Galileo Galilei first begins observing the moon with his telescope on November 30, 1609. He discovers that the surface of the moon is not smooth, as previously thought, but full of mountains and valleys, like Earth.
Galileo was an Italian astronomer and physicist. In 1609, he constructed his own telescope to observe the heavens. Dutch eyeglass makers invented the telescope in 1608, but Galileo improved on their original design significantly. He ground and polished his own lenses to achieve greater magnification, and used his invention to observe the surface of the moon.
Galileo was not the first or the only astronomer to use a telescope to study the moon, but he was the first to publish a detailed report of his findings. His report, Sidereus Nuncius, or Starry Messenger, was published in 1610 and made him famous throughout Europe. It included detailed sketches of the surface of the moon, which demonstrated that it had topography like Earth, with mountains, valleys and plains. These findings challenged prevailing ideas about the nature of heavenly bodies.
In Galileo's time, most scholars believed, based on the writings of Aristotle from the third century BCE, that heavenly bodies were perfect, unchanging spheres. The Earth alone was imperfect. The Aristotelian model also put the Earth at the center of the universe. Sidereus Nuncius suggested that both of these ideas were false: the moon was not a perfect sphere, and the Earth was not the center of the universe.
Galileo continued to publish groundbreaking studies of the solar system, but these brought him into conflict with the Roman Inquisition. The Roman Catholic Church had adopted the works of Aristotle into its teachings. The Church was willing to accept that the moon had terrain, like Earth, but unwilling to accept that the Earth orbited the sun. In 1633, Galileo was forced to recant his belief in heliocentrism and spent the rest of his life under house arrest in Florence.