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

Key Ideas:

Galileo Galilei was the first modern astronomer.

Important Discoveries with the telescope:

Confrontation with the Church

Galileo Galilei (1564-1642)

Galileo Galilei

Italian Contemporary of Kepler, and in many ways the first modern scientist

Preferred experimentation and measurement to philosophical rhetoric.

The Telescope

The telescope was invented in 1608 by Dutch spectacle makers. Tradition says this was Hans Lipperhey, but no one really knows who was the first.

Word spread fast:

Galileo's Telescope

Galileo learned about the telescope in 1609, but did not get a chance to see one.

He soon turned his telescope on the night sky.

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

In 1610, Galileo published his telescopic observations in the Sidereus Nucius (The Starry Messenger).

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

Among these observations were:

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

Galileo found craters and mountains on the Moon:

Conclusion: The Moon was another world like the Earth.

Spots on the Sun

Galileo observed sunspots:


The Moons of Jupiter

Galileo discovered 4 moons orbiting Jupiter

Conclusion: The Earth is not the only center of motion in the Universe.

The Phases of Venus

Venus goes through phases like the Moon

Schematic of Venus Phases
Click on the image to view full size (11Kb)

Conclusion: The Sun was also a center of motion.

The Impact of Galileo's Observations

The impact was immediate and forceful:

The discoveries brought Galileo immediate fame throughout Europe.

Perhaps most importantly, with the telescope everyone could literally "see 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 Trial of Galileo


Galileo faced two specific charges:

What was really going on in the background was that enemies of Galileo convinced Pope Urban VIII that a character in the Dialogue named Simplicio who ineptly defended the Ptolemaic system was a thinly veiled caricature of the Pope himself. This provided a pretext for making an example of Galileo, albeit on trumped up charges. Galileo was his own worst enemy in this situation, as he vastly overestimated his influence in Rome, and the degree to which his well-deserved fame would protect him.

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 experiments in mechanics.

Eppur si muove (and still, it moves)

Galileo died, blind and 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.

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Updated: 2006 October 4
Copyright Richard W. Pogge, All Rights Reserved.