He was an Italian philosopher, astronomer, and mathematician
who made fundamental contributions to the sciences of motion, astronomy, and
strength of materials and to the development of the scientific method. His
formulation of (circular) inertia, the law of falling bodies, and parabolic
trajectories marked the beginning of a fundamental change in the study of
motion. His insistence that the book of nature was written in the language of
mathematics changed natural philosophy from a verbal, qualitative account to a
mathematical one in which experimentation became a recognized method for
discovering the facts of nature. Finally, his discoveries with the telescope
revolutionized astronomy and paved the way for the acceptance of the Copernican
heliocentric system, but his advocacy of that system eventually resulted in an
Inquisition process against him. The heliocentric system is a cosmological model in which the Sun is assumed to lie at or near a central point (e.g., of the solar system or of the universe) while the Earth and other bodies revolve around it. In the 5th century BC the Greek philosophers Philolaus and Hicetas speculated separately that the Earth was a sphere revolving daily around some mystical “central fire” that regulated the universe. Two centuries later, Aristarchus of Samos extended this idea by proposing that the Earth and other planets moved around a definite central object, which he believed to be the Sun.
The heliocentric, or Sun-centerd, model of the solar system never gained wide support because its proponents could not explain why the relative positions of the stars seemed to remain the same despite the Earth's changing viewpoints as it moved around the Sun. In the 2nd century AD, Claudius Ptolemy of Alexandria suggested that this discrepancy could be resolved if it were assumed that the Earth was fixed in position, with the Sun and other bodies revolving around it. As a result, Ptolemy's geocentric (Earth-centerd) system dominated scientific thought for some 1,400 years.
In 1444 Nicholas of Cusa again argued for the rotation of the Earth and of other heavenly bodies, but it was not until the publication of Nicolaus Copernicus's De revolutionibus orbium coelestium libri VI (“Six Books Concerning the Revolutions of the Heavenly Orbs”) in 1543 that the heliocentric system began to be reestablished. Galilei's support of this model resulted in his famous trial before the Spanish Inquisition in 1633.
He was put under house arrest after being convicted of heresy for his beliefs and died of natural causes. (Adapted from Britannica Encyclopedia)