Early Astronomy in the University of Michigan Collections

On Comets

The appearance of three comets over all of Europe in 1618 generated a vigorous debate between Galileo and Orazio Grassi, a Jesuit mathematician of the Collegium Romanum and a supporter of the Tychonian model of the universe. In his De tribus cometis anni M.DC.XVIII. disputatio astronomica publice habita in Collegio Romano Societatis Iesu ab uno ex patribus eiusdem societatis (On the three Comets of the year 1618: An Astronomical Disputation Publicly Held by one of the Fathers of the Society of Jesus), Grassi argued that, due to the absence of an observable parallax, comets should be described as celestial bodies moving beyond the Moon in an orbit around the Sun and the Earth.

De Tribus Cometis


Discorso: Woodcut

Libra Astronomica

Libra Astronomica: Woodcuts


Select Bibliography

  • Galilei, Galileo, Orazio Grassi, and Johannes Kepler. 1960. The Controversy on the Comets of 1618: Galileo Galilei, Horatio Grassi, Johann Kepler. Ed. & trans. Stillman Drake and C. D. O’Malley. Philadelphia: University of Pennsylvania Press.

Image Viewer