A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Position-Finding
From: Frank Reed
Date: 2018 May 13, 13:32 -0700
Yes, you may well have seen one of 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.