The Medician Stars

Abstract

In the initial sections of The Sidereal Messenger, Galileo focused on his telescopic observations of the moon. Perhaps most importantly, he noticed that the moon, like Earth, possesses mountains and valleys. This seemingly mundane observation had great significance, as it clearly demonstrated the falsehood of Aristotle’s notion that the moon, like other heavenly bodies, must be made of a fifth—ethereal—substance, whose natural motion was circular. Aristotle’s physics, founded on the natural motion of the four earthly elements, was becoming increasingly difficult to defend. Now, in the following text selection, Galileo turns his attention to the stars and the planets. What additional observations did he make in this arena? And what new problems did his discoveries pose for the followers of Aristotle?

Although I believed them to belong to the number of fixed stars, yet they made me wonder somewhat, because they seemed to be arranged exactly in a straight line.

—Galileo Galilei