After Galileo built his first telescope in 1609, he observed and described the landscape of the moon, discovered four of the moons of Jupiter, discovered the phases of Venus, and discovered sunspots. Galileo's discovery of sunspots supported the idea of heliocentrism, a model that places the sun at the center of the universe with the planets orbiting around it.
Galileo studied and later taught mathematics at the University of Pisa. He helped to describe ballistics and presented his "Principle of Inertia." In 1632, Galileo presented his concept of basic relativity in physics, which later became the basis of Einstein's special theory of relativity.
In 1632, Galileo published a book openly supporting heliocentrism. He was later forced to publicly denounce his findings.