Crescent Venus
Galileo discovered that Venus and Mercury have phases just like our moon has phases. Here are some of his sketches of Venus:
Portrait of Galileo
Here is a page from his notebook.
Here is a picture of the crescent Moon and crescent Venus taken through a telescope from Bursa, Turkey in 1997.
1) The ancient Greeks realized that the phases of the Moon could be understood as a natural result of sunlight falling on a sphere. If you are looking at a full moon, where is the sun?2) If you are looking at a quarter moon where is the Sun?
The motions of the planets are complex, almost like a formal dance pattern. The planets change direction, approach each other, and retreat again. They seldom repeat the same arrangement of three or more. From Earth it appears that they travel among the stars. It was surely natural for the imaginative folks of years ago to invent stories to explain these motions, and to include the stars, or gods, as reasons for their travels. It took many years and the work of many intelligent people to figure out a simple explanation for all this complexity. In this unit and the next we'll trace a series of insights.
