Galileo was born on 15 February 1564 in Pisa, Italy. When he was eight years old his family returned to Florence, his father’s hometown. Galileo, however, remained in Pisa with a family member. Two years later he rejoined his family in Florence. At the age of eleven he went to school at the monastery at Vallombrosa (1038). When he was seventeen years old he enrolled at the University of Pisa (1343) where he studied medicine before turning to mathematics and physics.
He was, however, still officially enrolled as a medical student at Pisa and in 1585 he left without completing his degree. Galileo began teaching mathematics and in 1589 he was appointed to the chair of mathematics in Pisa. In 1592 he became professor of mathematics at the University of Padua (est.1222) beginning a period of eighteen years at the university. At Padua his duties were mainly to teach geometry and standard (geocentric) astronomy. However, Galileo argued against Aristotle’s view of astronomy in three public lectures he gave in connection with the appearance of a new star (now known as ‘Kepler’s supernova’) in 1604.
The use of lenses for eyeglasses began in the thirteenth century. Leonard Digges (c.1515-59) and his son Thomas Digges (c.1546-1595) living in England in the 1570s, may have made an instrument consisting of a convex lens and a mirror, but the first documentary evidence of such an instrument identifies the maker as Hans Lippershey (1570-1619) who presented it to Dutch authorities in September 1608. In 1609 Galileo received a letter telling him about a spyglass that a Dutchman had shown in Venice. From these reports, Galileo began to make a series of telescopes whose optical performance was much better than that of the Dutch instrument.
When Galileo first turned his telescope to look at the night sky he saw that the planets were proportionately enlarged as expected but the stars were not. He observed the Milky Way, previously believed to be nebulous, and found it to be a multitude of stars packed so densely that from Earth they appeared to be clouds. He located many other stars too distant to be visible with the naked eye. He also saw that the Moon’s surface was irregular, with mountains like those on Earth and was therefore not a perfect sphere as Aristotle had claimed.
On 7 January 1610 he pointed his telescope at Jupiter and saw what he thought at the time were three stars, strung out in a straight line through the planet. Observations made on subsequent nights showed that the positions of these ‘stars’ relative to Jupiter were changing in a way that was inexplicable if they had really been fixed stars. He concluded that they were moons orbiting Jupiter (Io, Europa and Callisto). On 13 January he discovered a fourth moon of Jupiter (Ganymede). This evidence proved that everything did not revolve around Earth. In March 1610 he published these early telescopic observations in a short treatise entitled Sidereus Nuncius (‘Starry Messenger’).
Galileo, with an eye to getting a position in Florence, named the four moons of Jupiter the Medicean stars. In June 1610 he resigned his post at Padua and became chief mathematician at Pisa and mathematician and philosopher to the Grand Duke of Tuscany Cosimo II de Medici (30; r.1609-21).
In 1612, having determined the orbital periods of Jupiter’s moons, Galileo proposed that with sufficiently accurate knowledge of their orbits one could use their positions as a universal clock, and this would make possible the determination of longitude. He worked on this problem from time to time during the remainder of his life; but the practical problems were severe.
Galileo first turned his telescope on Saturn on 25 July 1610 and it appeared as three bodies – his telescope was not good enough to show the rings but made them appear as lobes on either side of the planet. Continued observations puzzled Galileo as the bodies on either side of Saturn vanished when the ring system was edge on. The rings reappeared when he observed the planet in 1616, further confusing him.
Ptolemaic astronomy implied that Venus was always between Earth and the Sun and so an observer on Earth should never see the planet fully lit. The heliocentric (and the Tychonic) model predicted that all its phases would be visible since the orbit of Venus around the Sun would cause its illuminated hemisphere to face Earth when it was on the far side of the Sun and to face away from Earth when it was Earth-side of the Sun. From September 1610 Galileo observed Venus exhibiting a full set of phases similar to that of the Moon.
Sunspots can be seen with the naked eye but only when the conditions are suitable. The first observation of a sunspot through a telescope was made in 1610 by Thomas Harriot (c.1560-1621). Christoph Scheiner (1573/75-1650) believed the spots were satellites of the Sun. In 1611 Johann Fabricius (1587-1616) argued, from observing the change in the positions of sunspots, that the sunspots were on the surface of the Sun and that the Sun was therefore rotating. In his Letters on Sunspots (1613) Galileo argued that the sunspots were on the surface of the rotating Sun. The very existence of sunspots presented yet another difficulty with the assumption of unchanging perfection of the heavens in Aristotelian philosophy.
Galileo’s treatise on sunspots, in which he openly adhered to the Copernican theory, got him into trouble with the Catholic Church. A committee of consultants declared to the Roman Inquisition (1542-1860) that the proposition that the Sun is the centre of the Universe was a heresy. In February 1616 on the order of Pope Paul V (70; r.1605-21), Cardinal Maffeo Barberini called Galileo to his residence and administered a warning to him not to hold or defend the Copernican theory.
When Barberini became Pope Urban VIII (76, r.1623-44) he assured Galileo that he could write about Copernican theory as long as he treated it as a mathematical proposition. However, with the printing of Galileo’s book Dialogue Concerning the Two Chief World Systems (1632), in which the relative merits of Ptolemy and Copernicus are discussed, Galileo was called to Rome in 1633 to face the Inquisition again. Galileo was found guilty of heresy and was ordered to be imprisoned. After a period with the friendly Archbishop of Siena in Tuscany, Ascanio Piccolomini (81; r.1629-1671), Galileo was allowed to return to his villa at Arcetri, where he spent the remainder of his life under house arrest. In 1638 Galileo became totally blind and died on 8 January 1642.
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