A Classical Approach of Newtonian Mechanics

12 ORBITAL MOTION 12.2 Historical background

epicycle

deferant

planet

Figure 100: The Ptolemaic system.

centric model, since in the former model the Earth occupies a privileged

position in the Universe.

The geocentric model was first converted into a proper scientific theory, ca-
pable of accurate predictions, by the Alexandrian philosopher Claudius Ptolemy

(85– 165 AD). The theory that Ptolemy proposed in his famous book, now known

as the Almagest, remained the dominant scientific picture of the Solar System for

over a millennium. Basically, Ptolemy acquired and extended the extensive set

of planetary observations of his predecessor Hipparchus, and then constructed a

geocentric model capable of accounting for them. However, in order to fit the
observations, Ptolemy was forced to make some significant modifications to the

original model of Eudoxas. Let us discuss these modifications.

First, we need to introduce some terminology. As shown in Fig. 100 , deferants

are large circles centred on the Earth, and epicyles are small circles whose cen-
tres move around the circumferences of the deferants. In the Ptolemaic system,

instead of traveling around deferants, the planets move around the circumfer-

ence of epicycles, which, in turn, move around the circumference of deferants.

Ptolemy found, however, that this modification was insufficient to completely ac-
count for all of his data. Ptolemy’s second modification to Eudoxas’ model was

centre of deferant P^

Earth

+

C E

equant