Ptolemaic system

The Ptolemaic system was a model to explain the motions of the heavens in which the earth was the centre of the universe and all other celestial bodies revolved around it, espoused by Claudius Ptolemaeus in his work, the Almagest some time around the 2nd century, C.E., and accepted for over 1,000 years by the vast majority of Europeans to be the correct cosmological model. It may be also called the geocentric model. It was overthrown by the Copernican revolution after Galileo Galilei and Copernicus discovered that the planets orbited the sun.

Replacement with Copernican system

Ptolemy's model was finally disproved by Galileo, when, using his telescope, he discovered that Venus goes through phases, just like our moon does. Under the Ptolemaic system, however, Venus can only be either between Earth and the Sun, or on the other side of the Sun (Ptolemy placed it inside the orbit of the Sun, after Mercury, but this was completely arbitrary; he could just as easily swapped Venus and Mercury and put them on the other side, or any combination of placements of Venus and Mercury, as long as they were always colinear with Earth and Sun). If that was the case, however, it would not appear to go through all phases, as was observed. If it was between Earth and Sun, it would always appear mostly dark, since the light from the sun would be falling mainly where we can't see it. On the other hand, if it was on the far side, we would only be able to see the lit side. Galileo saw it small and full, and later large and crescent. The only (reasonable) way to explain that is by having Venus orbit the Sun, or to use a Tycho Brahe type geocentric system.

Related Topics:
Galileo - Telescope - Crescent - Tycho Brahe

~ ~ ~ ~ ~ ~ ~ ~ ~ ~