For most astronomers, vanishing moons are not especially extraordinary. If one knows when and where to look, and given a clear sky and the proper equipment, he or she could probably observe at least one disappearing moon each night of the year.
Without any visual assistance, we see our own moon disappear every month as it passes between us and the sun; a standard pair of binoculars easily reveals four of Jupiter’s moons regularly slipping in and out of visual range; and larger telescopes will unveil even some of the most distant known moons, like Titan, Oberon, or Triton, passing behind their giant hosts. When you think about it, disappearing moons really do seem rather ordinary.
There is one occurrence of vanishing moons, however, that is not so usual. It occurs just a few times each century and it typically lasts but a few short minutes. Fortunately, it is an event that is readily predicted and easily observed, and we will be watching it this week through the SLOOH telescopes.
At any given time, we can see the Galilean satellites accompanying Jupiter like tiny pearls on either side of a brilliant bauble. In fact, you really never see Jupiter without at least one of its four main moons nearby. Never, that is, unless you happen to be watching at just the right time, and one of those times happens to be this week.
Late on the night of May 21, all four of the Galilean moons will vanish from sight, and they will remain invisible together for nearly twenty minutes. For the first time in years, Jupiter will appear to be moon-less. SLOOH will feature this special event with a solid block of Jupiter missions, from 03:40 through 04:20, in Dome 2 on the Member Channel. We’ll also include live commentary from one of our SLOOH SkyGuides to discuss the recorded history of this special event and to explain more about exactly what’s happening with each of those four ‘invisible’ moons.
