On a rare night of 10/10 seeing, the ASNSW 17.5” Dobsonian was used with three stacked Barlows and a 4.8mm Nagler, yielding a magnification of over 2400x!
A small shadow only one tenth the diameter of Io's shadow was seen and was at first believed to be either an artefact of Jupiter's atmosphere or perhaps the shadow of one of Jupiter's smaller moons.
Initially, as the shadow was observed moving along the equatorial plane of Jupiter but at a faster rate than the planet's rotation, a satellite shadow was inferred.
However, after several minutes, observers were startled to see the shadow abruptly change course and move vertically to the south equatorial belt for about 20 percent of the planet's diameter, then reverse course once more and adopt its initial motion.
Certainly no satellite has ever exhibited such behaviour, and it is reliably rumoured that serious consideration is being given to the shadow being that of some gigantic space vehicle of fictional "Death Star" proportions.
Alternately, Professor Manuela Vögtswagen, Head of the Department of Astral Physics at the University of Bürgerweldt has proposed a more scientific explanation that the shadow is caused by a small, hitherto undiscovered volcanic moon squeezed between the gravitational fields of Io and Jupiter.
Being close to the latter, it has either so far escaped detection by being lost in Jupiter's glare or is a recently captured comet.
The motions then could be caused either by volcanic plumes, or by cometary jets acting like thruster-motors on a rocket and causing random and abrupt perturbations in the object's orbit.