Some time ago, I published an image of the shadow transits of Ganymede and Io I had taken on the 9th March 2014. The moon Ganymede was clearly visible but try as I might I could not find Io against the clouds of Jupiter. I used APS 'creatively' to try and convince myself that I had found it but eventually realised that if you clicked the sharpening tool enough times in any location on the cloud tops of Jupiter you could create a nice Io just about anywhere you wanted to!

Anyway and eventually, I stopped clicking my mouse and engaged my eyes and brain. I had always been aware that in my image, Ganymede's shadow was far from circular. I had dismissed this as an artifact of the curvature of the planet, the shadow was close to the limb so an elliptical rather than a circular shadow was not unexpected. However when I thought about it, if it was being caused by the curvature of the planet, you would expect the major axis of the ellipse to run perpendicular to a tangent drawn at the the limb and roughly through the centre of the face of Jupiter. Even a cursory inspection of the image showed this not to be the case. Enlarging the shadow showed that the pixels were centred at two points and one set of pixels was distinctly less bright. Could this be a partial solar eclipse of Io by the moon Ganymede? As Io orbits inside the orbit of Ganymede this would appear to be a theoretical possibility. Any thoughts and advice on this would be welcome.