Jupiter’s four Galilean moons are always doing a moon dance around its parent planet, so there will be nights when you will see all four of the moons strung out along Jupiter’s ecliptic in more or less a straight line. Sometimes, you will see a moon just pop into view or disappear like my photo above when it goes behind Jupiter. And sometimes you will see a dark shadow crossing the surface of the planet when a moon is transiting or crossing in front of the surface of the planet…it’s the Suns light, which causes the shadow of the moon to fall on the surface of Jupiter’s.
If you want to keep track of where Jupiter moons (Io, Europa, Ganymede and Callisto) are when observing the planet then you’ll find this information in a couple of places.
The first in written form in the Australian publication called Astronomy 2018, which is an astronomer’s bible for the night sky and you can buy it at any astronomy shop or bookstore. The next is in your country’s current astronomy and space magazine.
But my favourite place, if you have a smart phone or iPad is a FREE App called Gas Giants, which shows you in the field exactly what’s happening with the Galilean system while viewing the planet through your telescope, it can be found in the App store at:
My image was taken on the 11th August 2018 with a Meade 10inch LX200 telescope and a ZWO ASI 120 MC-S colour CCD camera with a 3x Barlow attached. Its just one AVI movie file (2500 frames) stacked in RegiStax6 and processed in PS CS4.