The next scene opens with a celebration at the harbor of Venice. Galileo’s “new” telescope (which they affectionately call his tube) has astonished everyone, and the entire town is celebrating.
Galileo doesn’t have much interest in these festivities, he tells his friend Sagredo, even though the telescope has made him money (including securing his raise from the university). He’s seen something in the telescope that makes such frivolity trivial: the moon doesn’t generate its own light. This discovery makes him wonder if the telescope can’t be used to disprove other theories as well. Sagredo warns him to be careful.
The moon appears to shine because its surface reflects sunlight back onto the Earth. In Galileo’s day, however, it was believed that the moon must (somehow) generate its own light, since both it and the Sun revolved around the Earth. This isn’t explained to the reader, which creates a useful confusion.
Virginia enters with Ludovico. Galileo mentions that he improved the telescope from the Danish version. Ludovico says he can tell: the Danish version was green, while Galileo’s is red.
Ludovico’s tongue-in-cheek admonishment of Galileo establishes Ludovico as a capitalist to Galileo’s more communistic understanding of intellectual property rights.