AY 101 Lecture Notes - Lecture 3: Nicolaus Copernicus, Galilean Moons, Inferior And Superior Planets
Foundations of Modern Astronomy
Nicolaus Copernicus and the Heliocentric hypothesis
Copernicus (14731547) was a Polish scholar who postulated an alternative description
of the solar system. Like the Ptolemaic geocentric (Earth‐centered) model of the solar
system, the Copernican heliocentric (Sun‐centered) model is an empirical
model. That is, it has no theoretical basis, but simply reproduces the observed motions
of objects in the sky.
In the heliocentric model, Copernicus assumed Earth rotated once a day to account for
the daily rise and set of the Sun and stars. Otherwise the Sun was in the center with
Earth and the five naked‐eye planets moving about it with uniform motion on circular
orbits (deferents, like the geocentric model of Ptolemy), with the center of each offset
slightly from Earth's position. The one exception to this model was that the Moon moved
about Earth. Finally, in this model, the stars lay outside the planets so far away that no
parallax could be observed.
Why did the Copernican model gain acceptance over the Ptolemaic model? The answer
is not accuracy, because the Copernican model is actually no more accurate than the
Ptolemaic modelboth have errors of a few minutes of arc. The Copernican model is
more attractive because the principles of geometry set the distance of the planets from
the Sun. The greatest angular displacements for Mercury and Venus (the two planets
that orbit closer to the Sun, the so‐called inferior planets) from the position of the Sun
( maximum elongation) yield right angle triangles that set their orbital sizes relative to
Earth's orbital size. After the orbital period of an outer planet (a planet with an orbital
size larger than the orbit of Earth is termed a superior planet) is known, the observed
time for a planet to move from a position directly opposite the sun ( opposition) to a
position 90 degrees from the Sun ( quadrature) also yields a right‐angle triangle, from
which the orbital distance from the Sun can be found for the planet.
If the Sun is placed in the center, astronomers find that planetary orbital periods
correlate with the distance from the Sun (as was assumed in the geocentric model of
Ptolemy). But its greater simplicity does not prove the correctness of the heliocentric
idea. And the fact that Earth is unique for having another object (the Moon) orbiting
around it is a discordant feature.
Galileo Galilei and the invention of the telescope
Settling the debate between the geocentric versus heliocentric ideas required new
information about the planets. Galileo did not invent the telescope but was one of the
first people to point the new invention at the sky, and is certainly the one who made it
famous. He discovered craters and mountains on the Moon, which challenged the old
Aristotelian concept that celestial bodies are perfect spheres. On the Sun he saw dark
spots that moved about it, proving that the Sun rotates. He observed that around Jupiter
traveled four moons (the Galilean satellites Io, Europa, Callisto, and Ganymede),
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Document Summary
Copernicus (1473(cid:1679)1547) was a polish scholar who postulated an alternative description model. That is, it has no theoretical basis, but simply reproduces the observed motions of objects in the sky. of the solar system. Like the ptolemaic geocentric ((cid:1688)earth centered(cid:1689)) model of the solar system, the copernican heliocentric ((cid:1688)sun centered(cid:1689)) model is an empirical. Earth and the five naked eye planets moving about it with uniform motion on circular. In the heliocentric model, copernicus assumed earth rotated once a day to account for the daily rise and set of the sun and stars. Otherwise the sun was in the center with orbits (deferents, like the geocentric model of ptolemy), with the center of each offset slightly from earth"s position. The one exception to this model was that the moon moved about earth. Finally, in this model, the stars lay outside the planets so far away that no parallax could be observed.
