Chapter 3: The Copernican Revolution

Portrait of Galileo Galilei by Justus Sustermans, painted in 1636. National Maritime Museum, Greenwich, London.

Galileo Galilei provided the critical evidence to support the Copernican theory. Galileo was known by his first name, like Tycho. He worked at the same time that Kepler was working on his orbital theory. As early as 1597, Galileo wrote to Kepler that "Like you, I accepted the Copernican position several years ago... I have not dared until now to bring [my ideas] into the open." In one of the most dramatic episodes in the history of science, Galileo's scientific views took him on a collision course with the Catholic Church. Galileo was born in Pisa, Italy, and although he had to leave school because of financial difficulties, he returned to complete his studies in mathematics. He became a professor at the University of Padua, where he stayed for 18 years.

Imaginative painting showing Galileo Galilei displaying his telescope to Leonardo Donato.

Around 1609, probably in Holland, makers of eyeglasses discovered that they could combine lenses to magnify. This discovery led to the invention of the telescope. Galileo's brilliant idea was to turn the newly invented telescope skyward for a systematic study of the heavens. His first telescope had a magnification of only a factor of 3 and could be duplicated today for a dollar's worth of materials. Even his best telescope, with a magnification of a factor of 30, was inferior to good quality modern binoculars. But his discoveries changed the way people thought of the universe forever.

Over the course of about a week in January 1610, Galileo made six observations of the major satellites of Jupiter and presented his observations in Sidereus Nuncius.

Galileo's sketches of the moon from Sidereus Nuncius, published in March 1610.

Galileo observed mountains and plains on the Moon, proving that it was not a polished sphere or supernatural orb of the "perfect" celestial realm, but a geological world like the Earth. He detected sunspots on the surface of the Sun and tracked their motion, deducing that the Sun rotated once every four weeks. The Sun was therefore not perfect either, and if it could rotate, why not the Earth? Equally important was his discovery and subsequent observations of four moons near Jupiter. He showed that these moons orbit Jupiter just as the Moon orbits the Earth. This disproved the idea that everything moves around the Earth.

The most important observation for the debate about the Copernican model involved the phases of Venus. In the Ptolemaic model, Venus is always between the Earth and the Sun, so it will always show a crescent phase since most of the lit side faces toward the Sun. In the Copernican model, Venus can sometimes lie between the Earth and Sun, when it will have a new or crescent phase. But there are parts of the orbit where it lies on the opposite side of the Sun as the Earth when we see a full phase as the lit side faces back at us. There is a major distinction between the geocentric and the heliocentric predictions. By studying the illumination of Venus over a period of months Galileo showed that Venus went through a full cycle of phases. Not only that, but the full phase occurred when the angular size of Venus was the smallest, as the Copernican model would predict. This was compelling evidence that the heliocentric view was correct.

A diagram illustrating Kepler's laws.

Why did Galileo's debate with the Church turn ugly? (Part of the reason was undoubtedly Galileo's cantankerous personality since he was never one to shy away from an argument or a confrontation.) During the time of Copernicus, the heliocentric model was a mathematical abstraction — a hypothesis that presented no real threat to the established order. Then Galileo used his telescope to dethrone Earth from its special place at the center of the cosmos. It was just one of a number of worlds scattered through space. Galileo lectured and published in Italian rather than Latin, which was the traditional scholarly language. In this way, he reached many people. He presented the Copernican model as an established fact.

During his long period of house arrest, Galileo completed his other major work on mechanics or the science of motion. Aristotle had taught that rest was the natural state of any object and that a heavier object would fall faster than a light one. Galileo showed that both of these ideas were wrong. He compared the motions of balls of different weights rolling down inclined planes, using his pulse as a timing device. The inclined plane was just a device to slow the motion down enough that he could measure it. He never actually dropped objects from the Leaning Tower of Pisa; that was a story told by a student after he died. Galileo realized that friction on a surface or air resistance would affect the motion of a falling object. If these effects were minimized, a ball rolling on a flat surface would keep rolling. Uniform motion was just as natural as rest. In addition, objects of any material would fall toward Earth in the same way. The acceleration, or rate of change of velocity, does not depend on the weight or composition of the object.

Many of Galileo's ideas in mechanics were relevant to the motions of objects in space. He came up with the concept of inertia, the resistance of any object to a change in its motion. If a ball is dropped from the mast of a tall ship, it will not fall behind the ship but will continue with the forward motion of the ship and land at the base of the mast. If the atmosphere is being carried with the rotation of the Earth then we might not feel our motion in space, Galileo supposed. Similarly, someone confined to the cabin of a smooth sailing ship could do experiments and never be aware of their motion. Galileo was convinced by these arguments that the Earth could be in motion without our being aware of it. In this way, yet another of the objections to the heliocentric model was overcome.

Why is Galileo important in the history of science? He was a pioneer of observational astronomy. He completed the Copernican revolution with his telescopic observations. In a broad sense, he was the first modern scientist. Rather than accept the established wisdom on any subject, he conducted his own experiments. As any modern scientist would, he preferred to read the "book of nature."