revolution in the cosmos, contradicting the doctrine that only the center of the universe could be the center of revolution and that center was the earth!

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Critics • false images from lens • violated teachings of the

church that the Earth was at the center of the universe

He did not declare his belief in the Sun-centered Copernican until 1613 in a paper The Letters on Sun Spots. Another blow to Aristotelian cosmology for it showed that the Sun was not perfect. In 1616 the Inquisition warned Galileo to stop teaching that the Copernican system was true rather than as a hypothesis.

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So he taught it informally as a hypothesis but in the form of a ‘dialog’ giving both points of view. In 1623 Cardinal Barberini, a friend, was elected Pope [Urban VIII]. After discussions with the Pope he felt he had permission to write a ‘careful’ book. Dialogue on the Two Chief World Systems. With the approval of the church censors it was published in 1632.

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Dialogue concerning the two chief world-systems

(Aristotle, Ptolemy, Copernicus)

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This work, published in Florence in 1632, was Galileo's scientific and literary masterpiece. In it he discussed the relative merits of the Ptolemaic and Copernican systems by means of three interlocutors:

1. Filippo Salviati (a committed Florentine Copernican),

2. Giovanfrancesco Sagredo (an open-minded Venetian, initially neutral with respect to the theories) and the Aristotelian

3. Simplicio (a defender of the Ptolemaic theory).

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The author imagines that the three speakers discuss the heliocentric system over four days. The work was written in Italian in order to increase its circulation. Portrayed as impartial. it was a thinly disguised polemic favoring the Copernican cosmos. It was immediately condemned along with Galileo himself. The Pope was incensed because he saw himself as Simplicio. The Inquisition summoned Galileo to Rome and forced

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him to publicly recant his scientific beliefs. He was place under house arrest and denied the Church sacraments. The Dialogue was banned until 1835. Galileo himself was not ‘unbanned’.

Pope John Paul II in 1979 expressed the wish that the Pontifical Academy of Sciences conduct an in-depth study of the celebrated case. A commission of scholars was convened, and they presented their report to the Pope on October 31, 1992.

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Kepler(1571-1630)

Kepler ‘stole’ Brahe’s notebooks.

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Kepler used the data of Brahe to attempt to show[ 4 years] that the Copernicus theory was correct but in fact it disagreed also[ 8 arc minutes of error!]. Both Ptolemy and Copernicus failed to explain the observations! The planetary motions were neither uniform nor circular!!!

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Keller spent 16 years constructing a new model.

Three Laws.

1. Orbit of a planet is an ellipse with the Sun at one focus.

2. Equal areas in equal

times 3. Period P and semi-

major axis a 2 3P a∝

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No explanation. Newton, born in 1642 one day after the death of Galileo would explain all. End Lecture 5