What observations did Galileo make that made more people think again about the geocentric version?

Answer 1

Which observations showed Galileo that the Earth-centered theory was wrong?

A) The presence of four Galilean moons around Jupiter

B) The phases of Venus

C) The phases of the Moon

D) The fact that Earth rotates from the west to the east

E) Newton's laws of gravity are incompatible with the Earth-centered theory

Answer 2

Galileo's discovery of four moons orbiting round Jupiter were the first objects to be found that clearly did not orbit round the Earth, so it opened the question up.

His discovery of Venus's gibbous phase as is it emerges from behind the Sun to become the "Evening Star" every 18-20 months (as we now know) was a direct indication that the Ptolemaic system that had stood for 1400 years was incorrect. In the Ptolemaic system Venus passes between the Earth and the Sun at both inferior and superior conjunction, and therefore it must appear as a crescent both times.

However he was wrong in his claim that this proved that the Copernican system, with the Sun at the centre, must be right. Tycho's alternative system had the planets Mercury and Venus orbiting the Sun and then the Sun and the other planets orbiting the Earth. This allowed the full range of Venus's phases as shown in Galileo's telescope, while still preserving the Earth's place at the centre.

All these three early systems were rejected in favour of Kepler's model of 1609, with the Sun at the centre, when it was shown by Newton in the late 1600s that planets moving under the force of gravity must obey Kepler's three laws of planetary motion.

Answer 3

Galileo observed the phases of Venus, which showed that Venus must orbit the Sun and not the Earth. He also discovered the four largest moons of Jupiter, which suggested that Earth was not the center of all orbital motion and that objects could orbit something other than Earth. These observations strongly supported the heliocentric model proposed by Copernicus.