At about the same time as Kepler was developing his laws of planetary motion, Galileo Galilei (Figure 2.13) was finding fame—and notoriety—as an outspoken proponent of the Copernican system. Galileo was an Italian mathematician and philosopher. By being willing to perform experiments to test his ideas—a rather radical approach in those days (see Interlude 2-2)—and by embracing the brand-new technology of the telescope, he revolutionized the way in which science was done, so much so that he is now widely regarded as the father of experimental science.

Figure 2.13 Galileo Galilei (15641642).


The telescope was invented in Holland in the early seventeenth century. Hearing of the invention (but without having seen one), Galileo built a telescope for himself in 1609 and aimed it at the sky. What he saw conflicted greatly with the philosophy of Aristotle and provided much new data to support the ideas of Copernicus.*

*In fact, Galileo had already abandoned Aristotle in favor of Copernicus, although he had not published these beliefs at the time he began his telescopic observations.

Using his telescope, Galileo discovered that the Moon had mountains, valleys, and craters—terrain in many ways reminiscent of that on Earth. Looking at the Sun (something that should never be done directly, and which eventually blinded Galileo), he found imperfections—dark blemishes now known as sunspots. Furthermore, by noting the changing appearance of these sunspots from day to day, he inferred that the Sun rotates, approximately once per month, around an axis roughly perpendicular to the ecliptic plane. These observations ran directly counter to the orthodox wisdom of the day.

In studying the planet Jupiter, Galileo saw four small points of light, invisible to the naked eye, orbiting it, and realized that they were moons. To Galileo, the fact that another planet had moons provided the strongest support for the Copernican model; clearly, Earth was not the center of all things. He also found that Venus shows a complete cycle of phases, like those of our Moon, a finding that could be explained only by the planet's motion around the Sun (Figure 2.14). These observations were further strong evidence that Earth is not the center of the solar system, and that at least one planet orbited the Sun.

Figure 2.14 (a) The phases of Venus, rendered at different points in the planet's orbit. If Venus orbits the Sun and is closer to the Sun than is Earth, as Copernicus maintained, then Venus should display phases, much as our Moon does. As shown here, when directly between Earth and the Sun, Venus's unlit side faces us, and the planet is invisible to us. As Venus moves in its orbit (at a faster speed than Earth moves in its orbit), progressively more of its illuminated face is visible from Earth. Note also the connection between orbital phase and the apparent size of the planet. Venus seems much larger in its crescent phase than when it is full because it is much closer to us during its crescent phase. (The insets at bottom left and right are actual photographs of Venus at two of its crescent phases.) (b) The Ptolemaic model (see also Figure 2.6) is unable to account for these observations. In particular, the full phase of the planet cannot be explained. Seen from Earth, Venus reaches only a "fat crescent" phase, then begins to wane as it nears the Sun.

Galileo published his findings, and his controversial conclusions supporting the Copernican theory, in 1610, in a book called Sidereus Nuncius (The Starry Messenger). In reporting these wondrous observations made with his new telescope, Galileo was directly challenging the scientific establishment and religious dogma of the time. He was (literally) playing with fire—he must certainly have been aware that only a few years earlier, in 1600, the astronomer Giordano Bruno had been burned at the stake in Rome for his heretical teaching that Earth orbited the Sun. However, by all accounts, Galileo delighted in publicly ridiculing and irritating his Aristotelian colleagues. In 1616 his ideas were judged heretical, Copernicus's works were banned by the Roman Church, and Galileo was instructed to abandon his cosmological pursuits.

But Galileo would not desist. In 1632 he raised the stakes by publishing Dialogue Concerning the Two Chief World Systems, which compared the Ptolemaic and Copernican models. The book presented a discussion among three people: one of them a dull-witted Aristotelian, whose views time and again were roundly defeated by the arguments of one of his two companions, an articulate proponent of the heliocentric system. To make the book accessible to a wide popular audience, Galileo wrote it in Italian rather than Latin. These actions brought Galileo into direct conflict with the Church. Eventually, the Inquisition forced him, under threat of torture, to retract his claim that Earth orbits the Sun, and he was placed under house arrest in 1633; he remained imprisoned for the rest of his life. Not until 1992 were Galileo's "crimes" publicly forgiven by the Church. But the damage to the orthodox view of the universe was done, and the Copernican genie was out of the bottle once and for all.


Although Renaissance scholars were correct, none of them could prove that our planetary system is centered on the Sun, or even that Earth moves through space. Direct evidence for this was obtained only in the early eighteenth century, when astronomers discovered the aberration of starlight—a slight (20" ) shift in the observed direction to a star, caused by Earth's motion perpendicular to the line of sight. Additional proof came in the mid-nineteenth century, with the first unambiguous measurement of stellar parallax. Further verification of the heliocentricity of the solar system came gradually, with innumerable observational tests that culminated with the expeditions of our unmanned space probes of the 1960s, 1970s, and 1980s. The development and eventual acceptance of the heliocentric model were milestones in human thinking. This removal of Earth from any position of great cosmic significance is generally known, even today, by the term Copernican principle.

The Copernican episode is a good example of how the scientific method, though affected at any given time by the subjective whims, human biases, and even sheer luck of researchers, does ultimately lead to a definite degree of objectivity. Over time, many groups of scientists checking, confirming, and refining experimental tests can neutralize the subjective attitudes of individuals. Usually one generation of scientists can bring sufficient objectivity to bear on a problem, though some especially revolutionary concepts are so swamped by tradition, religion, and politics that more time is necessary. In the case of heliocentricity, objective confirmation was not obtained until about three centuries after Copernicus published his work and more than 2000 years after Aristarchus had proposed the concept. Nonetheless, that objectivity did in fact eventually prevail.