The History of Astronomy

Part 4The Debate Heats Up

Tycho and Kepler

At first Copernicus's Simple Heliocentric model met opposition from great astronomers such as Tycho Brahe .

But some agreed and went further with ridiculous proposals…

Like Thomas Diggs and Giordano Bruno who proposed that

Stars are other Suns and planets may orbit them!

Tycho Brahe (1546-1601)Tycho Brahe was a Danish Astronomer He made the most accurate observations ever made

without the aide of a telescope.He also:• Corrected Standard Astronomical Tables• Studied a Supernova in Cassiopeia (1572) and

determined that it must be far beyond the supposed “spheres” of the planets and stars That is: Stars are at different distances.

• Proposed his own Model of the Universe

Tycho Brahe’s ModelTycho rejected Copernicus for the old reason:

Where’s the Parallax?So

Tycho left the Earth Stationary and had the Moon & Sun Orbiting it.

But he had all the other planets orbiting the Sun!

With Venus and Mercury closer to the sun than Earth.

Johannes Kepler (1571-1630)Kepler was a supporter of the Copernican model of

the UniverseHe …• wrote a treatise (1596) in support• suggested the Sun emitted a force that pulled on

the planets.• In 1600 he became Tycho Brahe’s assistant and

took over when he mysteriously died in 1601• used Tycho Brahe’s data to chart Mars’ motion and

showed that it and Earth both orbit the Sun…… but in ellipses!

Kepler’s LawsKepler’s improved Copernican model had orbits that

obey three laws:1. The Planets orbit in ellipses with the sun at one

focus.2. The speed of the planet varies so that a line

joining the sun and Planet will sweep out an equal area in an equal time.That is when closer faster and farther slower.

3. The period of revolution is related to the length of the semimajor axis by:

P² = a³