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Io Mystery

Hawksmoor

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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.

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I really want someone to be able to throw some light on this topic.   I am confused.....Which is normal.  Sorry to be of no help at all.

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Surely for one moon to partially eclipse another, they would have to be almost in alignment and that is clearly not the case here.

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Surely for one moon to partially eclipse another, they would have to be almost in alignment and that is clearly not the case here.

Thanks for your comment.  As you say, for an eclipse to occur the two moons and the sun would need to be in alignment but our position as observer on Earth does not need to be on the same line. My thought was that my view was  at an oblique angle to the line of the eclipse. Hopefully this makes sense of the image?

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Hi, I would think you could get a solar eclipse of one of the inner moons by one of the outer. I had a look at http://www.calsky.com/cs.cgi  and Io is visible above and to the left of Ganymede's shadow at about the same time as your image.  

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Hi, I would think you could get a solar eclipse of one of the inner moons by one of the outer. I had a look at http://www.calsky.com/cs.cgi  and Io is visible above and to the left of Ganymede's shadow at about the same time as your image.  

Thanks for your helpful comment.

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