Tycho Brahe and Johannes Kepler

Tycho Brahe and Johannes Kepler

1.

Tycho Brahe (1546-1601, Denmark)

Builds improved instruments, becomes greatest pre-telescopic observational astronomer
Smashes "celestial sphere" idea
1. 1558: Partial solar eclipse
2. 1572: Supernova in Cassiopeia
  - No measured parallax $\Rightarrow$ well beyond the moon in Aristotle's celestial realm
  - Heavenly change, contradicts Aristotelian cosmos
3. 1577: Comet
  - Limits on parallax $\Rightarrow$ at least 6 $\times$ farther than Moon
  - No mere "atmospheric phenomenon"
Tycho builds grandiose observatory Uraniborg, ``Castle of the Heavens''
Weak theoretician
Joins Kepler to help interpret data

2.

Johannes Kepler (1571-1630, Germany)

Excellent mathetician/theoretician
First outspoken defense of Copernicus (50 yrs after death!)
Carried out the analysis of Tycho's data and developed 3 empirical laws which described the behavior of the planets in their orbits.
1. Planetary orbits are ellipses with Sun at one focus
2. Orbits sweep out equal areas in equal times
3. Distance to the Sun (a) related to Period (P) about the Sun: $P^2 \propto a^3$
Smashes the dogma of uniform circular motion
No more epicycles
Searches for but does not discover the physical causes of planetary motion (i.e., gravity)
- Kepler thinks the planets are held in their orbits by "magnetic" force from Sun which "diminishes in ratio to distance as does the force of light"
- Kepler's cosmos and the Holy Trinity: Father--Sun at center, Son--Sphere of stars at circumference, Holy Ghost--force from Sun in intervening space