The Ptolemaic Model of Near-Earth Space:
Retrograde Motion

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In astronomy, as in all of science, necessity is the mother of invention. The astronomers of antiquity, who were affluent or lived lifestyles of relative leisure, spent their time making observations of the motion of the planets against the background of stars.

In ancient Greece and Rome, amongst other places, the Seven Wanderers had special significance. Without the aid of a telescope, one observed that seven objects changed their positions with time relative to the stars. We know these objects as the Sun, the Moon, Mars, Mercury, Jupiter, Venus, and Saturn. The days of the week are named after the gods of Roman Mythology associated with the Seven Wanderers. This fact is easier to recognize if you look at the days of the week in Spanish: Domingo, Lunes, Martes, Miercoles, Jueves, Viernes, Sabado.

Except for the Sun and Moon, the wanderers all exhibit retrograde motion when viewed night after night for a period of many months or years. As the planets orbit the sun, they seem to move slowly in our sky from west to east but periodically stop that progress and reverse course amongst the stars. In the Copernican (sun-centered) solar system as we understand it today, the planetary orbital periods get longer as you look farther out from the sun. This fact results in a faster planet catching up to and passing a slower planet as they orbit. From the point of view of a person on the faster planet, the slower one seems to stop and reverse course as it is being passed. It is an illusion, similar to what you might experience while sitting in a fast-moving car passing a slow-moving car: the background objects may briefly seem to move forward faster than the car you are passing.

In ancient Rome, and during much of the Middle Ages in Europe, the Earth was believed to be at the center of all things. This idea was combined with the observed retrograde motion of the wanderers by the Roman astronomer Ptolemy. To account for the observations, the planets Mars, Jupiter and Saturn can't go around the Earth in simple circles. Ptolemy constructed a model based on circles-within-circles called epicycles and deferents. While complicated, the model had plenty of appeal since it was based on the assumption that much of nature is governed by the "perfect" geometric figure: the circle.

You can see for yourself how the Ptolemaic model accounts for the observed wobble or retrograde motion by running this Java demonstration.

For a more detailed account of the development of our understanding of the Solar System, with links to further information on the history of astronomy, click here.

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