The Geocentric Universe

• The Earth as the fixed center fits our experience.

• Sun, Moon and planets appear to orbit the Earth, against a background of fixed stars on the rotating celestial sphere.

Two Time Scales

• The Sun, Moon, planets, and all the stars appear to revolve around the Earth once in 24 hrs.

• Furthermore, over a period of weeks/months the Sun, Moon and planets “wander” across the Celestial Sphere. (i.e. move at a different rate compared to the fixed stars.)

Geocentric Observations of Planetary Motion

• Planets (“Wanderers”) do not move in “perfect” circles, relative to us.

• Planets vary in brightness.• Never stray far from the ecliptic.• Generally traverse the celestial sphere

from west to east (eastward), like the sun.• Occasionally appear to loop backwards

and forwards, with respect to the stars.

Retrograde (Sidereal) Motion

General eastward or

“direct” motion of Mars has

apparent westward or “retrograde”

loops.

Explain That Behaviour!

• Earth is Fixed• Aristotle (and Plato) believed that perfect

motion must be circular• Uniform speed – no ‘change’

Problems: Could not explain brightness variations or retrograde motion.

Refine the theory!

Introduce Epicycles and Deferents

Planet moves on circular Epicycle, whose center moves on a circular deferent

‘Wheels within wheels.’

Planet moves on circular Epicycle, whose centermoves on a circular deferent –centered on the Earth

Problem Solved!

Vary speed of motion in Deferent and Epicycles to match the observed behaviour.

Ptolemy’s Model

• Motion of the planets were further refined (>80 circles!)

• Predictions were quite accurate.

• Survived for 1300 years!

Sensible? Plausible?

• Aristarchus (310-230BC) suggested that the Earth moved around the Sun!

• Aristotle gave ‘convincing’ arguments for geo-stationary view:

1. Earth doesn’t feel like it is moving!2. “Wind”, if moved, would blow us off!3. Why no stellar ‘parallax’?

Nicholas Copernicus 1474-1543

• Rediscovered Aristarchusheliocentric model.

• BUT, still based on many circles –simply a change in perspective.

Aristotle verses Copernicus!

Heliocentric system can also explain retrograde motion and brightness variations.

Tycho Brahe 1546-1601

• Remarkable Observational Astronomer!

• Pre-telescope era.• Provided data used

by Kepler.• Did not believe in

heliocentric system.

Galileo Galilei 1564-1642

• Adapted the recently invented telescope to look at the Moon, Venus, Sun and Jupiter.

• Provided direct experimental evidence to support heliocentric model.

The telescope that Galileo

used to make his first

observations.

One of Galileo's original drawing of the moon, compared to the real thing!

Galileo’s Drawing of Sun spots

Aristotle was wrong! The Sun (like the moon) is not

perfect – it has blemishes!

Jupiter and its ‘Moons’

Galileo’s observations:Evidence for ‘mini’ solar system.

The Phases of Venus – Heliocentric View

Possible to have a “full” Venus

The Phases of Venus – Geocentric View

Various phases and sizes,but impossible to have ‘full’ Venus!

The Phases of Venus – Observations!

Convincing Observational

Evidence!

But still nostellar parallax.