Galileo Galilei (1564-1642) was a pivotal figure in the development of modern astronomy, both because of his contributions directly to astronomy, and because of his work in physics and its relation to astronomy. He provided the crucial observations that proved the Copernican hypothesis, and also laid the foundations for a correct understanding of how objects moved on the surface of the earth (dynamics) and of gravity.

Newton, who was born the same year that Galileo died, would build on Galileo's ideas to demonstrate that the laws of motion in the heavens and the laws of motion on the earth were one and the same. Thus, Galileo began and Newton completed a synthesis of astronomy and physics in which the former was recognized as but a particular example of the latter, and that would banish the notions of Aristotle almost completely from both.

One could, with considerable justification, view Galileo as the father both of modern astronomy and of modern physics.

The Telescope

Sunspots

Galileo observed the Sun through his telescope and saw that the Sun had dark patches on it that we now call sunspots (he eventually went blind, perhaps from damage suffered by looking at the Sun with his telescope). Furthermore, he observed motion of the sunspots indicating that the Sun was rotating on an axis. These "blemishes" on the Sun were contrary to the doctrine of an unchanging perfect substance in the heavens, and the rotation of the Sun made it less strange that the Earth might rotate on an axis too, as required in the Copernican model. Both represented new facts that were unknown to Aristotle and Ptolemy.

Galileo observed 4 points of light that changed their positions with time around the planet Jupiter. He concluded that these were objects in orbit around Jupiter. Indeed, they were the 4 brightest moons of Jupiter, which are now commonly called the Galilean moons (Galileo himself called them the Medicea Siderea---the ``Medician Stars''). 

These observations again showed that there were new things in the heavens that Aristotle and Ptolemy had known nothing about. Furthermore, they demonstrated that a planet could have moons circling it that would not be left behind as the planet moved around its orbit. One of the arguments against the Copernican system (and the original heliocentric idea of Aristarchus) had been that if the moon were in orbit around the Earth and the Earth in orbit around the Sun, the Earth would leave the Moon behind as it moved around its orbit.

The Phases of Venus

The crucial point is the empirical fact that Venus is never very far from the Sun in our sky (see the earlier discussion of aspects & phases of the inferior planets). Thus, as the following diagrams indicate, in the Ptolemaic system Venus should always be in crescent phase as viewed from the Earth because as it moves around its epicycle it can never be far from the direction of the sun (which lies beyond it), but in the Copernican system Venus should exhibit a complete set of phases over time as viewed from the Earth because it is illuminated from the center of its orbit.

Phases of Venus in the Ptolemaic and Copernican systems

It is important to note that this was the first empirical evidence (coming almost a century after Copernicus) that allowed a definitive test of the two models. Until that point, both the Ptolemaic and Copernican models described the available data. The primary attraction of the Copernican system was that it described the data in a simpler fashion, but here finally was conclusive evidence that not only was the Ptolemaic universe more complicated, it also was incorrect.

Myriad Observations Showing Phenomena Unknown to Aristotle

1. Showing that the planets were disks, not points of light, as seen through the telescope.

2. Showing that the great "cloud" called the Milky Way (which we now know to be the disk of our spiral galaxy) was composed of enormous numbers of stars that had not been seen before.

3. Observing that the planet Saturn had "ears". We now know that Galileo was observing the rings of Saturn, but his telescope was not good enough to show them as more than extensions on either side of the planet.

4. Showing that the Moon was not smooth, as had been assumed, but was covered by mountains and craters.

Galileo and the Concept of Inertia

Most objects in a state of motion do NOT remain in that state of motion. For example, a block of wood pushed at constant speed across a table quickly comes to rest when we stop pushing. Thus, Aristotle held that objects at rest remained at rest unless a force acted on them, but that objects in motion did not remain in motion unless a force acted constantly on them. Galileo, by virtue of a series of experiments (many with objects sliding down inclined planes), realized that the analysis of Aristotle was incorrect because it failed to account properly for a hidden force: the frictional force between the surface and the object.

Thus, as we push the block of wood across the table, there are two opposing forces that act: the force associated with the push, and a force that is associated with the friction and that acts in the opposite direction. Galileo realized that as the frictional forces were decreased (for example, by placing oil on the table) the object would move further and further before stopping. From this he abstracted a basic form of the law of inertia: if the frictional forces could be reduced to exactly zero (not possible in a realistic experiment, but it can be approximated to high precision) an object pushed at constant speed across a frictionless surface of infinite extent will continue at that speed forever after we stop pushing, unless a new force acts on it at a later time.

Galileo and the Church

Information supplied by: http://csep10.phys.utk.edu