Benedetto Castelli (1578 – 9 April 1643), born Antonio Castelli, was an Italian mathematician. Benedetto was his name in religion on entering the Benedictine Order in 1595.
Castelli was involved in the discovery of the phases of Venus:
In December 1610, Galileo received a letter from Castelli, asking if the phases of Venus were observable through Galileo's new telescope.[1] Days later, Galileo wrote in a letter to Johannes Kepler saying that he'd observed Venus going through phases, but took complete credit for himself. It is unclear, lacking copies of any earlier correspondence, whether Castelli was telling Galileo of it for the first time, or responding to Galileo having previously informed him of it.[2]
He published Mensuration of Running Water, an important work on fluids in motion, and then his Geometrical Demonstrations of the Measure of Running Waters in which the publishing notes described him as Abbot of San Benedetto Aloysio and Mathematician to Pope Urban VIII, once a supporter of his mentor, Galileo.
He dedicated both publications to "the most Illustrious, and most Excellent Prince" Taddeo Barberini, a nephew of Pope Urban VIII.[3]