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Copernicus vs Ptolemy

How do we know what is "right" in Science

We can learn a lot by looking at the history of wrong ideas. In many cases, the wrong ideas have simply been forgotten. Also, correct ideas have taken a very long time to be accepted. Usually the scientific method works in the end, but prejudice, preconceptions, racism and plain stupidity have hindered scientific progress. Examples include

It is not possible for every generation of scientists to check every result, so the question is "why do we believe what we believe?"

Ptolemy and Copernicus

  1. (Easy) Why do you believe the earth is not flat?
  2. (Hard) Why do you believe the earth rotates?
  3. (Very hard) Why do you believe the earth orbits the sun?


Why the Earth is not flat:
shadow on moon is always curved
Why do you believe the earth orbits the sun?
Why the earth must be stationary: Suppose that the earth revolved round sun. Since the separation between the stars doesn't change, the earth must be stationary

What is wrong with this argument?


Simplest model is Geocentric

  1. Mercury & Venus never get far from sun.
  2. Retrograde Motion
  3. Changing brightness during year
so need to introduce epicycles

COPERNIKUS (1473-1543) Ptolemy by now required 40 epicycles:

Orbits circular (so still need epicycles)
motion of Mercury & Venus "unlocked" from sun.
Lack of parallax because fixed stars are very far away

So why did people at the time belive in Copernicus? Reasons for asserting the earth is motionless:
  1. David in Psalm 89: God has founded the earth and it shall not be moved.
  2. Joshua bade the sun stand still-which would not be notable were it already at rest.
  3. The earth is the heaviest element, therefore it more probably needs rest.
  4. Everything loose on the earth seeks its rest on the earth, why should not the whole earth itself be at rest?
  5. We always see half of the heavens and the fixed stars also in a great half circle, which we could not see if the earth moved, and especially if it declined to the north and south...
  6. A stone or an arrow shot straight up falls straight down. But if the earth turned under it, from west to east, it must fall west of its starting point.
  7. In such revolutions houses and towers would fall in heaps.
  8. High and low tide could not exist; the flying of birds and the swimming of fish would be hindered and all would be in a state of dizziness.

Reasons for the belief that the earth is moved:

  1. The sun, the most excellent, the greatest and the midmost star, rightly stands still like a king while all the other stars with the earth swing round it.
  2. That you believe that the heavens revolve is due to ocular deception similar to that of a man on a ship leaving shore.
  3. That Joshua bade the sun stand still Moses wrote for the people in accordance with the popular misconception.
  4. As the planets are each a special created thing in the heavens, so the earth is a similar creation and similarly revolves.
  5. The sun fitly rests at the center as the heart does in the middle of the human body.
  6. Since the earth has is itself its especial centrum, a stone or an arrow falls freely out of the air again to its own centrum as do all earthly things.
  7. The earth can move five miles in a second more readily than the sun can go forty miles in the same time.

Voight (1667). Der Kurstgunstein Einfalt Mathematisher Raritaten Erstes Hundert.

So is there really any reason to choose Copernicus vs. Ptolemy?

Copernicus is also wrong (the sun is not the centre of the solar system, and planets do not move in circles. However, the real tests come with Galileo (observation of the phases of Venus and the Moons of Jupiter) and Newton (supplied a theory which would allow the calculation in detail)
  1. (the warmup exercise!). There is no such thing as gravity: instead, each object has a demon attached to it who pushes down. To stop the demons, you must push upwards with an opposite force. This "theory" explains why Disprove this theory!
  2. (harder) There is no such thing as gravity: instead, the universe is filled with invisible particles, which travel in all directions. Large bodies "shadow" these particles: hence there will be an unbalanced pressure of particles.This gives a force that mimics gravity (i.e. it falls off as 1/r2).

    Disprove this theory!

    Now we should look at a very unusual view of the solar system