The Galilean Moons
If you go out after sunset at the moment and face East, you can’t miss Jupiter, the second brightest object out there after Venus. He is very bright at the moment because he is close to its nearest position from us.
If you have a small telescope with a x12 magnification (or even good 7x50 binoculars propped on something solid), point at Jupiter and you will see some of his biggest moons, known as the Galilean Moons, discovered by Galileo Galilei with his home made telescope around 1610. From Jupiter outwards, they are called Io, Europa, Ganymede and Callisto.
Slightly bigger than our Moon, Io has the most violent geology in the Solar System due to the huge tidal forces Jupiter hammers it with. It has over 400 active volcanoes. Its orbital period is about 42 hours (that the time it takes to go around Jupiter).
Europa is a bit smaller and a lot quieter. It is covered by a frozen water ocean that may be liquid under the surface. A good candidate for simple extra-terrestrial life, apparently. Its orbital period is EXACTLY twice that of Io. Weird!
Ganymede is huge, bigger than Mercury! And its orbital period is EXACTLY four times that of Io, or twice that of Europa. Weirder!
And finally Callisto is almost as big, and a bit slower that you may expect now. Its period is not 8 times that of Io, more like 9.4 times.
Jupiter has 79 moons and counting. But the Galileans run the show.
References:
Galilean Moons
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