Galileo Galilei, the famous Italian physicist, was born in Pisa on February 15, 1564, and died at Arcetri, near Florence on January 8, 1642. He uncovered the crucial evidence for the Earth's revolution about the Sun, and made many other crucial scientific discoveries.

Galileo's early life

Galileo's father, Vincenzo Galilei, was a financially troubled musician who made little impact on the world; but was of noble birth. This saved Galileo, and the five younger children of Vincenzo, from extreme poverty. One advantage Galileo had was that the family, although poor, lived in the centre of things. They moved from Pisa to the equally glorious city of Florence in the early 1570s.

Galileo's pendulum

The intellectual life of Galileo began to take off when he returned to his birthplace and enrolled as a student at the University of Pisa in 1581. His first magnificent experiment was performed during a very boring cathedral service. 

Galileo was staring at the ceiling, hoping for the sermon to end. when he spotted a violently swinging chandelier. He decided to experiment, and measured the duration of the chandelier's swing using the beat of his own heart. As Galileo expected, the swing slowed down and became gentle. But Galileo was surprised to find that the time it took to perform one swing remained the same.

Intrigued, Galileo began experimenting with pendulums at home. He found that found that the duration of swing was dependent on the string's length (and nothing else). This eventually led to Galileo's invention of the pendulum timer--effectively, the first pendulum clock.

Galileo's falling objects

Shortly after his pendulum experiments, Galileo turned his attention to freely falling objects and provided evidence against Aristotle's ideas about objects falling under gravity. The story is that Galileo dropped different objects from the leaning tower of Pisa and found they accelerated toward the ground at the same rate (9.8 ms^-2). This meant, of course, that they hit the ground at the same time when dropped together. This story may not be true, but Galileo certainly rolled cannon balls down wooden slopes and obtained a similar result.

• No one had made Galileo's observation before because lighter objects are very often seen to fall more slowly than heavier objects. Try dropping a book and a piece of paper and see if they hit the ground at the same time! A lead weight falls faster than a feather because air resistance has less effect upon it. In a dramatic demonstration of Galileo's principle, astronaut David R. Scott dropped a lead weight and a feather on the moon, and the lead weight and feather were seen to hit the ground simultaneously.

Galileo's family

Galileo was appointed professor of mathematics at the University of Padua in 1592. Around this time he had three children (out of wedlock) with Marina Gamba, a woman he had met on one of his many trips to Venice. His two daughters, Livia and Virginia, were placed in convents.

Galileo's telescope

In 1610 Galileo moved to the Court of the Medici in Florence, invited there because of the discoveries he was making with a new invention--the telescope.

Hans Lippershey, a Flemish spectacle maker, patented the first telescope in 1608. But within a few months, Galileo constructed his own - and his had a magnification of X60, as opposed to his rivals X10. He turned this impressive instrument on the Moon. Galileo found the moon to be "full of vast protuberances, deep chasms and sinuosities". It was far from the perfect sphere described by the Greeks. The imperfections of another heavenly body were shown up when Galileo's telescope revealed spots on the Sun.

In a series of observations in 1609, Galileo spotted four moons in orbit about Jupiter. This was the first time anyone had seen objects circling a heavenly body. Galileo had revealed that not everything had to orbit the earth, as Aristotle and Ptolemy had decreed.

Galileo and the sun-centred solar system

Both the geocentric and heliocentric theories predicted that Venus should have phases like the moon (e.g. full Venus, half Venus, crescent Venus). But, as Copernicus had shown, the phases would be different depending on whether the geocentric or heliocentric theory applied. Galileo's observations of the phases of Venus agreed with the predictions of the Copernican model of the solar system.

Around this time, Johannes Kepler improved Copernicus’s theory. The predicted motions of the planets now fully agreed with the observations that Galileo and others were making.

Galileo presented his heliocentric world view in a popular book entitled Dialogue Concerning the Two Chief World Systems. This is a dialogue between Salviati, (a heliocentrist) and Simplicio (a geocentrist). Another character, Sagredo, acts as moderator. Over the course of four days, they discuss the Ptolemaic and Copernican views of the universe. Simplicio is shown to be a buffoon, and is mocked by Sagredo (who sides with Salviati). Simplicio's pronouncements were similar to those made by the Pope at that time.

Galileo and the inquisition

Soon after publication of the Dialogue, Galileo was brought to trial by the Inquisition. They said Galileo was guilty of heresy for suggesting that the Earth moved around the Sun. The religious authorities of the time took a literal view of the Bible, and the Bible suggests that the Earth does not move. Galileo was sentenced to indefinite house arrest, and forced to deny the truth of his arguments to avoid execution. But after sentencing he couldn't resist muttering, "And yet it moves!".