After his discovery of Jupiter's moons, Galileo turned his telescope to the other planets. Mars and Venus were not well placed for observation and the appearance of Saturn puzzled him, but he concluded that there were no moons around the other planets.

Discovery Two: The Phases of Venus

In September of 1610 he received an offer from Grand Duke Cosimo de Medici to move to Florence. Eager for more discoveries, Galileo turned his attention back to Venus. At that time, Venus was an "evening star" and Galileo's poor optics showed little more than a fuzzy blob. But the very fact that Venus showed no evidence of a crescent was a blow against the Ptolemaic system.

According to the geocentric system of Ptolemy, Venus was always between the Earth and the Sun and so could only show a crescent phase. Of course, there was always the possibility that Venus gave off its own light and hence no phase effect would be seen.

By mid-December, Venus showed a half-moon phase and Galileo was convinced that Venus did indeed show phases. In a letter to Cosimo's brother, Galileo preserved his priority of discovery by including a Latin anagram which could later be unscrambled.

Why did he not announce his discovery then and there? At half-lit phase, Venus has a small angular diameter and the fuzzy image presented by his optically poor telescope was not at all convincing - especially to a non-believer! It wasn't until the crescent phase became obvious that Galileo deciphered his anagram.

You can follow Galileo's footsteps by examining the appearance of Venus in the late summer to early winter of 1610 by downloading the file Venus1610.snf and running time forward.

And just what did Galileo find puzzling about Saturn? Stay tuned for the next article to find out.