Galileo Galilei 1564-1642 /

A mathematical scientist . . .A Tuscan family ... Galileo's law of fall ... The pendulum .The basis of ballistics ... Nicolaus Copernicus and Tycho Brahe ... Galileo raises his telescope ... and makes enemies ... Trial by the Inquisition ... An Jctive old age ... PERSPECTlVE •••

Academies of Il1arning ... Opposition'to new ideas ... Instruments on land and sea ... Astrology - a science-art

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"He hath first over thrown all former astronomy ... and next all astrol- ogy . " So reported the English ambassador at Venice, when he learnt of the discoveries published three days earlier by an obscure professor of mathematics at the nearby University of Padua, Galileo Galilei. Never before, and seldom since, has scientific news caused such a stir as Galileo's first observations with his telescope. Beside the seven planets mown since the days of ancient Babylon, h ehad found four m o re : little ones which revolved around Jupiter. On the Moon he had 'seen mo unt a ins and plains like those on Earth. These were some of t he revelations o faslim book called The -Siderea(M.essenger" which appeared in March1610~ To a ileo'SCoiUemporaries, and perhaps to -us, Galileo is best known as the first man to raise a telescope to the sky, revealing something ofthe immensity ofthe Universe.

Prime mover of the Scientific Revolution Yet Galileo is much more than that. If anyone person can be said to have set the Scientific Revolution in motion and pulled modern sci- ence out of ancient natural philosophy, that man was Galileo. Of all the people of his time, he best realized that the old way of looking at the world would have to go; and he best knew how to begin construc- ting a new way. This he did by making physics mathematical. Events on Earth would help explain what could be seen in the sky, the sky could show us how things happened on Earth. Everywhere nature behaves in an orderly manner, which we can understand, provided our interpretation is couched in a mathematical language. For the proofs of geometry - so he thought - are absolutely certain, unlike other kinds of human reasoning. What is more, just as one theorem in geometry leads to the next, so one discovery will lead to another.

Some of Galileo's ideas are not wholly original, and can be traced back to the Middle Ages, even to ancient Greece. Although he often criticized Aristotle, Galileo realized that he had set out the basic ques- tions we must answer, if we want to know how the world works. How-

. ever, Aristotle's answers were inadequate, Galileo believed, because his physics was not mathematical. Galileo showed, too, how instru- ments designed according to the principles of optics, a mathematical science, could extend the powers of the human senses, making them stronger and more reliable. Above all, for him, unlike the ancient Greeks, geometry did not have to be restricted to the description of a

static world. The movements of bodies can also be analyzed by means of lines, triangles, circles and numbers. So time can be treated math- ematically in the same way as the three dimensions of space. Galileo bequeathed to later science the idea of "acceleration" as a mathemat- ically defined concept. That enabled him to demonstrate a "law of falling bodies" which became the foundation of all later dynamics (i.e. studies of how objects move).

2 .6. Galileo helped redefine our place in the cosmos.

MBthfHfllfticisn, physicillt, lUfronome, By the time of Galileo, the institutionalized, 2,000 year-old tradition of Aristotelian science was already breaking down. A new way of thinking was taking shape: all real knowledge was to be expressed in mathematical terms, which, it was now believed, constituted the only objective and reliable language. Theories about nature had to be put to the test of carefully controlled experiments, whose results should take the form of measurements. The change of thought took almost two centuries to become established in western Europe; today this prolonged crisis is known as the Scientific Revolution.

Since physics explains the basic characters of things, it was the foundation or "cutting edge" of the new science. But only after Nicolaus Copernicus realized that the Earth is a planet of the Sun, not the center of the Universe, did it become possible to reason from Earth to sky and sky to Earth and so to construct a physics that would apply universally. That is why the greatest figures of the Scientific Revolution, Galileo and Newton, were both physicists and astronomers.

Galileo thrust Copernicus's theory upon the general public and showed how crucial it was for an understanding of our place in the cosmos. By recording his observations with the newly invented telescope, he launched an astronomy that studied the features of celestial bodies.

Galileo established mathematical laws describing the motion of falling bodies. Understanding the forces that cause bodies to fall, and that hold the entire system together, he had to leave to his successors, notably Newton. But Galileo's dynamics, however elementary, remain the foundation of classical mechanics.