At the left is the standard simple model of the
Earth
and
Mars
as they orbit the
Sun
. The orbits are taken as circles, whereas, the true motions are ellipses with small eccentricities (
0.017
and
0.093
, respectively ) and both in the plain of the ecliptic. The earth defines the ecliptic and Mars' orbit is actually is inclined at about 2
o
to the eciptic. The view here is from above the Solar System in a reference frame at rest with respect to the Sun. The view from Earth is much different as can be seen below. But before looking at it, watch the annimation at the left and notice that every once-in-a-while the earth catches up with Mars and passes it. this is called
opposition
and it is when
retrograde (backwards) motion
occurs.
For clarity only the motion of Mars is shown.
The change
in the apparent size of Mars is approximate. Subject to the simple assumptions above the apparent motion should be quite accurate. It is also clear that the apparent motion is quite complicated. Keeping in mind that Ptolemy and his precursors where not interested in explaining the Universe but rather predicting its behavior, it should not be surprising that Ptolemy came up with a model based on a deferent circle and an epicycle to explain this motion.
The planet goes around the center of epicycle at one rate while the center of the epicycle goes around the deferent at yet another rate. The purpose is to predict the motion of Mars in a mechanical fashion. In other words Ptolemy created the ancient equivalent of a modern programable calculator. The model shown here is a very simple version consistent with the simple heliocentric model shown above. Learn more about how the simple Ptolemaic model worked
here
.
© 1999 Carl Adler
mailto:
Carl@Image-ination.com
