Il Saggiatore: Engraving
In this passage, and as shown in the figures of this engraving, Galileo explains how the telescope allows us to see Saturn, Jupiter, Mars, and Venus as clearly distinctive bodies in terms of shape and size. The lower part of the engraving shows the phases of Venus, including the variations in size as it travels through its cycle.
However, Il Saggiatore is best known for displaying a forceful defense of a modern scientific method based on the observation of the physical world and ruled by mathematical principles. This method is perfectly summarized by an often-quoted passage whereby Galileo describes the universe as an open book written in the language of mathematics, whose characters consist of geometrical figures.