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    Re: Additional altitude correction for Venus
    From: Stan K
    Date: 2015 May 14, 21:53 -0400

    Yes, I could have been more clear.  The part about why the older almanacs show this phenomenon but the newer ones do not is resolved.  But in a previous paragraph I said "And even considering a phase correction, I still do not see how it is a function of altitude, doing more than just cancelling out the effect of parallax, but that might just be a personal problem."

    For the benefit of those who are not lucky enough to have ancient almanacs, the full section reads:

    The additional corrections for Venus and Mars allow for parallax and phase, and are given by p cos H - k cos (theta), where H is the altitude, theta the angle at the planet between the vertical and the Sun, and p and k are, for Venus, for (the year):-

    (Table with eight values of p and eight values of k spread out across the year)

    The corrections given on page A2, and on the bookmark, are mean values applicable in the case of Venus only when the Sun is below the horizon.  For daylight observations of Venus the observed values of H and theta should be used to calculate the correction directly; the term - k cos (theta) is positive when the Sun is lower than Venus, zero when they have the same altitude, and negative when the Sun is higher.

    Yes, Frank, more confusing that it is worth, and certainly fishy, as you explained.  I can understand why they dropped phase.


    -----Original Message-----
    From: Frank Reed <NoReply_FrankReed@fer3.com>
    To: slk1000 <slk1000@aol.com>
    Sent: Thu, May 14, 2015 9:32 pm
    Subject: [NavList] Re: Additional altitude correction for Venus

    Well, it's interesting, yes, but I don't think this is really "resolved" (and in the interest of continuity, I've dropped that word from the subject). The explanation describes a factor of k·cos(theta), but the angle theta depends on the observing circumstances so how could they include it in a generic altitude correction table? Something's fishy. This offset for phase really belongs in the celestial coordinates themselves, which I believe is what they do today, or it could be put in a separate calculation or a small table entered with theta, if it's going to be used at all. At least the values of k seem to be in agreement with the model I described earlier today, so that's encouraging.
    Frank Reed
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