What two discoveries did Galileo make that support a Heliocentric system?

Answer 1

The most important discovery Galileo made to support the heliocentric model was his telescopic observation of the phases of Venus. From Earth, Venus appears in phases like those of the moon's - new, waxing crescent, waxing gibbous, full, waning gibbous, and waning crescent. In the Ptolemaic model Venus is never on the far side of the sun from Earth, and so it would never display the gibbous or full phases. On his first observation to check for Venusian phases he saw a gibbous Venus at once invalidating the Ptolemaic model. Over several months he continued his observations and saw Venus changing phase to half, and waning crescent confirming that Venus orbited the sun.

Answer 2

Galileo observed the orbital pattern of Venus and its phases, which reinforced the heliocentric model. He was the first to do so because he invented a telescope that allowed him to view the planets.

Answer 3

Galileo made discoveries with a telescope that raised doubts about the ancient geocentric model of the planets devised by Ptolemy.

Galileo discovered the moons of Jupiter, the first objects to be discovered that obviously did not orbit the Earth. Secondly he discovered the full range of Venus's phases, including a gibbous phase (more than half) which showed that the Ptolemaic model cannot be totally right.

Galileo assumed that this failure of the Ptolemaic system demonstrated that the heliocentric system of Copernicus must be correct. However Tycho produced a third model that was geocentric, and it allowed the full range of Venus's phases. So Galileo's discoveries were not conclusive.

The models by Ptolemy, Copernicus and Tycho used combinations of circles and epicycles to describe the planets' positions.

The final model devised by Kepler had the Sun at the centre but it abandoned circle and epicycles and instead had the planets in elliptical orbits. This model was validated by Newton's later theoretical discoveries and also by more and more accurate measurements of the planets' positions in later centuries.

Answer 4

He made no discoveries that directly support the heliocentric theory, but his discovery of the moons of Jupiter, circling Jupiter and not the Earth, opened the way to questions about what else might not be orbiting the Earth, like the Sun, for example. Some one hundred years after Galileo's time it was found that the Sun is in fact at the centre, but in Galileo's time science did not possess the knowledge to settle that question.

Answer 5

None, but Galileo's discoveries demonstrated faults with the old Ptolemaic system. These faults were removed in Tycho Brahe's model, which was also geocentric. However the heliocentric theory of Kepler, using elliptical orbits for the planets, was eventually accepted by everyone, but that happened many years after Galileo's lifetime.