This famous Italian astronomer believed in Copernicus’ view of a heliocentric universe. Galileo’s own observations of the stars and planets convinced him of it even more. But at the time such views were heretical and contrary to the account given in the Bible, and they brought Galileo into sharp conflict with the church, forcing him to recant his theory even though it was correct.
The Italian philosopher and cosmologist Giordano Bruno (1548–1600) was a follower of Copernicus. He deduced from Copernicus that the other planets in the solar system, like the Earth, were all worlds in their own right. He believed that if they orbited the Sun and were much the same size as the Earth, then it was a logical deduction that they were also inhabited. He also suggested that the stars were so far away they could, in fact, be distant suns. The Copernican system showed that our world was not unique in the universe. Bruno was not strictly a scientist nor an astronomer; he was simply someone who had reached the conclusion that the Earth may not be the only inhabited world as created by God. The Catholic Church had no doubt that Bruno was wrong in his assumption that other worlds could exist. In 1600 Giordano Bruno became a martyr to science when he was burned at the stake for refusing to renounce his opinions.
Galileo Galilei (1564–1642) was also a follower of Copernicus. Although he was not burned at the stake like Giordano Bruno, his beliefs brought him into conflict with the pope. Because of this he, too, will be remembered as someone who was persecuted for their scientific beliefs. Vincenzo Galilei, Galileo’s father, was a musician who moved from Florence to Pisa in about 1563, just before Galileo was born. When Galileo was about ten his family moved back to Florence. At the age of 17 we find Galileo back again in Pisa studying medicine at the university. Galileo therefore knew both Florence and Pisa very well. His interests did not lie in medicine, however; he was much more interested in mathematics and its application to physical science. The anecdotal story of Galileo watching the pendulum swing of the chandelier in the cathedral of Pisa is well known—he recognized that the period of the swing was constant and did not vary with the amplitude (the distance from one extremity of an oscillation to the middle point), and he deduced that the pendulum could therefore be used to regulate a clock.
In 1585 Galileo’s father experienced financial difficulties and as a result he could no longer support his eldest son at university. Galileo returned again from Pisa to Florence to help with the ailing family business, and he took work as a private tutor to add a little to the family finances. At this time he made a device called a hydrostatic balance, which could be used for measuring the specific gravity of bodies. It was based on the principle of Archimedes, the Greek philosopher from the third century BC and a man whom Galileo ranked far above Aristotle as a scientist. In 1589 Galileo returned again to Pisa; this time he came as professor of mathematics at the university. Although he had no formal qualifications for the job he was by this time well known, and he had frequently demonstrated his mathematical skills. He enjoyed his new profession and quickly immersed himself in the life of the university. As well as mathematics he taught astronomy, including the works of Ptolemy, and at the same time he was able to develop his interests in mechanics.
One of Galileo’s experiments involved rolling balls down inclined planes and then measuring the balls’ speeds as they passed various markers set up along the planes. The measurement of time was no easy matter; in the 16th century primitive watches existed, but there was no such thing as a stopwatch. In his youth, when timing the chandelier in the cathedral at Pisa, Galileo had used his own pulse to measure the time intervals. As a young man he measured small time intervals using a simple pendulum of his own—a simple device based on his observations of the one at Pisa. He was able to formulate the concepts of velocity and acceleration and to show that the speed of his rolling balls increased uniformly as they rolled down the inclined plane.
Galileo turned his thoughts to bodies in free fall. He reasoned that all bodies accelerated as they fell to Earth, but that they all fell at exactly the same rate. If, for example, a large object and a smaller, lighter object were dropped together from the same height, they would both strike the ground at much the same time. How could Galileo resist using the Leaning Tower of Pisa for his experiments? What better place could he have for testing his theories about falling bodies? He dropped a cannon ball and a musket ball from the same height from the highest level of the tower. We now know that the effects of air resistance would cause the heavier ball to reach the ground before the lighter one. So the question facing us is: did Galileo ever perform the experiment or is the story apocryphal? Viviani, Galileo’s first biographer, states that he repeated the experiment many times, and this seems in keeping with the truth, for it was his nature to experiment using different masses and other refinements.
Galileo never married, but he had two daughters and a son by a woman called Marina Gamba. His father died in 1591, and in that year he moved from Pisa to Padua university and soon afterward he moved from Padua to Venice.
It was during his time at Padua and Venice, in the first decade of the new century, that Galileo heard about a wonderful new instrument fashioned by a spectacle maker called Hans Lippershey (1570–1619), who had a practice at Middelburg in Holland. By peering through a tube containing lenses this instrument somehow made distant objects appear to be nearer and larger. This amazing device was the telescope. Soon Galileo had discovered how the telescope was constructed and was making his own version of the instrument, which he sold to merchants and others.
Galileo was pleased with his commercial success, but then went on to develop his instrument even further and to use it to examine the skies. He was not the first astronomer to use the telescope for this purpose, but he had a great flair for the instrument. He made his astronomical telescopes with two convex lenses. This required a longer tube but gave better results, even though the convex lenses produced an inverted image. He was soon making new discoveries.
Galileo turned his telescope toward the Moon, marveling at how close the telescope seemed to bring it. He saw craters and mountains and what he thought were seas. All these had been seen before but never with such brightness and detail. He looked at the planets, and on nearly every one of them he saw something new. By looking in the direction of Jupiter he could see four small spots of light near the planet. He observed Jupiter every night and discovered that the spots of light changed their positions. He had discovered the four largest moons of Jupiter, namely Io, Europa, Ganymede and Callisto. This in itself produced a problem for the geocentric traditionalists because it brought the number of bodies believed to be orbiting Earth to more than the sacred number seven. Then Galileo turned his telescope to look at Saturn. He discovered two strange companions, one on each side of the planet. What he was observing were the rings of Saturn, but the telescope was not powerful enough to resolve them properly. Over a period of time the companions grew smaller and disappeared, only to return again later in the year (an effect caused by the rings being seen edge on). When he observed the planet Mars he saw that it clearly displayed a disc. However, it was the planet Venus that offered the greatest surprise. Through the telescope Venus showed phases, just like the Moon. This observation more than any of the others convinced Galileo that the Copernican system was right; the phases of Venus exactly matched the motion of the planet around the Sun.
When Galileo turned his telescope to the stars he got another surprise. The stars still appeared as tiny spots of light—the telescope did not seem to bring them any nearer—but when he looked at the spaces between the stars, more and more stars appeared. When he trained his magic tube on the Milky Way he saw new stars appearing in their hundreds and thousands:
In order that you may see one or two proofs of the inconceivable manner in which they are crowded together, I have determined to make a case against two star-clusters, that from them as a specimen you may decide about the rest. As my first example I had determined to depict the entire constellation of Orion, but I was so overwhelmed by the vast quantity of stars and by want of time that I have deferred attempting this to another occasion, for there are adjacent to, or scattered among the old stars more than 500 new stars within the limits of one or two degrees … As a second example I have depicted the six stars in the constellation Taurus, called the Pleiades … near these lie more than forty others invisible to the naked eye, no one of which is more than half a degree off any of the aforesaid six, of these I have noticed only thirty-six in my diagram.
The universe obviously contained far more stars than anybody had ever imagined. The number of visible stars in the universe seemed to have increased a thousand-fold or more, just by the invention of the telescope. It seemed that it would take a hundred men a lifetime to catalog all of them.
Excited by his discoveries, Galileo wrote a small book called the Sidereus Nuncius, or Starry Messenger, in which he described them. The book appeared in print in 1610, and not surprisingly it came in for immediate criticism. The first salvo was fired in 1612 by a Dominican friar called Nicolo Lorini (fl. 1614) and the second in 1614 by another Dominican called Tommaso Caccini (1550–1618). The affair simmered for a few years, until Galileo was summoned to see the pope about his unconventional thoughts. In 1616 he was instructed very clearly to desist from putting forward his view that the Sun lay at the center of the universe. But Galileo knew that he was right. Instead of renouncing his ideas he simply gathered more evidence to support his case. The result was that Galileo’s Sidereus Nuncius was put, along with Copernicus’ De Revolutionibus Orbium Coelestium, on the list of prohibited books.
Undaunted by these setbacks Galileo started work on his next book. In this he had support from an old friend called Benedetto Castelli (1578–1643) who was appointed as the official mathematician to the pope and also had the approval and support of Cardinal Barberini (1568–1644). Galileo meant to call his book Dialogue on the Tides, but under pressure changed the title to Dialogue Concerning the Two Chief World Systems. The book’s subject was almost exactly what its title suggests, and was a trialogue with three main characters. One of the main characters was called Fillipo Salviati. He was a real person, an old friend of Galileo’s who had died in 1614 at the age of 31, and so could not be persecuted for his beliefs. In the book Salviati proposes Galileo’s views, putting forward the case for a heliocentric universe. He “uses” all the evidence that Galileo had collected from his study of Copernicus, from his own telescopic observations and from other sources. Salviati is, of course, essentially Galileo himself putting forward his own case for the system of the world—a heliocentric universe.
The second character is called Giovanni Francesco Sagredo. He was also a real person and another friend of Galileo’s. He, too, had died before the book was written. In Galileo’s Dialogue Sagredo does not hold strong opinions about the system of the world, but merely acts as a kind of mediator in the discussion between the two other parties. The third character is called Simplicio. He is the defender of the geocentric universe theory in which all the heavenly bodies revolved around the Earth. It was an unfortunate but quite deliberate choice of name by Galileo for it suggested a simpleton, and although Simplicio puts forward some very clever ideas and reasons for his beliefs, he is consistently defeated in his arguments by the better-educated and well-informed Salviati. It was but a small step to associate Simplicio with Pope Urban VIII, and Galileo must have realized that the pope would be offended by the parody presented in his character, but he still went ahead with the publication.
In Dialogue Salviati questions the shape of the Earth. He argues that it is a sphere just as the Sun and the Moon are spheres. Simplicio refutes the idea and quotes Aristotle as his authority:
It is vain to inquire as you inquire, as you do, what part of the globe of the Sun or Moon would do if separated from the whole, because what you inquire would be the consequence of impossibility. For, as Aristotle demonstrates, celestial bodies are invariant, impenetrable and unbreakable; hence such a case could never arise. And even if it should, and the separated part should return to the whole, it would not return thus because of being heavy or light, since Aristotle also proves that celestial bodies are neither heavy nor light.
This of course is exactly what Salviati, in the person of Galileo, wants to hear and he sets about deriding the attitude of the Greek scientists who were no more than armchair philosophers, too proud to take measurements or to seek out the truth for themselves.
The Dialogue was published in 1632. Galileo had no trouble getting his book past the Florentine censors, but when it reached Rome there was a sudden turn of the tide against him. Galileo knew he had enemies, but he thought that Barberini was on his side. Barberini, however, had become Pope Urban VIII in the 16 years since Galileo started writing his book, and now found the papal stance on the issue of what lay at the center of the universe ridiculed. Thus, although Galileo had gone to great lengths to obtain approval before publishing his views, he nevertheless found that his opponents were determined to trump up a charge against him. In September 1632 the holy office put Galileo on trial for heresy. He knew what had happened to Giordano Bruno and that he would face the Inquisition. He was found guilty of teaching the philosophy that the Earth moved. He was forced to read out a long and humiliating recantation of his views in the hall of the convent of Santa Maria Sopra Minerva before the entire congregation of the holy office. There may be no truth in the story that as he left the Inquisition the dispirited Galileo murmured “Eppur si muove” (And yet it moves), but the story is entirely in keeping with his views, and in the months that followed he must have said the words many times over to himself.
Galileo’s trial for heresy proved to be just as disastrous for his accusers, the Catholic Church. The church was expected to defend the version of creation as told in Genesis, and the trial of Galileo was the first occasion on which anything so profound had ever challenged the literal truth of the story told in the Bible. History shows that Galileo was not guilty of heresy, but merely seeking the truth, and many years later the Vatican offered a long-overdue apology.
Theologians had always argued and debated about the interpretation of the gospels, and this frequently led to the formation of new sects and religious orders. This was particularly true in the 16th century when the Protestants made the break with Rome. After this had taken place, astronomers in Protestant countries could build upon the work of Galileo without fear of persecution. Other evidence was waiting to undermine the church’s beliefs, however. For example, when geologists first began to challenge the age of the Earth from the dating of the rocks, and later in the 19th century when Charles Darwin first published his theory of evolution.