Jupiter has recently undergone an equinox which means that it’s equatorial plane appears pretty much edge on to us here on Earth at the moment. The four big moons of Jupiter, the so called Galilean moons, all orbit more-or-less in Jupiter’s equatorial plane, so around the time of a Jovian equinox, they can appear to interact with one another.
They’re not physically interacting of course but they can appear to pass in front of one another, partially hide bits of each other and even cast shadows on one another. Collectively, these line of sight interactions are known as “mutual events”.
On the night of April 8th 2015, I sat outside watching Jupiter under reasonable seeing conditions. The sky was actually a bit wobbly but after a recent run of jelly seeing what was delivered on the 8th was very welcome. The Great Red Spot was on disc as was the second Galilean moon, Europa. You can see Europa in this red-filtered image I took; it’s the dark dot in the centre of the planet.
As the minutes clicked by, Europa headed towards the edge of Jupiter’s disc. As the disc naturally darkens close to the limb, the more constant brightness of Europa switches it’s appearance from a dark dot to a bright one. The black dot you can see coming in from the left is Europa’s shadow.
The video has been constructed from 90 separate movie files (approximately 70Gb of component files were captured for this sequence) each distilled into a still frame. The frames have then been individually processed and the whole lot compiled into a single AVI movie file.
It’s still a work in progress and there are bits about it that need to be addressed. The ‘disturbance’ off Jupiter’s limb close to Europa, isn’t real – it’s an imaging artifact (seriously conspiracy theorists, it’s not of alien origin nor it is a sign that Jupiter’s about to explode!). Also, there is some noise from the individual frame edges which I’ll clean up at a later date.
The frames have had an artificial gamma boost which renders the planet looking a bit washed out. This was necessary to brighten the rather dark disc of Callisto which can be seen off to the right of Jupiter.
As Europa heads off disc, it heads over towards Callisto before passing right in front of it in a moon-moon transit, mutual event.