Galileo Galilei — Astronomer, Physicist, and Father of Modern Science

Galileo di Vincenzo Bonaiuti de' Galilei was born in Pisa, then part of the Duchy of Florence, the first of six children. His father Vincenzo Galilei was a leading lutenist, composer, and music theorist who was part of the Florentine Camerata. Galileo was initially educated at the Vallombrosa Abbey and seriously considered the priesthood as a young man.

At his father's urging, Galileo enrolled at the University of Pisa to study medicine. While attending a mass in the cathedral, he noticed a swinging chandelier and discovered the isochronism of the pendulum — that the time of each swing was the same regardless of the arc. He later experimented with pendulums to confirm this principle. After accidentally attending a geometry lecture, he persuaded his father to let him study mathematics and natural philosophy instead of medicine.

Galileo published a small book on the design of a hydrostatic balance he had invented, which first brought him to the attention of the scholarly world. He also created a thermoscope, a forerunner of the thermometer. In 1588, he obtained a teaching position at the Accademia delle Arti del Disegno in Florence, instructing in perspective and chiaroscuro.

Galileo was appointed to the chair of mathematics at the University of Pisa at age 25. In 1591, his father died, leaving Galileo responsible for his younger brother. During this period, he conducted experiments on falling bodies, reportedly dropping objects from the Leaning Tower of Pisa to demonstrate that objects of different weights fall at the same speed.

Galileo moved to the University of Padua, where he taught geometry, mechanics, and astronomy until 1610. This period was the most productive of his career. He made significant discoveries in both pure fundamental science and practical applied science, including securing a patent for a horse-powered water pump in 1594. He also fathered three children out of wedlock with Marina Gamba.

Galileo observed and discussed Kepler's Supernova (SN 1604). Since the new star displayed no detectable diurnal parallax, Galileo concluded that it was a distant star, disproving the Aristotelian belief in the immutability of the heavens. This was his first major challenge to the established cosmology.

Based on descriptions of the first practical telescope invented in the Netherlands, Galileo built an improved instrument with about 3x magnification, then versions up to 30x magnification. He demonstrated an 8x or 9x telescope to Venetian lawmakers. Telescopes became a profitable sideline, sold to merchants for use at sea. He published his first telescopic astronomical observations in the "Sidereus Nuncius" (Starry Messenger) in March 1610.

Galileo observed three small "stars" near Jupiter, and over subsequent nights tracked their motion. He concluded they were moons orbiting Jupiter — the first discovery of celestial bodies orbiting another planet. This provided powerful evidence for the Copernican heliocentric model, as it showed that not everything orbited Earth. He named them the "Medicean Stars" after his patron, Cosimo II de' Medici. They are now known as the Galilean moons: Io, Europa, Ganymede, and Callisto.

Galileo observed that Venus exhibits a full set of phases similar to the Moon, from crescent to gibbous to full. This was impossible under the Ptolemaic geocentric model, which predicted Venus could only show crescent and new phases. The observation provided conclusive evidence for the Copernican heliocentric model and caused most astronomers to abandon the Ptolemaic system.

Galileo made telescopic studies of sunspots, proving that the heavens were not perfect and unchanging as Aristotle taught. The apparent annual variation in their trajectories provided additional arguments against the Ptolemaic and Tychonic systems. He also observed Saturn's rings, initially mistaking them for planets, and observed the Milky Way, finding it to be a multitude of stars.

The Roman Inquisition investigated Galileo's advocacy of Copernican heliocentrism. Cardinal Bellarmine informed Galileo that he could not defend or hold Copernican theory as fact, but only as a mathematical hypothesis. The Inquisition concluded that the doctrine that the Earth moves was "foolish and absurd in philosophy, and formally heretical."

After years of cautious work, Galileo published his masterpiece, "Dialogue Concerning the Two Chief World Systems," which compared the Copernican and Ptolemaic systems. Written in Italian rather than Latin, the book reached a wide audience. The character Simplicio, who defended the geocentric view, was widely seen as a caricature of Pope Urban VIII, deeply offending the pope who had previously supported Galileo.

Galileo was tried by the Roman Inquisition, found "vehemently suspect of heresy," and forced to recant his Copernican views. Under threat of torture, he knelt and declared that the Earth did not move. According to legend, after recanting he muttered "E pur si muove" ("And yet it moves"). He was sentenced to house arrest for the remainder of his life. His "Dialogue" was banned.

While under house arrest at his villa in Arcetri, Galileo published "Discourses and Mathematical Demonstrations Relating to Two New Sciences," summarizing his life's work on kinematics and the strength of materials. The book was smuggled out of Italy and published in Leiden, Netherlands. It became the foundation of modern physics and was studied by Newton and later scientists.

Galileo died at age 77 in Arcetri, still under house arrest. The Pope refused permission for a public monument, and he was buried discreetly in the Basilica of Santa Croce in Florence. In 1737, his remains were transferred to a monumental tomb opposite Michelangelo's. The Catholic Church formally acknowledged its error in 1992. Galileo is remembered as the father of modern science, observational astronomy, and the scientific method.