3.3 Galileo’s Observations Supported the Heliocentric Model
Galileo Galilei (1564–1642—Figure 3.17) was the first to use a telescope to make and report significant discoveries about astronomical objects. Galileo’s telescopes were small, yet they were sufficient for him to observe spots on the Sun, the uneven surface and craters of the Moon, and the many stars in the band of light in the sky called the Milky Way.
Galileo’s Observations
Galileo provided the first observational evidence that some objects in the sky do not orbit Earth. When Galileo turned his telescope on Jupiter, he observed four “stars” in a line near the planet. Over time he observed that the objects remained near Jupiter, but changed position from night to night (Figure 3.18). Galileo hypothesized that those objects were moons orbiting Jupiter. Those four “Galilean moons” are the largest of Jupiter’s many moons. Galileo also estimated the relative distance of each moon from Jupiter and the periods of their orbits, and he showed that the square of the period was proportional to the cube of the radius of the orbit for each moon, consistent with Kepler’s third law.
Galileo also observed that Venus went through an entire set of phases like the Moon. He noticed that as the phases of Venus changed, so did the size of Venus in his telescope. In a geocentric model in which Venus orbits Earth like the Moon does, the apparent size of Venus would change only slightly, when it looped an epicycle. In the heliocentric model, however, the Earth–Venus distance varies by a lot, and Venus’s size changes accordingly. When Venus is in its gibbous to full phases, it is farther away, on the other side of the Sun from Earth, and smaller in the sky. When Venus is in its crescent to new phases, it is closer, on the same side of the Sun as Earth, and larger in the sky (Figure 3.19). In the experiment that opens this chapter, you can build a model that demonstrates this relationship. Galileo’s observations of Jupiter’s moons and the phases of Venus in particular convinced Galileo that Copernicus was correct to place the Sun at the center of the Solar System.
In addition to his astronomical observations, Galileo did important work on the motion of objects. Unlike natural philosophers, who thought about objects in motion but did not actually experiment with them, Galileo conducted experiments with falling and rolling objects. As with his telescopes, Galileo improved or developed new technology to enable him to conduct those experiments. For example, by carefully rolling balls down an inclined plane and by dropping various objects from a height, he found that a falling object travels a distance proportional to the square of the time it has been falling. If he simultaneously dropped two objects of different masses, they reached the ground at the same time, showing that all objects falling to Earth accelerate at the same rate, independent of their mass.
Galileo’s observations and experiments with many types of moving objects, such as carts and balls, led him to disagree with the Greek philosophers about when and why objects continue to move or come to rest. Before Galileo, it was thought that an object’s natural state was to be at rest. But he found that an object naturally does what it was doing until a force acts on it. That is, an object in motion continues moving along a straight line with a constant speed until a force acts on it to change that state of motion. That idea of inertia, which Newton later adopted as his first law of motion, has implications for not only the motion of carts and balls but also the orbits of planets.
what if . . .
What if Galileo had found that instead of obeying Kepler’s third law, the moons of Jupiter behave in such a way that the orbital period is proportional to the radius of the orbit? Does this observation falsify Kepler’s third law for the planets, and what can you conclude from the difference between these results?
Dialogue Concerning the Two Chief World Systems
Galileo faced considerable danger because of his work. His later life was consumed by conflict with the Catholic Church over his support of the Copernican system. In 1632, Galileo published his best-selling book, Dialogo sopra i due massimi sistemi del mondo (“Dialogue Concerning the Two Chief World Systems”). The Dialogo presents a brilliant philosopher named Salviati as the champion of the Copernican heliocentric view of the universe. The defender of an Earth-centered universe, Simplicio—who uses arguments made by the classical Greek philosophers and the pope—sounds silly and ignorant.
Galileo, a religious man with two daughters in a convent, thought he had the Catholic Church’s tacit approval for his book. But when he placed several of the pope’s geocentric arguments in the unflattering mouth of Simplicio, the perceived attack on the pope got the church’s attention. Galileo was put on trial for heresy, sentenced to prison, and eventually placed under house arrest. To escape a harsher sentence, Galileo was forced to publicly recant his belief in the Copernican theory that Earth moves around the Sun. According to one story, as he left the courtroom, Galileo stamped his foot on the ground and muttered, “And yet it moves!”
The Dialogo was placed on the pope’s Index of Prohibited Books, along with Copernicus’s De Revolutionibus. Nevertheless, Galileo’s work traveled across Europe, was translated into other languages, and was read by other scientists. Galileo spent his final years compiling his research on inertia and other ideas into the book Discourses and Mathematical Demonstrations Relating to Two New Sciences, which was published in 1638 in Holland, outside the Catholic Church’s jurisdiction. (In 1992, Pope John Paul II apologized for the “Galileo Case.”)
CHECK YOUR UNDERSTANDING 3.3
Which of Galileo’s astronomical observations did Copernicus’s model explain better than Ptolemy’s? (a) sunspots; (b) craters on the Moon; (c) the moons of Jupiter; (d) the apparent size and phases of Venus
Answer
Answer
d