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Jupiter's moons

Jupiter's 4 brightest satellites can be seen with binoculars:

They can be identified for any given date & time on my web site:

We now know that Jupiter has dozens of moons orbiting around it but four of them are large and bright enough to be seen with very little optical aid: binoculars or a telephoto lens. These 4 were seen by Galileo and they are called Io, Europa, Ganymede and Callisto.

I first became interested in astronomy in my teens by observing these moons myself with a very small telescope (a toy, little more than an inch in diameter). Because Jupiter is very massive the moons orbit very quickly, changing position in a matter of hours. This can be fascinating to watch, especially as they cast shadows on the planet and sometimes on each other, or eclipse each other. It is like a miniature solar system.

The moons can be identified for any given date & time by means of a program on my web site: see

Europa is sometimes in the news because it almost certainly has a large ocean under its icy crust (icy because it is a long way from the Sun, 5 times the Earth-Sun distance). The water is heated by tidal forces as Europa orbits in Jupiter's very strong gravity. It is therefore thought to be the most likely place to find life elsewhere in our solar system.

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