In 1609 Galileo heard of a new invention shown at an exposition in Venice. It was an "optical tube" that would bring distant objects close. Galileo set out to reinvent such an instrument and quickly succeeded. During the next few months Galileo made some of the most important discoveries of astronomy. His observations also established the Copernican system as physical reality.

He observed the craters and mountains on the Moon and was struck by the Earthlike appearance of the Moon's landscape. He also noticed that the dark side of the Moon was faintly illuminated by Earth, a phenomenon known as Earthshine. With this he proved that Earth "shines" as much as the other planets and thus must reflect the light from the Sun.

Directing his telescope on the planets, Galileo discovered that, unlike the stars, which remained pointlike in his telescope, the planets appeared like small disks, or, in the case of Venus, as a miniature crescent. This convinced him that the stars are much farther away from Earth than the planets. He also discovered that the Milky Way consists of a large number of stars.

Observing Jupiter, Galileo discovered that it is surrounded by four "stars" (at that time planets, as well as fixed stars, were just called stars). He called these natural satellites or moons, as we now know them to be, Medicean stars in honor of the Medici, rulers of Florence. He started observing their motion around Jupiter and came to the conclusion that they must be circling the planet in the same way as the Moon orbits Earth. This discovery was an important one because it showed that the Earth-Moon system is not a unique anomaly, which many people thought an important objection to the Copernican system.

The strongest proof for the validity of the Copernican system came from Galileo's observations of the phases of Venus. Not only did it become clear that Venus reflects light from the Sun and does not shine by its own light, but also that the size of Venus changes with the changes of its phase. When the planet was visible as a complete disk, it was very small, and when it became a narrow crescent, its apparent diameter became large. This could only be explained by assuming that the planet orbits the Sun.

Finally, by observing the motion of sunspots, Galileo demonstrated the rotation of the Sun.

History of Science and Technology, edited by Bryan Bunch and Alexander Hellemans. Copyright © 2004 by Houghton Mifflin Company. Published by Houghton Mifflin Company. All rights reserved.