Galileo's Telescope

Kevin T. Kelly

Department of Philosophy

Carnegie Mellon University

1543: De Revolutionibus.

Steady acceptance of Copernicus.

1572: Tycho finds no parallax in a nova. Tycho used huge quadrants, so the accuracy was unprecedented.

1604: Galileo finds no parallax in nova.

1609: Kepler's Astronomia Nova: epicycles eliminated from Copernicus. Physical hypotheses.

Galileo changes the debate by means of telescopic observations. He heard about the telescope from Holland.

1000 X magnification.

Problems: spherical and chromatic aberrations.

Starry Messenger. Popular, vernacular tract.

1. Mountains and valleys on moon even larger than on Earth: not immutable and smooth. Earth would look the same: reddish glow in lunar eclipse is Earth-shine.
2. Milky way is made of stars. Stars look like points in telescope, placing them very far away. This corroborates the lack of parallax due to the Earth's motion.
3. Moons of Jupiter (Medician stars): a model of the solar system around another planet. Eliminated peculiar status of Earth's moon in Copernicus' system. Got him a job with Cosimo de Medici.

Other discoveries:

1. Phases of Venus: incompatible with epicycle theory. Also, illustrated another planet shining by reflected light.
2. Saturn has funny "ears"!
3. Sun has spots moving over its surface! Nice argument showing they are not planets.

Sensational impact on popular culture

1. John Milton already suggests other stars and worlds in the universe.
2. John Donne: the Earth is lost in immense space.