Missives from a fly bottle
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Last revised 31 August 2017
28. The shadow

Here are two principles about shadows that most people would accept to be true:

1) Nothing can cast a shadow unless light first falls upon it.

2) A shadow cannot “pass through” an opaque object. (For example, lit from above, a bowl on a wooden table will cast a shadow upon the table top, and not upon the floor below, because the bowl’s shadow cannot “pass through” the table top and land on the floor.)

But now, in a rare astronomical conjunction, two of Jupiter’s moons, Callisto and Europa, line up perfectly with the sun and a shadow
is cast upon Jupiter’s surface as shown.

But whose shadow is it?

It cannot be Europa’s shadow since, by the first principle, nothing can cast a shadow unless light first falls upon it, but no light falls upon Europa, being shielded completely by Callisto.

Nor can it be Callisto’s shadow since, by the second principle, Callisto’s shadow cannot be cast through Europa and land on Jupiter’s surface.

From Bas van Fraasen’s Laws and Symmetry.