Astronomy 161
Introduction to Solar System Astronomy
Prof. Paul Martini

Lecture 14: Galileo

Key Ideas

Galileo Galilei: the first modern astronomer
Important discoveries with the telescope:: Moons of Jupiter; Phases of Venus; Craters and Mountains on the Moon; Sunspots
Confrontation with the Church

Galileo Galilei (1564-1642)

Italian contemporary of Kepler: Gifted mathematician; Brilliant observer and experimenter
Preferred experimentation and measurement to philosophical rhetoric: Staunch anti-Aristotelian; Often at odds with the scholarly establishment

The Telescope

The telescope was invented in 1608 by Hans Lipperhey, a Dutch spectacle maker
The invention spread fast:: In April 1609 you could buy one in eyeglass shops in Paris; First one reached Italy in August of 1609

Galileo's Telescope

Galileo learned about the telescope in 1609, but did not get a chance to see one: Based on a description, and acquainted with lens making, he made his own; Solved many technical problems, and came up with a design that produced good-quality images with a magnifying power of 20x
Soon pointed his telescope at the night sky...

"...a most beautiful and delightful sight"

1610: published his observations as the Sidereus Nuncius (The Starry Messenger): Later observations were published in letters and in a longer work, The Assayer, in 1623
The most important observations were:: Craters and Mountains on the Moon; Sunspots and Solar Rotation; Moons of Jupiter; Phases of Venus

Moon Craters and Mountains

Galileo saw craters and mountains on the Moon: The Moon was not a smooth, perfect sphere as taught by Aristotle and Ptolemy; The surface was "...rough and uneven, and just like the surface of the Earth itself..."; Galileo was able to measure the heights of lunar mountains using their shadows
The Moon was another world like the Earth

Spots on the Sun

Galileo observed sunspots: The bright surface of the Sun had dark spots that changed and moved across its surface; He discerned solar rotation (about 27 days)
The Sun was not a perfect body and it was rotating about its axis
If a huge object like the Sun is rotating, why not the Earth?

Moons of Jupiter

Galileo discovered 4 moons orbiting Jupiter: Jupiter appeared as a disk; 4 points of light follow it and move around it; Deduced that they were moons of Jupiter
Concluded that the Earth is not the only center of motion in the Universe

Phases of Venus

Venus goes though phases like the Moon: Sequence of phases and changes in diameter proved conclusively that Venus orbits the Sun; Ptolemaic theory predicts the opposite of what is observed with the telescope
The Sun was also a center of Motion: Didn't prove the Copernican system (it was also consistent with the Tychonic system)
Profoundly damaging to the Ptolemaic system

Galileo's drawing of the phases of Venus from The Assayer (1623)

Impact of Galileo's Observations

The impact was immediate and forceful:: Kepler was delighted, and soon got his own telescope, as did many others; Many scholars began to take the Copernican system seriously as a physical description; Hardcore skeptics claimed that the telescope was lying (and Galileo too) and entrenched
With the telescope, everyone could literally "see for themselves"

Troubles with the Church

1616:: The Church officially declared that the heliocentric theory is "philosophically false and at least an erroneous belief"; De Revolutionibus is officially banned
Galileo was called to an audience with Cardinal Robert Bellarmine, who cautioned him verbally to stop teaching and defending the Copernican model in public

Galileo's "Dialogue"

1624:: Galileo writes "A Dialogue on the Two Cheif World Systems," ably defending the Copernican System; Seeks permission from Pope Urban VIII to publish it, but is only allowed to discuss it as a mathematical proposition
1632:: Galileo's "Dialogue" is published in Florence, written in Italian (Tuscan), not Latin; It was an instant success and widely acclaimed

The Trial of Galileo

1633:: Galileo is summmoned by the Roman Inquisition and a document is produced alleging that Bellarmine in 1616 specifically forbade him to discuss the Copernican system in any way
Galileo faced two charges:: Disobedience of Bellarmine's 1616 order; Misleading the censors who granted permission to publish the Dialogue

Errors and Heresies

Enemies of Galileo convinced Pope Urban VIII that a character in the Dialogue named Simplicio (who ineptly defended Ptolemy) was a thinly veiled caricature of the Pope
Publicly humiliated and threatened with torture, Galileo had no choice but to admit guilt, and "abjure, curse, and detest the aforesaid errors and heresies..."

House Arrest

Galileo was placed under house arrest at his villa in Arcetri near Florence until his death in 1642: Despite this, in 1636 he finished "The Two New Sciences" describing his earlier experiments in mechanics; They were published in Protestant Leyden in 1638
Laid the foundations of classical physics

Eppur si muove (and still, it moves)

Galileo spent his final 4 years in blindness, and died under house arrest, on January 8, 1642
On Christmas Day of that same year, Isaac Newton was born in Woolsthorpe, England
In 1992, 350 years later, Pope John Paul II officially declared Galileo innocent

See A Note about Graphics to learn why some of the graphics shown in the lectures are not reproduced with these notes.