[Chris]

Chris asked the Naked Scientists:
   
Love the show, listen to the podcast every week, you guys are great.

My question is, how did Ptolemy and others who believed in a
geocentric model of the universe account for the phases of the moon?

--Chris
Springville, UT

What do you think?

[LeeE]

I think that the first thing you've got to remember in regard to this question is that the phases of the Moon are not so much a consequence of a geocentric or heliocentric model of the solar system but are a result of spherical celestial bodies, and by the time of Ptolemy it had long been generally accepted that the Earth and other celestial objects were spherical and not flat (there are some indications that the spherical nature of Earth had been suspected in the 7-8th century BC and Eratosthenes had actually made a pretty good attempt at estimating its circumference in 240 BC.  Ptolemy lived in the 2nd century AD, a good few hundred years later).

In Ptolemy's model the Earth sat at the center of a series of concentric 'shells', with the shells between the innermost and outermost each carrying either the Moon, Sun, or one of the planets (the innermost shell carried the meteorological phenomenon and the outermost carried the distant stars).

In this model the Moon's shell was closer to the Earth than the shell that carried the Sun so it was possible for the Sun to pass behind the spherical Moon and consequently light it from any angle to produce the lunar phases, just as you get when you light any spherical body from different directions.  In fact, this geocentric model works just as well as the Heliocentric model regarding the lunar phases because it allows for the same relative positions to occur (Ptolemy was quite a bit out in his estimated sizes and distances though, and I think his model would have required things to speed up and slow down a bit and not move at constant speeds).

[PhysBang]

It is with the phases of the planets, like Venus, that there is a difference in expected phases between the two systems.