Galileo

Galileo Galilei lived between the years 1564 AD and 1642 AD. Known by his first name, Galileo is considered to be one of the greatest scientists of all times.His work on inertia and the constant acceleration of falling bodies is well known, and contributed to the later success of Sir Isaac Newton's development of his three Laws of Motion. His observation of the temporal constancy of a pendulum's swings led to the development of the first accurate clock. Galileo is considered by many historians to be the first true experimental scientist. Galileo's conclusions regarding various aspects of nature came from unbiased observations of that nature, and not from uncritical acceptance of Aristotle's and other's unsubstantiated philosophical musings on that nature.Galileo's work in Astronomy cannot be underestimated. After he made his own telescope (based on a Dutchdesign), Galileo looked at various heavenly bodies. He noticed that the Sun had spots on it, and the Moon had mountains and valleys, both indications of their "imperfections". He was the first to observe that Venus' illumination exhibited phases, like the Moon, an indication that it must be a planet circling the Sun interior to Earth's orbit. Galileo was also the first person to observe the four main moons of Jupiter (Io, Callisto, Europa and Ganymede), now known as the Galilean Moons or satellites of Jupiter.Galileo barely missed discovering the rings of Saturn, and the (at that time unknown) planet Neptune.

This body of astronomical observations ineluctably led Galileo to the conclusion that Copernicus was right, that the Sun was the center of the Solar System. For publishing this viewpoint, he spent much of the latter years of his life under house arrest.

In the image below, we see how a full variation of phases of Venus' illumination can only be explained by a heliocentric model of the Solar System. Click on the image to view it in its original size.