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George wrote:

> There is no way to time a lunar eclipse with "absolute precision", as
> anyone who has watched one will know. The whole event is fuzzy and
> indefinite. [snip] the moment of the Moon entering completely into full
> shadow is indeterminate


I have watched a number of lunar eclipses and what still surprises me is
that the umbra has a rather sharp edge -- not sufficient to satisfy
those who like to fuss over the precise moment that one of the moons of
Jupiter passes into the shadow of its parent but very much sharper than
if the penumbra shaded steadily into the umbra. Why this should be so, I
have never figured out. But it is.

George thereafter noted:

> Menzies tells us that the key event that had to be timed (with a star), was
> what he calls U3, at the first signs of emergence from full umbra. This
> seems surprising, as U2, the entry into full umbra, would have been equally
> useful, and the combination of the two more useful still.


I would suggest that the observer has no warning that U2 is coming until
it has arrived. It would be some little while, even a few minutes, after
U2 before anyone would know that it had been passed. U3, in contrast,
can be anticipated as the patch of umbra visible on the Moon's disc
becomes smaller and smaller.


And thanks to Richard Langley for pointing out that the date should be
1421. Something in the back of my head said that 1491 was much too close
to Columbus' date but the warning didn't press on my consciousness
sufficiently for me to check.


Trevor Kenchington


--
Trevor J. Kenchington PhD                         Gadus@iStar.ca
Gadus Associates,                                 Office(902) 889-9250
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Nova Scotia  B0J 2L0, CANADA                      Home  (902) 889-3555

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