Astronomy 161: An Introduction to Solar System Astronomy Prof. Richard Pogge, MTWThF 2:30

Lecture 16: "The Starry Messenger":
Galileo Galilei & the Telescope

Key Ideas:

Galileo Galilei was 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, and in many ways the first modern scientist

• Gifted mathematician
• Brilliant observer and experimenter

Preferred experimentation and measurement to philosophical rhetoric.

• Staunch anti-Aristotelian in many ways
• Often at odds with the scholarly establishment

The Telescope

Word 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

• Based on a description, and acquainted with lens making, he made his own.
• He solved many technical problems, and came up with a design that produced good-quality images with a magnifying power of 20x.

"...a most beautiful & delightful sight."

Later observations were published in letters, and in a longer work, The Assayer, in 1623.

Among these observations were:

• Craters and Mountains on the Moon
• Sunspots & Solar Rotation
• Moons of Jupiter
• Phases of Venus

Note: Galileo made other important observations, including that the Milky Way is made of myriads of faint stars, and and the odd shape of Saturn. The latter turned out to be Saturn's Rings which were just beyond the ability of Galileo's telescope to discern. They were correctly recognized as rings by the astronomer Hevelius in 1656. Neither of these observations concern us directly here.

Moon Craters & Mountains

• Moon was not a smooth, perfect sphere as taught by the Aristotle and Ptolemy.
• 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.

Spots on the Sun

• Bright surface of the Sun had dark spots that changed and moved across its surface.
• Discerned solar rotation over time.
• Sun was not a perfect body, and was in fact rotating.
• Contemporaries also reported them (they appear in Chinese records as early as 28 BC, but nobody in the west had access to this knowledge).

The Moons of Jupiter

• Jupiter appeared as a disk.
• 4 points of light that followed it and moved in orbits around it.
• Deduced that they were moons (We now call them the Galilean Satellites of Jupiter)

• There was a solar system in miniature around Jupiter for all to see!

The Phases of Venus

Click on the image to view full size (11Kb)

• Order of phases and change in diameter proved conclusively that Venus orbited the Sun.
• Ptolemaic theory predicted the opposite behavior.

• This itself did not prove the Copernican system (it was also consistent with the Tychonic system), but it was inconsistent with the prevailing Ptolemaic system.

The Impact of Galileo's Observations

• Skeptics claimed that the telescope was lying (and Galileo, too) and entrenched.
• 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 system.

Perhaps most importantly, with the telescope everyone could literally see it for themselves. This, more than all the profound philosophical speculations or obscure mathematical arguments, was behind the tremendous impact of Galileo's observations. These phenomena were observed facts of nature, and had to be confronted.

Galileo knew he could hold his own on the ground of scholarly confrontation. He was now by far the most famous scientist in Europe, and immensely influential. This position led him to miscalculate his own influence, and to badly misjudge the influence of his enemies.

Troubles with the Church.

• The Church officially declares that the heliocentric theory is "philosophically false and at least an erroneous belief."
• De Revolutionibus was 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.

1624:

• Galileo writes "A Dialogue on the Two Chief World Systems", ably defending the Copernican system.
• Seeks permission from Pope Urban VIII to publish it, but is rebuffed.

1632:

• Galileo's Dialogue is published in Florence, written in Italian (Tuscan), not Latin.
• It was an instant success & widely acclaimed.
• Galileo unfortunately played directly into the hand of his enemies.

The Trial of Galileo

• Galileo is summoned 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 (modern scholarship has shown that this document is a forgery, or at best trumped up).

Galileo faced two specific charges:

• Disobedience of Bellarmine's 1616 order.
• Misleading censors who published his book.

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

Despite this, in 1636 he finished "The Two New Sciences" describing his experiments in mechanics.

• Unable to get published due to his conviction, the manuscript was smuggled out of Italy and published in Protestant Leyden in 1638.

• This book helped lay the foundations of classical physics.

Eppur si muove (and still, it moves)

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.