How did Galileo 's observation of Jupiter and Venus support Copernicus' model?

Answer 1

Galileo's discoveries with the telescope sowed doubts about the ancient Ptolemaic model which had the Earth at the centre. First, Jupiter's moons orbited round Jupiter and not the Earth, the first objects that had been discovered that were not orbiting the Earth. Secondly the discovery of the phases of Venus raised doubt about the Ptolemaic model, particularly the gibbous phase when, we now know, Venus is round behind the Sun as seen from Earth. In the Ptolemaic model Venus never goes behind

the Sun (as seen from Earth) and the gibbous phase cannot happen.

By implying in the title of his book 'Dialogue of the two world systems' that there were only two models in contention, those of Ptolemy and Copernicus, Galielo insinuated that if the Ptolemaic model was incorrect the Copernican system must be correct. This was logically flawed because at the time there were two other systems in the public domain that both explained Venus's phases. These were the models of Tycho, which was geocentric, and finally Kepler's which was heliocentric.

Of these four, Kepler's was eventually accepted by everyone after Newton's theoretical discoveries showed that Kepler's elliptical orbits for the planets agreed with the new theory of dynamics.