Page 34 - GALIET PHYSICS BLOSSOMS I+

If we follow the line numerically, our line of sight moves backward relative to the stars between spot 5 and 7 where we witness Jupiter’s apparent retrograde motion. When we are in spot 7, we witness the beginnings of the apparent prograde motion that continues in 8, 9, 10 and 11.
Our line of sight changes because Earth, on the inner orbit, moves faster than Jupiter (Mars).
In reality, what appears as retrograde and prograde planetary motion is due to the motion of the Earth and not the planets. Indeed, the heliocentric model gives a natural explanation for retrograde planetary motion as a consequence of earth’s motion, which changed our understanding of the universe and our place in it.27
The Ptolemaic and Copernican models are mathematically equivalent; they predict the same motion of the planets with great accuracy. Ptolemy calibrated his model by adjusting the size and speed of the epicycles and deferents; Copernicus calibrated his model by adjusting the size and speed of a planet's orbit. Despite Copernicus’ displeasure with the equant model, which he thought to deviate from uniform circular motion, Copernicus retained the epicycle on deferent model. This model allowed him to predict future planetary positions with an accuracy comparable to Ptolemy. The apparent retrograde motion still exists, but modern astronomy has given us a natural explanation.
Copernicus’ Revolution:28
1. Earth is not at the centeràthe sun is.
2. All planets (spheres) revolve around the sun.
3. The stars are father much farther away than the sun.
4. The firmament does not move, the earth does.
a. Earth rotates around its fixed poles.
b. The firmament is immobile.
5. Apparent planetary retrograde + forward motion is not planetary motion, but the
earth’s.
27 Chaisson, McMillan. Astronomy Today. 7th Edition. Figure 2.9. 39. 28 Chaisson, McMillan. Astronomy Today. 7th Edition. 38.
•34•