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    Re: Lunar Distances with Alex's SNO-T
    From: Bill B
    Date: 2006 Nov 09, 02:08 -0500

    Left town unexpectedly for family matters.  I apologize for the delay in
    response.
    
    Frank wrote:
    
    > So there's no
    > problem calculating its exact angular size in the sky. BUT you can't use the
    > value in the Nautical Almanac. Is that sacrilege? Isn't the Nautical Almanac
    > nearly Holy Scripture for celestial navigation? Some people treat it that way
    > sometimes, but in fact, the modern Nautical Almanac is no more than a
    > well-honed tool for a particular class of celestial navigation observations,
    > namely, ordinary altitude observations. And here and there in the tables of
    > the Nautical Almanac, you will find that some quantities are inaccurate by as
    > much as three-tenths of a minute of arc, because that level of error is not
    > critical for ordinary altitude observations.
    >
    > To get the correct semi-diameter of the Moon, just take out the Moon's HP from
    > the almanac for the correct hour of observation. Multiply that by 27.27%. Then
    > apply the augmentation. You can calculate this or just use a short lookup
    > table as follows: if the Moon's altitude is between 10 and 30 degrees, add 0.1
    > minutes of arc, between 30 and 60 add 0.2', above 60 add 0.3' (see "Easy
    > Lunars" on my web site). That's close enough in most cases. Note that if the
    > Moon is lower than 15 degrees or so, you have to take refractional flattening
    > into account, but you could just as well wait until the Moon's higher than
    > that.
    
    I'll take the word of a guy that knows the exact diameter of the moon and
    its distance from the observer at any given time.  And I would bet the farm
    you know how to determine that.
    
    For those of us OCD in other areas, if I recall you stated your formula was
    approximate.
    
    You said 27.27%, Alex posted 27.277. Is his added digit meaningful?
    
    How "approximate" is "approximate?" You wrote, "Actually, the correct
    horizon SD at the time of the observation was 15.9 (calculated from 27.27%
    of the HP --you should always use a calculated SD when doing lunars). Then
    you need to augment it for altitude which gives 16.0 minutes of arc. The
    result is then quite a bit closer, but yes, a little off --an error of 0.14
    minutes of arc in the Moon's diameter.
    
    "0.14 minutes of arc." Off what standard, please?
    
    Thanks again
    
    Bill
    
    
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