It is not difficult to photograph the Galilean satellites as long as you have a telescope or zoom lens (which is effectively a small telescope for your camera). In October 2021, we are just coming towards the end of one series of mutual events around that date. The last time Jupiter was at an equinox was on. Or one may disappear into the shadow of another. For example, one Moon can occult, or pass in front of, another. But every six years, when Jupiter reaches one of its two equinoxes during its 12-year orbit, the plane of the Galilean satellites’ orbits appears edge-on to us.įor a few months around each equinox, mutual events can occur between these moons. Jupiter’s big four satellites orbit in virtually the same orbital plane (imagine the orbit as a disk that cuts through the planet).įor most of the time, that plane, or disk, is tilted up or down as viewed from the Earth. Mutual events – eclipses and occultations Incidentally, Jupiter’s remaining moons are much smaller bodies and so not easy for the amateur astronomer to spot. So Ganymede may appear closer to Jupiter than Io if it is on the far side of its orbit, for example. You will find that they may well be displayed in a different order from that of distance. Or you may see fewer than four moons if one or more are in front of or behind the planet.Ī smartphone or computer app such as Stellarium will help tell you which moon is which. You may see two moons each side of Jupiter, or one on one side and three on the other. Look from one night to the next and their positions will have changed completely, as shown in the two images on this page, taken 24 hours apart. It may surprise you to see how quickly these moons travel in their orbits. Jupiter and satellites (from left) Europa, Io, Ganymede and Callisto, photographed on the night of June 19th, 2018. How the arrangement of the Galilean moons changed over 24 hours.
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