Greek views of the earth and the stars

Where is the wise man? Where is the scholar? Where is the philosopher of this age? Has not God made foolish the wisdom of the world? For since in the wisdom of God the world through its wisdom did not know him, God was pleased through the foolishness of what was preached to save those who believe. Jews demand miraculous signs and Greeks look for wisdom, but we preach Christ crucified: a stumbling block to Jews and foolishness to Gentiles, but to those whom God has called, both Jews and Greeks, Christ the power of God and the wisdom of God. For the foolishness of God is wiser than man's wisdom, and the weakness of God is stronger than man's strength.
-- 1 Corinthians: 20-25 (NIV)

Or, as Paul (MR) paraphrases it: "Greeks look for patterns"

Mathematical mysticism

Pythagoras (6th century BC) noticed a connection between numbers and music:

Ratio of string lengths of 1:2 $\rightarrow$ interval of precisely 1 octave.

Ratio of string lengths 0f 2:3 $\rightarrow$ interval of precisely 1 fifth

Patterns--simple patterns

Bring Viola

And his followers had a great reverence for the integer numbers {1, 2, 3, 4, ....}.

MonadThe representation of one, the monad, was a circle, a geometrical figure with one side.

See: Pythagoras: Music and Space

Stars in motion

Leave a camera lens open long enough, and you'll see this pattern of star motion in the heavens above...

Which way are they moving?

[Writing - Rotating Stars]

What else do you notice about the motions of the stars?

More patterns in the night sky

With the naked eye you can watch the stars, and notice..."constellations". . What is a constellation?

What happens as night progresses to the constellations? Are constellations just temporary? Do stars leave / enter a constellation? Let's see....

  • Stars do not come and go from constellations. The constellations are constant groups of stars.
  • $\rightarrow$ The stars don't (appear to) move relative to each other.

So what about that model of all the stars attached to a big crystal sphere?

What about the moon?

Is the moon also part of a constellation?
What does it look like?
Video of 'moonrise'

Aside from the moon, there are 6 other objects that you can see with the naked eye that behave similarly...

These objects, called "the wanderers" by the Greeks are named Mercury, Venus, Mars, Saturn, and Jupiter--the planets. Oh, the 6th one is the Sun.

It is important to remember that the planets do not appear to behave any differently from the stars on any given night. It is only when you watch them over the course of weeks or months that you might start to notice that they're not staying in any one constellation.

Earth-centered Universe

SpheresThe ancient Greeks described the universe as a series of concentric shells centered on Earth.

You might wonder a bit.... why were the stars all on the same shell? Could no relative motion of "star against star" be seen?

Well, except for Aristarchos...

Aristarchos of SamosAristarchus (approx. 310 BC - 230 BC) proposed a sun-centered universe.

Why do you think his model was not accepted at the time??

BTW, Aristarchus also successfully calculated the distance to the moon.

The planets (the 'wanderers')


Unlike the sun and the moon which move uniformly (constant speed) relative to the 'background' of stars, the planets move erratically.

Their motion is now faster, now slower, and sometimes even backwards ('retrograde').

They also get brighter and dimmer.


Ptolemy's universePtolemy wrote up the prevailing Greek view in his Almagest (ca. 150 A.D.), which managed pretty good agreement with observations made at the time.

Telescopes were unknown until Galileo. The Greeks made measurements by sighting along rods.

Why do we believe.... the earth is round?

The Earth as seen from Apollo 17

Really ancient Greeks

Monad...Such as Pythagoras, believed, for aesthetic reasons, that circles and spheres were perfect forms, and that therefore the earth was most likely spherical.

Ships dropping below the horizon

Ship dropping below the horizon

looking over the horizon

New constellations

Aristotle travelled from Greece south to Egypt, and noted that different constellations could be seen, which could not be seen from Greece.

[We tried to demonstrate in class *why* this is evidence for a round earth, rather than a flat one.]

Demonstrate this with two student: Most of the class are the traditional constellations. As a student moves parallel to the front of the classroom (to simulate being on the flat earth).

The "new constellation" student is positioned slightly behind the moving student, and cannot be seen for parallel motion. But if the student pivots while moving...


Suggested exercises

Conceptual Exercises (CE) in Chapter 1: 1, 4, 6

Concept Checks in Chapter 1: 1-6, using this picture for #5 and #6.

Image credits

Anton, Dr. Manuel, NASA, joiseyshowaa

Do a demonstration where one student puts hands on back of head as if lying on grass.  Position a second student behind (out of view).

Move first student left-right to show "same stars".  (flat earth).

Now, swing first student around, and show that they can now see second student.


  Write a few sentences of reaction:  "It was unreasonable for the Greeks to believe that stars rotated about the earth."

  Turn in the drawings you made to convince each other of the direction of rotation of the earth.

  Draw the phases you'd expect to see at 1,2,3,4 with the epicycle model
  Draw the phases you'd expect to see at 1,2,3,4 with a sun-centered model