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    Re: 1491 The year China discovered longitude
    From: Trevor Kenchington
    Date: 2004 May 8, 12:27 +0000

    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{at}iStar.ca
    Gadus Associates,                                 Office(902) 889-9250
    R.R.#1, Musquodoboit Harbour,                     Fax   (902) 889-9251
    Nova Scotia  B0J 2L0, CANADA                      Home  (902) 889-3555
    
                         Science Serving the Fisheries
                          http://home.istar.ca/~gadus
    
    
    

       
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