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Galileo, Tycho, and Kepler
Galileo Galilei (1564-1642)
Galileo is considered the father of modern
physics, and even modern science. He
performed a variety of experiments, such as:
• Dropping balls to measure gravity
• Rolling balls to examine inertia
• Observing the sky through a telescope
What Galileo Saw
• An imperfect Sun (sunspots)
What Galileo Saw
• An imperfect Sun (sunspots)
• A Moon with mountains and craters
What Galileo Saw
• An imperfect Sun (sunspots)
• A Moon with mountains and craters
• The “ears of Saturn”
What Galileo Saw
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An imperfect Sun (sunspots)
A Moon with mountains and craters
The “ears of Saturn”
Four moons orbiting Jupiter
What Galileo Saw
•
•
•
•
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An imperfect Sun (sunspots)
A Moon with mountains and craters
The “ears of Saturn”
Four moons orbiting Jupiter
The phases of Venus
Tycho Brahe (1546-1601)
Tycho Brahe (without a telescope) made extremely
accurate measurements of the positions of the stars and
planets over the course of 20 years.
Tycho Brahe (1546-1601)
Tycho believed that the
other planets orbit the Sun,
and the Sun and those
planets together orbit the
Earth. He hoped that his
measurements of the
motions of the planets would
confirm this model of the
solar system.
Johannes Kepler (1571-1630)
Tycho hired a young
mathematician, Johannes Kepler,
to show that his measurements
of the planets’ motions could be
reproduced by his preferred
model for the solar system.
Kepler tried to do this, but failed.
After Tycho died, Kepler
continued to search for a model
that would match the observed
motions of the planets. He
eventually succeeded with a
variation of Copernicus’ model in
which all of the planets, including
Earth, orbit the Sun.
Kepler’s Laws
1) Planets move in ellipses with
the Sun at one focus
2) Planets sweep out equal areas
in equal times as they orbit
(as a result, planets farther
from the Sun move slower)
3) The period, P, of an orbit (in
years) and its radius, a (in units
of the Earth-Sun distance) are
related by P2 = a3
These laws perfectly predicted the positions of the planets,
but they were just math, and Kepler didn’t have a physical
understanding of why they were true.
http://astro.unl.edu/naap/pos/animations/kepler.html