Galileo Galilei

Prior to Galilei (1564–1642) all natural sciences were based only on direct observations and pure thinking. He changed the methodology of observations by using tools like a telescope or microscope and he also created artificial situations in experiments to investigate certain phenomena (e.g. using an inclined plane to simulate and measure the speed of falling objects).

In 1611 the scholars of the church decided to accepted the fact that indirect measuring methods, like observations made by using a telescope, produce valid results.

Galilei's most significant observations with the telescope were confirming the Copernican helio-centric concept and hence challenged the church:

• The discovery that moons orbit Jupiter, which meant that a moving planet (Jupiter) can have satellites moving around it. This made it plausible that also the Earth (which was obviously orbited by our moon) could actually move as well. Prior to this discovery a steady earth was assumed as the only possible option to explain the fixed orbit of our moon around the earth. Those observations were only explained later by Newton's law of universal gravitation.
• The discovery of hills and valleys on the moon. Prior to these findings all of heaven was assumed   unchangeable and of ideal divine nature. Comets and meteors were assumed local weather phenomena on Earth (that's were the word Meteorology has got it's origin from). The observation of topography on the moon's surface questioned the dichotomy between earthly and heavenly objects, meaning earth was not unique, but a celestial object like any other.
• The most important discoveries were the phases of venus. Galilei saw that Venus displayed phases, just like the Moon, and that the phases of Venus were only understandable if Venus orbited the Sun. The observed phases were, in fact, impossible in a Ptolemic geocentric system. 

Galileo Galilei therefore came to the conclusion, that the earth could not be the centre of the universe. Later he withdrew from supporting the new copernican model to escape a more severe punishment from the inquisition. In 1633 he was sentenced to house arrest in which he remained for the rest of his life.

Galileo allegedly muttered the rebellious phrase "And yet it moves", but there is no evidence that he actually said this or anything similar. It might have been Giordano Bruno who said these words when being burned at the stick as a heretic in February 1600.

Galilei's experiments around motion and gravity together with the work of Johannes Kepler (1571–1630, three Kepler's laws) and René Descartes (1596–1650) formed the basis for the laws of classical mechanics, for which Isaac Newton (1642–1726) is renowned.

(For further information refer to Wikipedia, Wikiquote or this article in a German online magazine.)