The History of Astronomy

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Ptolemy: Geocentric model, including epicycles

1. Imperfect, changeable Earth,

2. Perfect Heavens (described by spheres)

Central guiding principles:

Slide 11

What were the epicycles in Ptolemy’s model supposed to explain?

The fact that planets are moving against the background of the stars.

The fact that the sun is moving against the background of the stars.

The fact that planets are moving eastward for a short amount of time, while they are usually moving westward.

The fact that planets are moving westward for a short amount of time, while they are usually moving eastward.

The fact that planets seem to remain stationary for substantial amounts of time.

Slide 12

Epicycles

The ptolemaic system was considered the “standard model” of the Universe until the Copernican Revolution.

Introduced to explain retrograde (westward) motion of planets

Slide 13

At the time of Ptolemy, the introduction of epicycles was considered a very elegant idea because …

it explained the motion of the planets to the accuracy observable at the time.

it was consistent with the prevailing geocentric world view.

it explained the apparently irregular motion of the planets in the sky with “perfect” circles.

because it did not openly contradict the teaching of the previous authorities.

All of the above.

Slide 14

The Copernican Revolution

Nicolaus Copernicus (1473 – 1543):

Heliocentric Universe (Sun in the Center)

Slide 15

New (and correct) explanation for retrograde motion of the planets:

This made Ptolemy’s epicycles unnecessary.

Retrograde (westward) motion of a planet occurs when the Earth passes the planet.

Described in Copernicus’ famous book “De Revolutionibus Orbium Coelestium” (“About the revolutions of celestial objects”)

Slide 16

In the Copernikan “Universe”, the orbits of planets and moons were …

Perfect Circles

Ellipses

Spirals

Epicycles

None of the above.

Slide 17

Johannes Kepler (1571 – 163)

Used the precise observational tables of Tycho Brahe (1546 – 161) to study planetary motion mathematically.

Circular motion and

Planets move around the sun on elliptical paths, with non-uniform velocities.

Found a consistent description by abandoning both

Uniform motion.