Geocentric view
This simulation shows
the planets' paths relative to the Earth as they appear to move against the
background of the stars. The simulation is calculated using exactly the same
Newtonian (even heliocentric) laws as the other simulations - we've just
displayed the results as they appear relative to the Earth.
Three views are displayed; an overall view of this Earth-centred model of
the universe, and a pair of views from Earth towards a celestial body of
your choice; one tracing the object's apparent motion, the other a close-up
showing the Sun's shadow and changes in visual size. [The close-up
views from Earth show the planets at all the same magnification - and the
Moon and Sun at a lower magnification. Apart from Earth itself, all celestial
objects are displayed without orbiting moons or surface
detail.]
In this simulation the Earth spins (try zooming in to see it) - whereas in
a full-blooded geocentric model the Moon, Sun, planets and stars all circle
the Earth once a day in addition to the movements shown here.
Planetary motion
Instead of following (relatively simple) elliptical orbits around the Sun
(today's heliocentric model) - the planets take a more complex wandering
path around the geocentric model's fixed Earth. Each planet's overall motion
is to circle the Earth in the same west-to-east direction - but each
also regularly backtracks in a period of "retrograde motion".
Geocentric theory (elaborated by Claudius Ptolemy ~ 90-170 AD) explains this
strange motion with the idea that each planet, in addition to circling the
Earth (on a "deferent" circle) also moves around a smaller "epicycle".
Diagrams
of Ptolemy's Planetary models illustrate some of the intracacy of the
theory.
Alfonso X of Castille and Leon (1221-84) complained on having the Ptolemaic
system explained to him: "If the Lord Almighty had consulted me before embarking
upon Creation, I should have recommended something simpler."
In his 1543 deathbed publication of "On the Revolutions of the Heavenly
Bodies" the Polish astronomer Copernicus proposed a heliocentric
model, reviving the ideas of Aristarchus of Samos (200 BC). Copernicus'
heliocentric model didn't need epicycles to explain retrograde planetary
motion (but he did use them to make his planets' circular paths a closer
match to the ellipical reality).
Overcomplexity wasn't Ptolemy's geocentric theory's only problem - it seriously
misrepresented the motions of certain planets - as Galileo observed
with a telescope in the early 1600s.
What did Galileo see (you don't need a telescope to observe it in the simulation)
and how did it conflict with
Ptolemy's
model?
Which celestial bodies don't exhibit retrograde motion? Why don't they?
What was Tycho Brahe's (1546-1601) model of the universe and why did he adopt
it?
Links:
The
Scriptural Basis for a Geocentric Cosmology
The universe of Aristotle and Ptolemy
The Copernican Model:A Sun-Centered Solar System
Ancient
Greek astronomy
The
heliocentric revolution
Renaissance Astronomy and the Acceptance of the Heliocentric
Model
Ptolemaic system
Ptolemy
Diagrams
of Ptolemy's Planetary models
Copernicus
Suggests a Heliocentric Cosmology
Copernicus
and the Aesthetic Impulse
Renaissance astronomy