Who Discovered that other planets in the Solar System besides Earth have Moons and When?

Galileo discovered that other planets have moons and thus extended human understanding beyond our own planet. His careful work with the telescopes he built launched modern astronomy. His discoveries were the first astronomical discoveries using the telescope.

Galileo proved that Earth is not unique among planets of the universe. He turned specks of light in the night sky into fascinating spherical objects, into places, rather than pinpricks of light. In so doing, he proved that Polish astronomer Nicholaus Copernicus had been right when he claimed that the sun was the center of the solar system.

With his simple telescope Galileo single-handedly brought the solar system, galaxy, and greater universe within our grasp. His telescope provided vistas and understanding that did not exist before and could not exist without the telescope.

This was a discovery made possible by an invention, the telescope. Galileo saw his first telescope in late 1608 and instantly recognized that a more powerful telescope could be the answer to the prayers of every astronomer. By late 1609 Galileo had produced a 40-power, two-lens telescope. That 1609 telescope was the first practical telescope for scientific use.

A paper by Johannes Kepler describing the orbits of the planets convinced Galileo to believe the theory of Polish astronomer Nicholaus Copernicus, who first claimed that the sun was the center of the universe, not the earth. Believing Copernicus was a dangerous thing to do. Friar Giordano Bruno had been burned at the stake for believing Copernicus. Galileo decided to use his new telescope to prove that Copernicus was right by more accurately charting the motion of the planets.

Galileo first turned his telescope on the moon. There he clearly saw mountains and valleys. He saw deep craters with tall, jagged rims slicing like serrated knives into the lunar sky. The moon that Galileo saw was radically different from the perfectly smooth sphere that Aristotle and Ptolemy said it was (the two Greek astronomers whose teachings still formed the basis of all science in 1610). Both the all-powerful Catholic Church and every university and scientist in Europe believed Aristotle and Ptolemy.

In one night’s viewing of the moon’s surface through his telescope, Galileo proved Aristotle wrong, again. The last time Galileo’s observations had contradicted Aristotle’s teachings, Galileo had been fired from his teaching position for being right when he proved that all objects fall at the same rate regardless of their weight.

Galileo next aimed his telescope at Jupiter, the biggest planet, planning to carefully chart its motion over several months. Through his telescope (the name is a combination of the Greek words for “distant” and “looking”) Galileo saw a magnified view of the heavens no human eye had ever seen. He saw Jupiter clearly, and, to his amazement, he found moons circling the giant planet. Aristotle had said (and all scientists believed) that Earth was the only planet in the universe that had a moon. Within days, Galileo discovered four of Jupiter’s moons. These were the first discovered moons other than our own.

Aristotle was wrong again.

Still, old beliefs do not die easily. In 1616 the Council of Cardinals forbade Galileo ever again to teach or promote Copernicus’s theories. Many senior church officials refused to look through a telescope, claiming it was a magician’s trick and that the moons were in the telescope.

When Galileo ignored their warning, he was summoned to Rome by the Church’s all-powerful Inquisition. A grueling trial followed. Galileo was condemned by the Church and forced to publicly recant his views and findings. He was placed under house arrest for the rest of his life, dying in 1640 without hearing even one voice other than his own proclaim that his discoveries were true. The Church did not rescind the condemnation of Galileo and his discoveries until October 1992, 376 years after they incorrectly condemned him.

Galileo would have been astonished to learn that Jupiter resembles a star in composition. In fact, if it had been about 80 times more massive, it would have been classified as a star rather than a planet.