In the days before Jupiter’s Opposition (14th July), shadows of its Moons appear on the disc before they transit. Around 7.30 tonight (28th June), Io’s shadow will be upon Jupiter’s disc, whereas Io does not begin to transit till around 7.40pm.
This order reverses after Opposition, so it is worth having a look at these events before and after opposition to gain a 3D sense of the alignments.
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