Ancient History of Astronomy

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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

Slide 15

New (and correct) explanation f Retrograde (westward) motion of a planet occurs when the Earth passes the planet.

or retrograde motion of the planets: This made Ptolemy’s epicycles unnecessary Described in Copernicus’ famous book “De Revolutionibus Orbium Coelestium” (“About the revolutions of celestial objects”)

Slide 16

• In the Copernikan “Universe”, the orbits of pl anets and moons were … Perfect Circles

• Ellipses

• Spirals

• Epicycles

• None of the above.

Slide 17

Johannes Kepler (1571 – 1630)

Used the precise observational tables of Tycho Brahe (1546 – 1601) to study planetary motion mathematically. Found a consistent description by abandoning both

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

Slide 18

Kepler’s Laws of Planetary Motion

The orbits of the planets are ellipses with the sun at one focus.

Slide 19

Eccentricities of Ellipses

Slide 20

Eccentricities Orbits of planets are virtually indistinguishable from circlesof planetary orbits

Slide 21

A line from a planet to the sun sweeps over equal areas in equal intervals of time.

Slide 22

• Are all four seasons equally long? Yes.

• No, summer is the longest; winter is the shortest.

• No, fall is the longest; spring is the shortest.

• No, winter is the longest; summer is the shortest.

• No, spring is the longest; fall is the shortest.

Slide 23