Galileo Galilei (15 February 1564–8 January 1642) was an Italian astronomer, mathematician, and natural philosopher whose work helped transform astronomy and physics during the Scientific Revolution. His telescopic discoveries challenged established accounts of the heavens, while his mathematical investigations of motion contributed to the development of classical mechanics. His advocacy of a moving Earth also brought him into conflict with Catholic authorities. (plato.stanford.edu)
Life and career
Galileo was born in Pisa to Vincenzo Galilei, a musician and music theorist, and Giulia Ammannati. He studied at the University of Pisa, initially pursuing medicine before turning toward mathematics, and left without obtaining a degree. He held Pisa’s mathematics chair from 1589 to 1592, then taught at the University of Padua until 1610. These appointments provided the setting for much of his early research into motion and practical instruments. (catalogue.museogalileo.it)
At Padua, Galileo developed a geometric and military compass, a calculating instrument used for mathematical and practical problems, and constructed a thermoscope, a predecessor of the thermometer. In 1610 he became mathematician and philosopher to the grand duke of Tuscany, moving to Florence. His connection with the Medici family linked scientific investigation to court patronage. In 1611 he joined the Accademia dei Lincei, an early scientific academy in Rome. (catalogue.museogalileo.it)
Telescopic discoveries
In 1609 Galileo learned of the recently invented telescope and began constructing improved versions. He did not invent the original instrument. His achievement lay in improving its performance and making sustained astronomical observations. His telescopes combined a convex objective lens with a concave eyepiece, producing upright magnified images but a narrow field of view. Their usefulness depended on careful workmanship as well as observing skill. (galileo.library.rice.edu)
Galileo observed mountains and depressions on the Moon, contradicting the view that celestial bodies were perfectly smooth. In January 1610 he identified four satellites orbiting Jupiter, demonstrating that not every celestial body circled Earth. These satellites are now called the Galilean moons. He also resolved portions of the Milky Way into numerous individual stars. He announced major discoveries in Sidereus Nuncius (Starry Messenger), published in March 1610. (science.nasa.gov)
Later in 1610, Galileo observed the phases of Venus. Their full sequence showed that Venus travelled around the Sun and was incompatible with the traditional arrangement associated with Ptolemy. However, the observations did not uniquely establish heliocentrism: Tycho Brahe’s system, in which the planets orbited the Sun while the Sun orbited a stationary Earth, also accommodated them. Galileo nevertheless interpreted the discovery as support for Nicolaus Copernicus. (catalogue.museogalileo.it)
Motion and mathematical explanation
Galileo investigated motion through mathematical reasoning and experiments. His published account of inclined-plane experiments described rolling a bronze ball down a grooved board and measuring elapsed time with flowing water. Slowing the descent made its changing speed easier to investigate. He established that, for uniformly accelerated motion starting from rest, distance increases in proportion to the square of elapsed time rather than simply to time itself. (galileo.library.rice.edu)
His account of falling bodies replaced important features of the physics inherited from Aristotle. In modern notation, idealized free fall from rest is expressed as , where is distance, is time, and is gravitational acceleration. He also showed that combining uniform horizontal motion with uniformly accelerated vertical motion produces a parabolic projectile trajectory. These treatments abstracted from complications such as air resistance. (it.wikisource.org)
Galileo’s discussions of motion included thought experiments. His example of a stone falling from the mast of a uniformly moving ship explained why shared horizontal motion could persist during a fall. Consequently, the apparent stability of objects on Earth did not by itself establish that Earth was motionless. This reasoning contributed to the principle commonly called Galilean relativity. (plato.stanford.edu)
Copernicanism and the trial
Galileo’s support for Copernican astronomy raised questions about the interpretation of Scripture. In 1616 Cardinal Robert Bellarmine admonished him against teaching or defending the Copernican position. His Dialogue Concerning the Two Chief World Systems, published in 1632, compared the Ptolemaic and Copernican systems through conversations among three speakers and presented arguments favouring Earth’s motion. Publication was followed by an order to appear before the Roman Inquisition. (museogalileo.it)
On 22 June 1633, Galileo was condemned on grounds of vehement suspicion of heresy and required to abjure. His punishment became house arrest. After a period in Siena, he returned to his villa at Arcetri near Florence in December 1633, where his contacts and activities remained restricted. The proceedings concerned both Copernican doctrine and his compliance with the earlier prohibition. (museogalileo.it)
Later work
Despite confinement, Galileo completed Discourses and Mathematical Demonstrations Relating to Two New Sciences, published in Leiden in 1638. Its two principal subjects were the strength of materials and motion. Alongside falling bodies and projectiles, it examined how changes in size affect structural strength: enlarging an object geometrically increases its weight more rapidly than the cross-sectional area supporting it. This analysis also explained why larger animals require differently proportioned supporting structures rather than merely enlarged versions of smaller ones. (galileo.ou.edu)
By 1638 Galileo was blind. He died at his Arcetri villa on 8 January 1642. His final book preserved investigations developed over decades, making his mechanics available beyond the circumstances of his condemnation. (catalogue.museogalileo.it)