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 –
centered on
the Earth
Planet moves on
circular
Epicycle, whose
center moves on
a circular
deferent
‘Wheels within
wheels.’
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
Aristarchus
heliocentric 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 no
stellar parallax.