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Kepler and Galileo

11 September 2003

Today:

• How Johannes Kepler found a much more accurate scheme for predicting planetary positions, based on new mathematical laws

• How Galileo, using the newly invented telescope, discovered startling facts about the sun, moon, stars, and planets

Tycho BraheDanish astronomer, 1546 - 1601

Tycho made highly accurate (within 1/60 degree) naked-eye observations of the positions of stars and planets over many years.

He proposed a compromise model in which all the other planets orbit the sun, but the sun orbits the earth.

Johannes KeplerGerman astronomer, 1571 - 1630

Kepler worked under Tycho during 1600 - 1601, then inherited the records of Tycho’s detailed observations.

He accepted the Copernican hypothesis, but abandoned the assumption that all motions are circular.

The Orbit of Marsbased on Tycho’s observations

Earth

Toward Mars

The Orbit of Mars

Earth

1.9 years later

The Orbit of Mars

The Orbit is an Ellipse!

Focus Focus

The sun is at one focus; there’s nothing at the other.

Planets go faster when they’re close to the sun

A line drawn from the sun to the planet sweeps out equal areas in equal times.

Outer planets move slower than inner planets.

Time (yr) Radius (AU)

Mercury 0.24 0.39

Venus 0.61 0.72

Earth 1.00 1.00

Mars 1.88 1.52

Jupiter 11.86 5.20

Saturn 29.46 9.54

Outer planets move slower than inner planets.

Time (yr) Radius (AU) (Time)2 (Radius)3

Mercury 0.24 0.39

Venus 0.61 0.72 0.37 0.37

Earth 1.00 1.00

Mars 1.88 1.52

Jupiter 11.86 5.20

Saturn 29.46 9.54

Outer planets move slower than inner planets.

Time (yr) Radius (AU) (Time)2 (Radius)3

Mercury 0.24 0.39 0.058 0.059

Venus 0.61 0.72 0.37 0.37

Earth 1.00 1.00 1.00 1.00

Mars 1.88 1.52 3.53 3.51

Jupiter 11.86 5.20 140.7 140.6

Saturn 29.46 9.54 867.9 868.3

(Time in years)2 = (Radius in AU)3

Kepler’s Laws

1. Orbits are ellipses, with sun at one focus.

2. Equal areas in equal times (faster when close to sun)

3. (Time in years)2 = (Radius in AU)3

Bottom line: The universe speaks math!

Galileo GalileiItalian scientist, 1564 - 1642

Made one of the first telescopes, and pointed it at the heavens . . .

Siderius Nuncius (The Starry Messenger)Venice, 1610

(Written in Italian, not Latin!)

Lunar topography . . . Sunspots . . .

Many new stars . . .

New “planets” orbiting Jupiter . . .

At least some planets don’t orbit the earth; and earth isn’t the only planet with a “moon”!

Phases of Venus . . .

Venus, at least, must orbit the sun, not the earth.

What happened to Galileo?

• He became famous (not just among astronomers)

• He started advocating Copernicanism (and ridiculing those who disagreed)

• Condemned by the Church, placed under house arrest

A final thought . . .

Copernicus was Polish, Tycho Danish, Kepler German, Galileo Italian, Newton English.

So what?

Science belongs to everyone.