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Jupiter's umbra at the distance of Io and Ganymede respectively, 


            Niall MacNeill

Jupiter's umbra at the distance of Io and Ganymede respectively

Technical card

Resolution: 3555x3122

Locations: Home property, Wattle Flat, NSW, Australia

Data source: Backyard


I recently imaged Jupiter on two separate occasions with two of its moons entering and emerging from its shadow. It struck me that these occultations show the extent of Jupiter's shadow (umbra) behind the planet. It seems amazing to me that this huge shadow extends back behind Jupiter. As we are on approach to opposition, this umbra is a cone extending back to the right of the planet. At opposition the shadow is directly behind the planet and then it swings to the other side.
Io orbits at a distance of 422,000 kms, where Ganymede is more than twice as far away at 1.1 million kms. Therefore they cut the cone of Jupiter's umbra at different points behind the planet. This graphic shows the shape of the cone of Jupiter's umbra at the set back distance of each planet.
You may observe that the circles are almost identical in size. Even at a distance of 1.1 million kms Jupiter's umbra is still 98.8% of the planet's diameter!! For Io it is 99.5% of the planet's diameter. So yes it is a cone but one that extends a very long way back from the planet. In fact it can be shown that this umbral cone is 50% of Jupiter's diameter at a distance of 43.5 million kms.
Note the bottom image was taken on 26th April and the top one on the 8th of May. In fact, the shadow will have moved somewhat in that time. Therefore the bottom image shows the shadow further displaced from the planet than it would have been on the date the top image was taken. As we move towards opposition it is approaching the planet each day that passes.



Niall MacNeill
License: None (All rights reserved)


Jupiter's umbra at the distance of Io and Ganymede respectively, 


            Niall MacNeill