Galileo Galilei (1564 – 1642)

Highlights of his life:

• (1564 – 1589) Born and educated in Piza, Italy
• (1589) Becomes Professor of mathematics at the University of Piza. Here he begins investigations into the properties of gravity.
• (1592) Becomes professor of mathematics at the University of Padua (near Venice). Here he became a convert to the Copernican solar system and began a correspondence with Johannes Kepler.
• (1609) Becomes aware of the telescope invented by Hans Lipperhey in Holland and begins to make his own. After some experimentation he increases the magnification of the telescope to about 20 X and begins astronomical observations. He shows the moon and Jupiter to skeptical philosophers at Padua. Some of them insist that the images seen in the telescope are due to imperfections in the instrument.
• (1610) Publishes Sidereus Nuncius (The Starry Messenger). Here he describes his observations of the surface of the moon and the moons of Jupiter. The book becomes a sensation and others begin making their own telescopes.
• (1613) Publishes Letters on Sunspots where, in addition to sunspots, he describes the phases of Venus. The phases observed are explainable only if Venus revolves around the sun. The accumulation of these observations lead Galileo to assert that the Copernican system is true physically, not just a neat way of calculating planetary positions. This begins to be controversial within some circles of academe and the church.
• (1615) Cardinal Robert Bellarmine urges Galileo to describe the Copernican system as a hypothesis only.
• (1616) Copernicus’ book is banned by the Holy Office. Galileo is again warned not to defend Copernicus as fact.
• (1632) Galileo publishes Dialogue on the Two Chief World Systems where three philosophers discuss the merits of Ptolemy and Copernicus. Although ostensibly equal weight is given to both sides, it is clear that the book favors Copernicus. Pope Urban VIII considers this a direct affront to the church’s prohibition and orders Galileo to Rome.
• (1633) Galileo put on trial and his Dialogue banned. He is put under house arrest in Florence for the rest of his life and forbidden to publish anything about the solar system ever again.
• (1642) Galileo dies.

Galileo’s Greatest Hits

1. The surface of the moon
1. A rough surface, having the appearance of a landscape seen from above.
2. Demonstrated that the moon is a “world” like the earth
3. Contradicted the Aristotelian assumption that objects in the heavens were perfect spheres
2. Spots on the sun
1. Demonstrated that the sun was rotating
2. Again, contradicted Aristotle’s idea that the sun was an unblemished sphere
3. The Milky Way resolved into stars
1. Implied that that the universe was three dimensional, since the new stars discovered by the telescope must be more distant than the stars bright enough to be seen with the naked eye.
2. This implied that the crystalline spheres of Aristotle did not exist
3. Gave weight to the idea that the reason that no parallax is seen is because the stars are much farther away than hitherto believed.
4. Planets were disks in the telescope
1. The planets looked something like the moon in a telescope.
2. Since the moon in the telescope appeared to be a world like the earth, this implied that the planets were also worlds.
5. Saturn appeared to be oblong or to have “ears”. This was something Galileo could not explain, foreshadowing discoveries to come of objects in the heavens hitherto unknown.
6. The phases of Venus
1. Venus went through a full set of phases from full to new, just like the moon.
2. This implied that Venus is orbiting the sun and is, thus, is sometimes between the sun and the earth and sometimes on the opposite of the sun from the earth. In Ptolemy’s system Venus was always between the earth and the sun and could never show the full phase.
7. The moons of Jupiter
1. Jupiter was revealed to have four “stars” which revolved around the planet in a systematic manner. Galileo called them satellites, a term that originally referred to patrons of aristocrats.
2. The fact that anything was moving around an object other than the earth directly contradicted Aristotle’s physics.
3. Here was a solar system in miniature, just as described by Copernicus and Kepler.