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    Re: Lunars.
    From: Herbert Prinz
    Date: 2001 Jul 09, 1:47 PM

    Dear Nigel and Steven,
    It find it hard to understand how the concept of "cheating" would apply to taking
    a lunar distance. LDs were and are not a sport, but rather a method of solving a
    technical problem with any tool available. Presetting the sextant was not only
    permissible, but in fact common practice. Not only did it save the trouble of
    moving the star across the sky diagonally, but it also helped with the body's
    identification. See, for instance, what Bowditch, American Practical Navigator,
    1909 (or similar editions), Appendix V has to say on the subject.
    The LD was not a method to find time and longitude out of the blue. It rather
    provided a corrective to an erroneous chronometer. By checking the latter
    regularly against LDs, time was always known to a reasonably good degree of
    The field of view of the sextant telescope that I normally use is ca. 3 deg of
    arc. The moon moves 1/2 deg per hour. One would have to have an unrealistically
    high chronometer error of 6 hours in order to not find the star in the telescope
    after presetting the sextant. On the other hand, knowing GMT only to the nearest
    hour will get you the star within a diameter of the moon.
    Strictly speaking, one always needs GMT beforehand anyway in order to obtain SD,
    HP and declination of the moon from the almanac.  However, in practice, the
    potential error resulting from a bad estimate of time will normally be
    insignificant; So, this would be my weakest argument.
    Best regards
    Herbert Prinz (from 1368950/-4603950/4182550 ECEF)
    >  Nigel wrote:
    > > Regarding watches, calcs etc, my own view is that if one wishes to use such
    > > exercises as LD's then they should be done by hand with logs of the time,
    > > usually Norie's 1805 edition (I do have earlier tables but they are a bit
    > > fragile)
    > Steven Wepster replied:
    > Yep. I saw what I did as a first experiment. One of the major difficulties is
    > getting an accurate measurement. It's relatively easy to bring a body down on
    > the horizon, but it's something completely different to bring it diagonally to
    > the moon. I didn't want to pre-set the sextant to the approx. right angle
    > because that is a _serious_ form of cheating, in my opinion. You need to know
    > Greenwich Time [for] it.

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