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Three days ago (Sunday 11st March) in London using my new telescope I was able to see Jupiter and three of its moons. It was very clear and easy to spot. How can I determine the names of the moons I saw (and which one was which)? I would assume they were the Galilean moons - as they are the largest, making the easily visible, however, I am not sure which moons I would have seen.

This is how they lined up through the ‘scope :



where .=moon & O= Jupiter

Any help would be appreciated.


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Jupiter's inner moons move fast enough that, in addition to the date, you really need the time (in UTC or with local timezone) as well. – Dan Neely Mar 13 '12 at 17:30
up vote 3 down vote accepted

There are a large number of programs called planetarium programs, which animate the solar system, so that you can view the positions of any objects, such as the Galilean moons, at any point in time. These are so accurate that you can enter the times Galileo himself observed the moons in 1609 and 1610 and see them plotted accurately. My personal favourite for many years has been Starry Night, so much so that I now work for the company that produces it as a technical writer.

If you just want to identify the moons, my friend Akkana Peck has written a little aplet which plots them online for any date and time:

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the best way to figure it out is to install a program called Stellarium on your computer. You can find Jupiter and zoom right in to the point that you can see the moons. Then you can reverse time or fast forward, and that will show you which moons were which at the time you were observing.

You can also use this to look forward in time to see a point that one of the moons transits in front of Jupiter and plan your time accordingly.

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