What is the title of Galileo's 1623 work on comets?

Galileo's 1623 work on comets was The Assayer. Its Italian title is Il Saggiatore, and it was published in Rome in October 1623 as Galileo's forceful response to a dispute with the Jesuit mathematician Orazio Grassi.

The controversy began after the appearance of several comets in 1618. Grassi argued that comets were celestial bodies located beyond the Moon. Galileo, whose arguments were presented partly through the work of Mario Guiducci, challenged Grassi's conclusions and later answered them directly in The Assayer. Galileo's own interpretation—that comets might be optical phenomena—was scientifically wrong, but the book's discussion of method became highly influential.

The Assayer was more than a narrow astronomical pamphlet. Galileo attacked reliance on authority and argued that nature should be investigated through observation, experiment, and mathematical reasoning. The work is often treated as an early manifesto for mathematical science and includes his famous image of nature as a book written in mathematical language.

It is easy to confuse this title with Galileo's other major books: The Starry Messenger reported telescopic discoveries, the Dialogue addressed competing world systems, and Two New Sciences treated motion and strength of materials.