In 1609, Galileo Galilei, a professor of mathematics at the University of Padua, learned of a new device being used in Holland: a telescope consisting of two pieces of curved glass in a tube, which could make distant objects seem near. Galileo constructed an instrument of his own which allowed him to view stars invisible to the naked eye and to resolve individual stars in the Milky Way. As he turned his telescope to the heavens, Galileo would make numerous discoveries that would support the Copernican theory.

Examining Jupiter in January 1610, Galileo noticed that the planet was surrounded by several little stars arranged in a straight line. Successive nights of observation revealed that these “stars” must be moons orbiting Jupiter, just as the Moon revolves around the Earth in the Copernican system. Galileo quickly published his discoveries in a small book entitled Siderius Nuncius, or Starry Messenger. Over the next two years, Galileo would also observe sunspots, the “appendages” of Saturn (which astronomer Christiaan Huygens would show to be the planet’s rings in 1656), and lunar topography. He also observed that the planet Venus went through a complete sequence of phases, just like the Moon.

Though the Catholic Church had censured Copernicanism in 1616, Galileo’s friend Maffeo Barberini was elected Pope in 1623, and Galileo decided it was safe to write a book comparing the Ptolemaic and Copernican cosmologies. With his belief in the sun-centered theory strengthened by his telescopic observations, Galileo produced a vernacular work, the Dialogue on the two great world systems, published in 1632. The Dialogue explained the advantages of Copernicus’ theory and the evidence which supported it. Galileo also included a discussion of motion and the tides to prove that the Earth indeed moved around the Sun. But the Dialogue angered Church authorities, and Galileo was brought before the Inquisition on charges of disobeying the ban on defending Copernicanism. Galileo recanted his belief in the Copernican system before the court and was sentenced to house arrest. His books, as well as all heliocentric works, would be suppressed in Catholic countries for well over a century.