Glossary of Astronomical Terms/Galilean satellites
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The Galilean satellites are the four largest satellites that orbit the planet Jupiter. The Italian astronomer Galileo Galilei discovered them on January 7, 1610, in the period when he first used a telescope to observe the heavens. The moons were named by Simon Marius, another early astronomer.
The four Galilean satellites are:
The orbits of the Galilean satellites had been well-known for decades after Galileo's discovery. It was therefore a shock to the Danish astronomer Ole Roemer, who was working in Paris in 1675, to see that Jupiter did not eclipse the moons as predicted. The eclipses came earlier than they had 6 months previous. Roemer correctly deduced that the difference was related to the speed of light, which he calculated at approximately 200,000 kilometers a second (200,000,000 meters a second, or 2 x 10^9 m/s in SI units). Although the figure was too low, Roemer was correct to one order of magnitude.
