Jupiter and the Galilean Moons

Montage of Jupiter’s four Galilean moons,
in a composite image from top to bottom:
Io, Europa, Ganymede, Callisto

On January 7, 1610, physicist and astronomer Galileo Galilei turned his new telescope to the nocturnal sky to watch the planet Jupiter and discovered the eponymous four moons of Jupiter, Io, Europa, Ganimede, and Callisto.

Based only on uncertain descriptions of the first practical telescope which Hans Lippershey tried to patent in the Netherlands in 1608, Galileo Galilei, in the following year was able to improve the telescope, with a magnifying capability of about 20×. Moreover, Galileo was also one of the very first to try the telescope not only for terrestrial observations but also towards the nightly sky. With his improved telescope, he was able to see celestial bodies more distinctly than was ever possible before. This allowed Galilei to discover sometime between December 1609 and January 1610 what came to be known as the Galilean moons.

On 7 January 1610 Galileo observed with his telescope what he described at the time as “three fixed stars, totally invisible by their smallness“, all close to Jupiter, and lying on a straight line through it. On the very same day he wrote a letter containing the first mention of Jupiter’s moons. At the time, he saw only three of them, and he believed them to be fixed stars near Jupiter. He continued to observe these celestial orbs from January 8 to March 2, 1610. In these observations, he discovered a fourth body, and also observed that the positions of these “stars” relative to Jupiter were changing in a way that would have been inexplicable if they had really been fixed stars. Within a few days he concluded that they must be orbiting Jupiter. Galileo named the group of four the Medicean stars, in honour of his patron, Cosimo II de’ Medici and Cosimo’s three brothers. Later astronomers, however, renamed them Galilean satellites in honour of their discoverer. Today, these satellites are called Io, Europa, Ganymede, and Callisto.

Galileo’s discovery proved the importance of the telescope as a tool for astronomers by showing that there were objects in space to be discovered that until then had remained unseen by the naked eye. More importantly, the discovery of celestial bodies orbiting something other than the Earth dealt a blow to the canonical Ptolemaic world system, which held that the Earth was at the center of the universe and all other celestial bodies revolved around it. In his writings Sidereus Nuncius (Starry Messenger), which announced his new celestial observations, Galileo does not explicitly mention the new Copernican heliocentrism, a theory that placed the Sun at the center of the universe. Nevertheless, Galileo accepted the Copernican theory. As a result of these discoveries, Galileo was able to develop a method of determining the current longitude based on the timing of the orbits of the Galilean moons.

But, Galileo might not have been the first to get a glimpse of the moons of Jupiter. A Chinese historian of astronomy, Xi Zezong, has claimed that a “small reddish star” observed near Jupiter in 362 BC by Chinese astronomer Gan De may have been Ganymede, predating Galileo’s discovery by around two millennia.

At yovisto you can learn more about Jupiter and also about the Galilean moons in the NASA documentary on the voyager space probe ‘Voyager – Humanities Farthest Journey

References and Further Reading


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