


Wandering stars

Early astronomers observed that a few of the brighter stars seemed to wander in the sky over the course of many months. Attributing this behaviour to a form of extreme power, these wandering bodies were assumed to be deities by the Babylonians. The Greeks named them “wandering stars” or “astēr planētēs” – the root of the word we use today: planets.

On 1 May 2011 five of our Solar System’s eight planets and the Moon could be seen. Image: G.Hüdepohl The geocentric model couldn’t yet explain why planets would appear brighter and darker at different times. The most striking issue was how to explain why the planets would occasionally stop and reverse directions. This is known as retrograde motion and would be impossible if the planets were the same distance to us at all times. Below is an image of the geocentric model which assumed that all planets travel around the earth in perfect circles.

Andreas Cellarius, "Ptolemaic orbits",  Harmonia Macrocosmica , 1661.The red planet Mars was particularly difficult to explain. If we assume a geocentric universe and map out the wandering motion of the Red Planet we’d see this pattern

Retrograde motion of mars when viewed from EarthIt was assumed that something must be wrong with the model. For early astronomers, there was a strong need to keep the Earth at the center of the universe. This led to some very creative ways of justifying the geocentric model of the universe. How could they explain why these pretzel shaped orbits occur if everything travels in circular orbits? Below is a representation of the apparent motion of the sun and planets from the earth. It shows the orbit of Venus over 8 years and Mars over 7 years. Notice the preztel shaped orbits which emerge:

Image: Encyclopædia Britannica (1777)


The Greek geometer and astronomer Apollonius of Perga (262 - 190 BCE) came up with a model to explain these changes in position and brightness. He believed that this cyclical variation could be represented visually by mini orbits, or epicycles, revolving around larger circular orbits, or deferents. The following animation shows Mars traveling around a red orbit, while the red orbit is also orbiting around the Earth on a secondary orbit.

Image: Peter CollingridgeUnfortunately, the epicycles made the new geocentric model incredibly complicated, and some planets even needed epi-epicycles for the model to match astronomical observations. There was, however, a competing theory that greatly simplified things. For this theory to gain support, astronomers as well as political and religious leaders needed to give up the dominant view that the Earth was at the center of the universe. This was not going to be an easy task.