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I tried it once, with a 3" telescope. I think that my time of immersion was off by about a minute or a minute and a half. I found it very difficult to be exact because the observed moon does not simply wink off or on; the change in brightness is gradual over several minutes, and to a novice like myself it seemed very arbitrary as to when I decided the moon had entered the shadow.

Io and Europa should give the best results, because they are in lower orbits and transverse the shadow boundary more quickly. Ganymede and Callisto orbit more slowly and are larger, making the transition in brightness significantly longer (tens of minutes, I believe).

Don Seltzer
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