What did Galilieo use his telescope to prove?

Answer 1

Galileo was the first person to study the night sky with a telescope, record the results and draw scientific conclusions, some of which were of fundamental importance.

There is no evidence that he set out to prove anything, but two of his discoveries raised serious doubts about the accuracy of the ancient theory of the planets' movements among the stars, the Ptolemaic theory, which was based on the idea of the Earth being fixed at the centre, with the Sun and planets moving round it in paths based on combinations of circles.

Galileo's discovery of the four large moons of Jupiter was important because they were clearly in orbit round Jupiter and not the Earth, so they became the first objects found that did not orbit the Earth.

His other discovery with implications for planetary theory was the discovery of the full range of phases of Venus. In Ptolemy's theory Venus's orbit passes inside the Sun's, so Venus's phase must be a crescent, or half-phase at the most. Galileo's discovery of an additional gibbous phase was important because the Ptolemaic theory could not account for it.

Some time before Galileo came on the scene, Nicholas Copernicus published an alternative theory of the planets in1543 which used the circle and epicycles from Ptolemy's theory but placed the Sun at the centre instead of the Earth. This simplified the planets' paths, but its importance to Galileo was that it allowed Venus to pass behind the Sun so that it could have a gibbous phase. That caused Galileo to become convinced that Copernicus must be right.

Meanwhile in Denmark Tycho Brahe made a famous new set of measurements of the planets' positions using new highly accurate equipment of his own design. These measurements were taken by his mathematical assistant Johannes Kepler and used to produce a new theory in 1609, which retained the Copernican idea of having the Sun at the centre. In all other respects the new theory was novel in that Kepler had the planets in elliptical orbits and all the old circles and epicycles were thrown out.

Galieo did not accept Kepler's model and he continued to promote the Copernican theory aggressively. Eventually he was charged with heresy for re-interpreting The Bible, and he was found guilty and recanted. However in modern times Kepler's model is generally accepted as right, subject to minor corrections from relativity.