Dr. Bozidar Mitrovic: We discussed the nature of the motion of planets.
Most of the time, it's from west to east prograde but every
now and then, they reverse the direction of motion.
The question is how to fit this into the Aristotelian model of geocentric
universe.
They actually had to invent a system of epicycles to describe the
motion of planets.
Let me explain what epicycles are.
Imagine that you have a bicycle wheel mounted horizontally.
At the edge of the bicycle wheel, you have a platform in the
form of a disk that is also spinning, and there is a little ball
mounted at the edge of the disk.
What would be, if both the wheel and the disk are spinning in the
same direction, the trajectory in space of this ball?
It would form these loops, these epicycles.
In other words...in epicycle system, you have that planets are moving along small
circles whose centers are moving along a bigger circle.
As a result, the trajectory of the planet has this loopy shape.
Basically, the prograde motion corresponds to this direction of
motion.
When they reverse the direction of motion, the planets are
actually undergoing this motion along the loop, and so on.
Again, prograde motion.
Here, it reverses and moves in retrograde motion
and so on.
This is the system of epicycles that they had to invent
in order to explain the motion of the planets.
This thing tells you that your theory, the geocentric model, is in
trouble.
In science, as in everyday life, once you need to
complicate things more and more in order to explain things
happening around you, usually means that your starting assumption
is wrong.
A hallmark of a correct theory, of correct assumption, is that,
actually, you don't need to do anything to it in order to account
for many, many things that you observe, also predict new
observations, account for observations in the future.
If you need to need to modify the theory, to tweak it
all the time to account for larger number of events, the odds are
that your starting assumption, in this case the assumption of
the Earth-centered universe, is wrong.
Then you have to actually revisit your basic assumption and see
what is happening.
Heliocentric model, we can understand this retrograde motion of planets much more naturally.
It turns out that the closer the planet is to the Sun, the faster
it moves, the bigger its orbital speed is.
As the distance from the Sun increases, the orbital speed drops.
In this case, of the Earth and the Mars, the Earth is moving faster
in its orbit around the Sun than Mars.
At some point, when Earth is here and Mars is here, we see it in the background
of stars to be located at this position one.
Sometime later, when the Earth is here at two, it has moved this distance.
Because the Mars is slower, it has progressed smaller distance.
Nevertheless, we see Mars to be here against the background of
stars.
Between four and six, if you will, the Earth is overtaking Mars.
From the vantage point of Earth, it looks to us as if the Mars is
lagging behind.
What is happening, the Earth is simply...because it's moving faster, overtaking the Mars, and
to us, moving together with Earth, it appears as if the Mars started
moving backward, but it hasn't.
It simply continues to move along its orbit around the
Sun, it just been overtaken by the Earth.
Then we have for a while, from one to four, we have prograde motion
from west to east.
From four to six, we have retrograde motion from
east to west.
Then things normalize again after six, starting from
seven, eight and so on.
To us, it appears that Mars continued moving along the same direction.
This is how the retrograde motion of planets is understood within the heliocentric
model.
Epicycles basically are...If this is the Earth, the trajectory of
the planet is obtained as follows.
The planets are moving along small circles, whose centers move along the
bigger circle.
The end result is basically that you get this looping
trajectory.
These are the epicycles.