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Lecture 8

ASTA01H3 Lecture Notes - Lecture 8: Astronomia Nova, Ellipse, Semi-Major And Semi-Minor Axes

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Kristen Menou

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Lecture 8 ASTA
However, Copernicus hesitated to publish even though other scientists, and even
church officials including Pope Clement VII, concerned about reform of the calendar,
knew about his work, sought his advice, and encouraged the publication.
In 1542, he finally sent the manuscript off to be printed.
Copernicus died in 1543 before the printing was finished, but saw the first prints on his
The most important idea in the book was placing the Sun at the centre of the universe
The retrograde motion of the planets was immediately explained in a straightforward
way without the epicycles that Ptolemy used
In the Copernican model, Earth moves faster along its orbit than the planets that lie
farther from the Sun
The same thing happens as Earth passes a planet such as Mars.
Although Mars moves steadily along its orbit, as seen from Earth, it seems to slow to a
stop and move westward (retrograde) relative to the background stars as Earth passes it
Coperius’s odel as siple ad straightforard opared ith the multiple off-
centre circles of the Ptolemaic model
However, De Revolutionibus failed to disprove the geocentric model immediately for
one critical reason
The Copernican model could not predict the positions of the planets any more
accurately than the Ptolemaic model!
Dislike toward the eccentric motion and the ugly equant made Copernicus return to a
strong but incorrect belief in uniform circular motion.
Therefore, even though his model put the Sun correctly at the centre of the solar
system, it could not accurately predict the positions of the planets as seen from Earth
The Copernican model is inaccurate.
It includes uniform circular motion and thus does not precisely describe the motions of
the planets.
However, the Copernican hypothesis that the solar system is heliocentric is correct.
The planets circle the Sun, not Earth.
The most important factor, though, may be the simplicity (beauty) of the idea.
Also, through a dual motion of the Earth (spin + orbit or rotation + revolution) many
seemingly disconnected phenomena could be explained without resorting to separate
explanations for each of them: rotation of the sky, loopy paths of the planets, phases of
planets, ...
In the Ptolemaic model, Mercury and Venus had to be treated differently from the rest
of the planets.
Their epicycles had to remain centred on the Earth-Sun line.
I Coperius’s odel, all the plaets ere treated the sae.
They all followed orbits that circled the Sun at the centre
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