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.