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    Re: Ganimede through a 30*6 scope?
    From: Frank Reed
    Date: 2018 May 13, 13:32 -0700

    Yes, you may well have seen one of Jupiter's moons! 

    At good elongations from the planet, both Ganymede and Callisto can be seen with small telescopes, spotting scopes, binoculars, medium-power sextant scopes, and even "naked eye" if you have good vision and good seeing. There is also a star of similar brightness, 21 Lib, that has been near Jupiter recently, so there's a chance (small, I would say) that you saw this star. It's easy to check the moons as well as nearby stars in Stellarium (see PS) and other "planetarium" software and apps. If you prefer something more lightweight, you can try this javascript app (very old but still functional): S&T Jupiter's Moons.

    Note: there was a suggestion just a few years ago that early (non-western) celestial navigators might have used visual observation of Jupiter's moons to get their longitude. This is a huge confusion. Yes, you can use Jupiter's moons to get absolute time (and thence longitude). Yes, you can sometimes see one of Jupiter's moons without a telescope. But it's not the same moons. Only the outermost of the four Galilean moons can be detected without a telescope --and just barely under extraordinary conditions, and they move far too slowly to determine absolute time. The innermost moons, especially Io, must be observed at high magnification as they are eclipsed by the planet or transit it to get absolute time. It's their rapid motion and the rapidly varying events of eclipses, transits, etc. that enabled surveyors and land navigators in earlier centuries to get longitude from Jupiter's moons.

    Frank Reed

    PS: Suppose you want to see Jupiter's moons in Stellarium. Open the app (desktop preferred; the smartphone version is a separate "fork" of the project) and set for the right date, time, and location. Then, if necessary, hit ctrl-F and type "Jupiter" to display the planet. Next tap the spacebar to lock the view on Jupiter. Then zoom in. You do this with various gestures and mouse functions. If you have a trackpad on your computer, try pulling with two fingers towards you. Zoom in or out until you can see Jupiter and the four great moons in a small area on-screen. Now play the app forward or backward in time. Pay particular attention to any "field stars" that might get confused with the moons. Of course, Stellarium is an open-source project and prone to more errors than centrally-controlled projects. It's a good idea to check the configurations of the moons with some other tool, like the S&T javascript app linked above.

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