**Galileo’s Issues with the Roman Catholic Church and Benedetto Castelli’s Involvement**
Galileo Galilei’s disputes with the Roman Catholic Church prominently commenced with the release of his “Letter to Castelli.” In December 1613, at a dinner in Pisa attended by Grand Duke Cosimo II de’ Medici, philosopher Cosimo Boscaglia contested the heliocentric theory, asserting its inconsistency with the Bible. Benedetto Castelli advocated for the heliocentric perspective. Dowager Duchess Christina subsequently challenged Castelli with scriptural arguments against the movement of the Earth. Castelli conveyed this discussion to Galileo, leading to Galileo’s reply in what became known as the “Letter to Castelli” dated December 21, 1613. In this letter, Galileo contended that the Bible should not be utilized for scientific inquiries and proposed interpretations that harmonized scripture with heliocentrism.
Galileo’s biblical interpretation posed a challenge to the Church’s authority, a delicate issue particularly during the Reformation/Counter Reformation division. Although the letter was not printed, it circulated extensively in manuscript form, ultimately resulting in a confrontation with the Church. Galileo later asserted that the Inquisition had an altered version of the letter, while he held that it was his original text.
Benedetto Castelli (1578–1643), often seen merely as Galileo’s pupil, was a remarkable scientist who made significant contributions to early 17th-century science, deserving acknowledgment beyond being a mere extension of Galileo’s efforts. Born Antonio, he entered the Benedictine order at age seventeen, taking on the name Benedetto. By the early 17th century, Castelli had joined Galileo in Florence after studying at Padua under Galileo’s guidance. A 1610 communication to Galileo, where Castelli asked about the phases of Venus, may have motivated Galileo’s later observations that confirmed heliocentrism, although Galileo claimed exclusive credit for those discoveries.
Among his achievements, Castelli developed a technique for documenting sunspots using a telescope and co-wrote responses to Aristotelian critics regarding buoyancy alongside Galileo. He was appointed as a mathematics instructor at the University of Pisa. His work “Delia misura dell’acque Correnti” established foundational concepts for modern hydraulics by correlating river cross-section areas with water volumes over time. Castelli also created what is likely the first rain gauge in Europe to monitor water levels in Trasimeno Lake. His writings amended previous works by Roman engineer Frontinus concerning aqueducts and hydrology.
As a papal advisor on hydraulics in Rome, Castelli educated students who made important contributions to the mathematical sciences, including Bonaventura Cavalieri, Raffaello Magiotti, and Evangelista Torricelli. Castelli’s scientific and educational legacy significantly shaped the scientific landscape of the 17th century.