![]() This mosaic shows the surfaces of Jupiter’s four Galilean moons images, as seen by NASA’s Galileo spacecraft. Astronomers and observers alike collectively refer to those four moons as the “Galilean satellites,” or “Galilean moons.”Īnd today, we know so much about each Galilean moon that they are almost considered planets in their own right. But of all Jupiter’s many dozens of moons, the four discovered by Galileo still remain the most observed, as well as the most fascinating. That Brough Jupiter’s total number of moons to 92, enough for the king of planets to leapfrog over Saturn at last. But in January 2023, researchers announced the discovery of a dozen new moons around it. The Galilean moons come into focusįour centuries later, we know that those points of light seen fluttering in the eyepiece of Galileo’s telescope were, in fact, the moons Io, Europa, Ganymede, and Callisto - the four largest of Jupiter’s extended family of moons.įor many years, Jupiter trailed behind Saturn for the record number of moons. But that meant that Jupiter didn’t go around the Earth, as everyone had thought, and Earth wasn’t really the center of the universe. There was only one explanation, Galileo surmised: The “companion stars” were objects going around Jupiter. On some evenings, as many as four companion stars were visible, arranged in pairs on either side of Jupiter, or three on one side and one on the other, or even all four on the same side. Intriguingly, their numbers changed from night to night, as did their arrangement. Like any good observer, Galileo recorded what he saw, pondered on it for a while, then likely swung his telescope to other targets.īut on subsequent clear nights, Galileo returned to Jupiter again and again, each time seeing strange companions close to it. There were several small, pinprick stars flanking it - two on one side, one on the other - all bathed in the bright planet’s metallic glare. ![]() ![]() When it eventually did, Galileo was amazed to see Jupiter wasn’t alone. As Galileo peered into the telescope’s eyepiece and jiggled the instrument around, searching for the planet, he must have been very impatient for it to finally swing into view. But the modified telescope he swung toward Jupiter that January evening had a very respectable magnification of 20x. Galileo’s first crude telescope only magnified objects a few times, barely enough to change their appearance. Courtesy of the Wellcome Collection, Attribution 4.0 International (CC BY 4.0) Line engraving by Boutrois & Jouannin after J. Galileo sets his telescopic sights on Jupiter This line engraving of Galileo depicts the astronomer alongside a drawing that shows Earth’s orbit around the Sun, as well as the Moon’s orbit around Earth. With great eagerness, he swung his new and improved telescope toward it, curious to learn what he would see. To the brilliant Moon’s upper-right, he saw a blue-white star, brighter by far than any of the stars around it.Īs a scholar of science who was familiar with the sky and its workings, Galileo knew that this point of light was no star it was the planet Jupiter. That man was scientist, inventor, and visionary Galileo Galilei.Īs darkness fell over Padua, Galileo gazed out over the city rooftops to the east to where the constellation Orion, and above it, a nearly Full Moon, were on the rise. 7, 1610, none of them could have known that somewhere in their beautiful city, standing on a balcony high above them, a man was preparing to not only make history, but transform our very understanding of our place in the universe. ![]() GillĪs the citizens of Padua, Italy, watched the Sun set on the evening of Jan. Moons: NASA/JPL/Galileo Jupiter: NASA/JPL/Space Science Institute Image data processing and layout: Kevin M. From bottom to top is Io, Europa, Ganymede, and Callisto. But new research suggests a promising pathway for how they may have formed. The origins of Jupiter's Galilean moons is a long-standing mystery.
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