Exhibit Guide Navigation

Galileo, Il Saggiatore (Rome, 1623), 1st ed., later state.

The Assayer

Although Galileo eloquently championed mathematical methods in science, the main target of his wit and sarcasm in The Assayer was Grassi, a fellow astronomer, whose mathematical methods proved that comets move above the Moon. Galileo countered that comets are an optical illusion, produced by reflected light, so it becomes a meaningless question to ask where they are located. 

This copy, with expanded errata, is the usual state of the first edition.

Galileo argued that no proof was available for Grassi’s assumption that comets are physical objects in motion rather than merely an optical effect of sunlight reflecting from vapor. Therefore parallax arguments prove nothing. 

Galileo’s provocative sarcasm did nothing to encourage good relations between Galileo and Jesuit astronomers. On the other hand, the book was read with delight at the dinner table by Urban, who wrote a poem praising Galileo for the witty performance.

Galileo's World Exhibition Location

Source: History of Science Collections

Section: Controversy

Section Number: 3

Object Number: 33

Subject Area(s): Mathematics, History of the Book, Astronomy, Philosophy, Religious Studies, Physics, Chemistry

Time Period: 17th Century

Region(s): Europe, Italy

Click any Image for a larger version

Exhibit Gallery OERs

Phases of Venus: Riccioli, New Almagest

Phases of Venus-Riccioli

The phases of Venus were an item of discussion in early modern Europe as scientists sought to determine whether it was evidence of the heliocentric system. Yet among the scientists it was anything but conclusive that this evidence proved the sun-centered universe. Learn more in this learning leaflet.


Six Cosmological Systems: Phases of Venus

Kircher-Cosmological Systems-Venus

It is often thought that Galileo's discovery of the phases of venus demonstrated the contested heliocentric model of the universe. However, such an understanding is overly simplistic of the early modern account of astronomy. Use this learning leaflet to learn more.


Elisabeth Hevelius: Observational Astronomer

Elisabeth Hevelius Learning Leaflet

How does the Sextant symbolize the person who worked on the famed Hevelius star catalog and star atlas throughout its production, from observation to publication? 

Elisabeth Hevelius, wife of Johann Hevelius, was an astronomer in her own right. They worked together in the observatory of their Gdansk home to measure angular widths and distances with a great sextant, which required two observers at a time. The Sextant was among the new constellations they proposed in Uranographia (1690), the most detailed and influential celestial atlas of the 17th century. The Uranographia contains 54 beautiful double page engraved plates of 73 constellations, and 2 oversized folding plates of planispheres.