August 25, 1609 (a Tuesday)

Bell Tower in St. Mark’s Square

On this date, the Italian mathematician Galileo Galilei marched the Doge of Venice (Leonardo Donato), his counsellor, the Chiefs of the Council of Ten, and the Sages of the Order, who commanded the Venetian navy, up the Bell Tower (Campanile) in St. Mark’s Square in Venice, Italy. Once at the top, Galileo showed them views of distant cities, ships on the horizon, and parishioners entering a church on the island of Murano – all of which had been invisible to the eye alone – with the aid of his first telescope. The Doge was awestruck. The military had a powerful new secret weapon. Venice was confirmed again as a triumph. Galileo presented the Doge with the telescope on his knees and received a doubled salary, a lifetime appointment, and a bonus amounting to a year’s wages.

Throughout the the rest of 1609, particularly during the winter, Galileo made many astronomical studies. On January 7, 1610 Galileo observed with his telescope what he described at the time as “three fixed stars, totally invisible by their smallness,” all close to Jupiter, and lying on a straight line through it. Observations on subsequent nights showed that the positions of these “stars” relative to Jupiter were changing in a way that would have been inexplicable if they had really been fixed stars. On January 10 Galileo noted that one of them had disappeared, an observation which he attributed to its being hidden behind Jupiter. Within a few days he concluded that they were orbiting Jupiter. He had discovered three of Jupiter’s four largest satellites (moons): Io, Europa, and Callisto. He discovered the fourth, Ganymede, on January 13. Galileo named the four satellites he had discovered Medicean stars, in honor of his future patron, Cosimo II de’ Medici, Grand Duke of Tuscany, and Cosimo’s three brothers. [Later astronomers, however, renamed them the Galilean satellites in honor of Galileo himself.] On March 12, 1610 Galileo published the results of his studies in a brief treatise entitled Sidereus Nuncius (Starry Messenger).

These observations over a six night period, from January 7 through January 13, provided a view to Galileo that revealed that perhaps not everything orbited the Earth (geocentric model), as Ptolemy as well as the Catholic Church had adopted. And, if these small, but bright points of light went around Jupiter and not the Earth, perhaps there were other objects that did not orbit the Earth. His findings allowed him to confirm the Sun-centered theory of Copernicus. This short period of time from the summer of 1609 through to March of 1610, when Siderius Nuncius was published, had a revolutionary impact on astronomy almost overnight and it catapulted Galileo into the scientific spotlight and into the fire and wrath of the Catholic Church.

The Catholic Church condemned Galileo for his theories on June 22, 1633. He was forced to disown them and to live on his own for the rest of his life. In the following century the Vatican began changing its attitude. A mausoleum was built in 1734 to honor him. In 1822 Pope Pius VII gave permission for Galileo’s theory to be taught in schools. In 1968 Pope Paul VI had the trial against Galileo reassessed, then Pope John Paul II took the final step in the Church’s rehabilitation of the scientist in 1984 when he formally acknowledged that the Catholic Church had erred when it condemned the Italian astronomer for maintaining that Earth revolved around the Sun.