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Geocentric and heliocentric phases of Venus

[Opera Laboratori Fiorentini]

In Ptolemy's system, Venus moved along an epicycle whose centre laid on the straight line that joined the Earth to the Sun. Moreover, Venus' epicycle always remained below the Sun. In such geometrical structure, the planet would have always looked like a very thin crescent. On the contrary, in Copernicus' system, Venus moved along an heliocentric orbit. In such geometrical structure, just like the Moon, the planet would have to display a complete series of phases. Here one can observe the Ptolemaic and the Copernican phases of Venus looking at the images of the planet reflecting in two mirrors. Galileo observed with the telescope exactly a complete series of phases, finding out that the Copernican system was the true one.

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