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    Re: Moon rise and moon set time
    From: Hanno Ix
    Date: 2015 May 14, 10:06 -0700
    Frank,

    allow me another question about another  "Gedanken-experiment":

    One should be able, in theory, to determine longitude by the difference in time of the culminations
    of the moon and the sun, say. It is not easy to observe the culmination of the moon but it
    could be calculated from the culmination of the moon's rim. The accuracy could be improved
    by a number of repetitions.

    Can this be done in practice at a stationary location? I believe it is not - if so, why not?

    H

    On Wed, May 13, 2015 at 2:01 PM, Frank Reed <NoReply_FrankReed@fer3.com> wrote:

    Hanno, you wrote:
    "In theory, can I determine my longitude by he time of the rising / setting of the moon at my location?"

    In principle, it's a special case of longitude by lunar altitude instead of the standard "lunar" employing lunar "distances" from other celestial bodies. This lunar altitude method has been re-discovered numerous times and publicized as something that might make longitude by the Moon accessible to a broader group of navigators. I know of a paper from the 1840s describing this technique, and in the 1960s John S. Letcher invented a version which he wrote up in his excellent book "Self-Contained Celestial Navigation with H.O. 208". The trouble with using lunar altitudes instead of lunar distances is that it introduces the sea horizon into the problem, and the sea horizon is uncertain at a level of about 0.25' - 0.5' in good conditions and worse under difficult conditions. The rising or setting of the Moon could serve as a sextant-free surrogate except for one little detail: the Moon is almost impossible to see when it's right at the horizon. Even under the best of circumstances with a full moon and excellent sky transparency right down to the horizon, the lowest portion of the Moon becomes lost in extinction near the horizon. The rising or setting Moon could serve as a crude measure of longitude, but it's of no practical use (and was not even considered as a backup historically).

    Frank Reed
    ReedNavigation.com
    Conanicut Island USA


       
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