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    Re: Longitude by Sunset
    From: Gary LaPook
    Date: 2012 May 8, 00:52 -0700
    The answer to your question  is "nothing." Mr. van Asten really does not understand celnav. The "Parallax in altitude" correction for the moon merely is a computation to convert the altitude actually measured by an observer on the earth's surface to what would have been measured by an observer located at the center of the earth since this is the angle that is needed for celnav and, for the Moon, this can amount to 61'. For bodies other than the Moon we can (for practical navigation) ignore this adjustment because it is very small due to the much greater distances to the other celestial bodies. For precise navigation you can make the same adjustment for  Venus, which can reach 0.5', and for Mars, which can reach 0.3'. The P in A correction for the sun is incorporated into the sun correction tables and this factor can reach 0.15'.
    But none of this affects the actual observation of the body. Mr. van Asten does bring up a theoretically  interesting point in that the observed altitude (Ho) of the Moon can never be zero even though the sextant altitude (Hs) can be (assuming that it is visible and not dimmed out by the atmosphere as we are discussing.) But the Ho is the altitude measured at the center of the Earth and there are not very many observations actually taken at that spot. For example, the semi-diameter of the Moon now is 17' and the refraction at the horizon is 34.5'. So without allowing for P in A (or in this case the horizontal parallax (H.P.)) an upper limb observation of the moon at Moonset, Hs = 0, would place the center of the Moon at 51.5' below the horizon and that is where it really is.  But as observed from the center of the Earth we have to add the H.P. of 61' so the Ho will be 9.5' above the theoretical horizontal. So Mr. van Asten must be taking his observations from a very deep hole.

    gl

    --- On Mon, 5/7/12, Alexandre E Eremenko <eremenko---.edu> wrote:

    From: Alexandre E Eremenko <eremenko---.edu>
    Subject: [NavList] Re: Longitude by Sunset
    To: NavList@fer3.com
    Date: Monday, May 7, 2012, 7:16 PM


    Marcel,
    And I still do not understand at all: what does parallax have to do at all
    with visibility of Moon at a moonrise/set?
    To me his explanation simply makes no sense.
    Sorry for my stupidity, perhaps someone can explain, or give a reference?
    Preferably with a picture of the last ray from the upper edge still visible, and the next ray which is already not.

    It seems to me that there whould be the horizon between these two rays:-)


    Alex.




       
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