Jupiter’s Moons
Year of Discovery: 1610
What Is It? Other planets (besides Earth) have moons.
Who Discovered It? Galileo Galilei
Why Is This One of the 100 Greatest?
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.
How Was It Discovered?
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.
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