Galileo Galilei (1564-1642, Italy)

Major contributions

• Galileo was a Contemporary of Kepler.
• Worked on the physics of motion.
• Founder of scientific method.
• Used experiment, rather than speculation, to study nature.
• Using the newly-invented telescope, made critical observations which demonstrated once and for all the correctness of the heliocentric model.
• After Galileo's death, Isaac Newton was able to deduce the physical laws which explain why the planets move as they do.

   

Observations with a telescope

• Galileo was the first to use the telescope to study the heavens.
• Galileo built his first telescope in 1609
• Galileo did not invent the telescope (first one constructed in Netherlands in 1608 by Hans Lippershey)
• Galileo discovered that:
  • Jupiter has 4 moons
  • Venus has phases (like the Moon)
  • Moon surface is rough and irregular
  • Sun has sunspots

Galileo's discoveries contradicted ancient cosmology

• Celestial objects, like the Sun and Moon, are not necessarily perfect.
• Earth is not the only center.
• Venus goes through a full set of phases, just like our Moon: full, gibbous, quarter, crescent.
• Size of Venus changes with the phases.
• Geocentric model predicts that Venus will always appear in a crescent phase, which is not borne out by the observations.

Galileo and the Church

• In 1616, the Church banned Galileo from defending the Copernican View.
• Galileo published the “Dialogue on the Two Chief Systems of the World, the Ptolemaic and the Copernican” as a discussion of the pros and cons.
• The book clearly supported the Copernican ideas through characters (Salvati -- a Copernican , Simplicio -- an Aristotelian, Sagredo -- supposedly neutral)
• Roman Inquisition forced Galileo to plead guilty to holding false doctrines.
• In 1633, Galileo received a life sentence at age 70 (house arrest). He died 7 years later, in 1642.
• In 1992, 359 years after the Galileo trial and 340 years after his death, Pope John Paul II established a commission that ultimately issued an apology, lifting the edict of Inquisition against Galileo.

Problems with Galileo’s Work in the Opinion of his Contemporaries

• Absence of observation of parallax (did not come until 1729).
• Explanation of the tides (came with Newton).
• Absence of proof that the Earth spins on its axis (did not come until 1851).

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