Download Greek Astronomy

History of Astronomy
Day Four
Greeks were really smart…
Figured out…
 Size
of the Earth
 Size
of the Moon
 Distance
to the Moon
 Distance
to the Sun
 Size
of the Sun
Early Greek Theory
Science needs both observations and theories, but the
observations must match the theories!
Greeks had some holes in their early theory.
 What to do???

Greek Problems
 Their
theories couldn’t predict the motion of the planets or
explain retrograde motion. The planets looked like they
would start moving backward!
https://youtu.be/0bkxcf0vwQ4
Retrograde Motion
All planets orbit the sun in the same direction (eastward). The planets appear to
move backward because the planet closer to the sun (the inner planet) moves
faster. When the inner planet passes the outer planet, each sees the other as
moving backwards (relative to its usual motion).
Scientists in hundreds of cultures all over the world tried to predict and explain
planetary motion.
Greek Problems
The Greeks also could not explain why summer was longer than winter.
Greeks were really smart… but they had issues.
What are your options when your theory is not quite correct
1.
Throw out Observations
2.
Throw out the Theory
3.
Modify the Theory
The Greeks chose to modify their geocentric
theory.
Problem #1: Ptolemy solved their issues
predicting the motion of the planets with
epicycles . . . sort of.
Greek Idealism
Recall…theory called for…
 Perfect Unchanging Heavens
 Objects in the heavens must move “perfectly”
 . . . so what about retrograde motion???
.
. . they never said much more about that . . .
Late Geocentric Theory
Problem 2: They had no explanation why summer is longer
than winter.
So, instead of developing a new theory, they just shifted the orbits off
center to explain how summer was longer than winter.
Is their solution getting simpler or more complex?
 The simple explanation tends to be the right explanation.


Not to mention the fact they were trying to prove the Earth
was at the center . . . and then they moved it out of the
center, to prove that it was at the center?!
However . . . .