Galileo Galilei is one of the central figures of the 17th century’s Scientific Revolution and of the history of science as a whole. He was an astronomer, physicist, engineer, and inventor. His many discoveries include the four largest moons of Jupiter (now commonly called the “Galilean moons”), the phases of Venus, the mountains on our Moon, and the rings of Saturn. He also invented the telescope he used to make these discoveries.

He also made significant discoveries in mechanical physics. He determined that objects of different weights fall at the same speed. He also conducted many experiments with pendulums and discovered that a pendulum has the same period (the time it takes to return to where it started) regardless of its amplitude (how high or low it starts). Galileo conducted physical experiments, recorded what he observed, and drew conclusions from those observations. In his “Letter,” he ardently defends direct observation and extrapolating theories from facts.

He was born in Pisa, Italy, in 1564. He studied mathematics and worked as a private tutor before becoming the chair of mathematics at the University of Padua. In Padua, he published his first scientific treatises and soon became a member of the Duke of Florence’s court.