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Ptolemy – To – Copernicus – To – Kepler
A Revolution in Our World View
Seeds of Change:
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For 1500 years, the geocentric model was accepted as the “true” nature of
the cosmos.
In 1543 Nicholas Copernicus (1473-1543) proposed a heliocentric model
for the Universe — in which Earth and the other planets orbited the Sun.
The Copernican Revolution involved more than just a model of planetary
motion; conceptually it replaced the Earth as the immobile center of the
Universe. As such it conflicted with religious doctrine of the day.
The heliocentric model was simple and elegant, replacing the complex
geocentric model, with its multiple epicycles.
Copernicus did not promote his model, and his ideas were considered little
more than philosophical concepts until observational evidence supporting
the heliocentric model was discovered, more than 70 years after his death.
During the time that Copernicus ideas “gestated”, the Danish astronomer
Tycho Brahe provided decades of detailed positional and brightness data on
the planets. Brahe was the last great pre-telescopic astronomer, and worked
to improve the geocentric model.
The heliocentric model was finally adopted in the first half of the 17th century,
amidst the turmoil of the Renaissance, when new observational and
theoretical information became known.
Galileo’s New Worlds:
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Galileo Galilei provided this new information by “inventing” the telescope in
1610. A technological advance opened up new worlds to humanity.
What did Galileo see ??
The telescope demonstrated the the Universe was far more complex than had
been known previously.
 Venus exhibits phases, as predicted in the heliocentric model, counter to the
predictions of the geocentric model.
 Jupiter had 4 smaller moons orbiting around the planet, clearly showing that
not all celestial objects orbit Earth.
 The Moon showed structures on its surface: mountains and craters. The
Moon was a world, perhaps not like Earth, but neverless, a place, which
might be visited.
 The Sun displayed “spots” – it was not the perfect orb of antiquity.
 The Milky Way dissolved into millions of previously unknown stars.
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Galileo became notorious for his views but persisted in promoting the Suncentered universe. He was brought before the church as a heretic and kept
under house arrest for the last part of his life.
Kepler and his “laws”:
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The German astronomer Johannes Kepler served as Tycho Brahe’s last
assistant, and he tackled the problem of predicting planetary positions.
In the early 1600s, after some 20 years of work, Kepler synthesized Brahe’s
observational data into three “laws” of planetary motion:
1. The orbits of the planets are ellipses, NOT circles.
2. Equal areas within an orbit are swept out in equal intervals of time.
3. The square of a planet’s orbital period is related to the cube of its semimajor axis (the radius of the orbit).
Kepler’s laws are empirical relations — they are not derived from a
physical theory, but simply observed facts.
By dispensing with perfect circles, the revised heliocentric model was able to
predict planetary positions with much greater accuracy than the geocentric
model.
Kepler’s mathematical explanation for the orbits of the planets, together with
the new worlds revealed by the telescope, provided a complete framework for
the heliocentric model, which cemented acceptance of this new world view.