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    Re: Shadow sights for longitude
    From: Geoffrey Kolbe
    Date: 2018 Jan 13, 23:40 -0800

    From: Brad Morris
    Date: 2018 Jan 13, 15:05 -0500

    Hello Geoffrey 
    I found your description quite compelling and read it several times over to be assured that I properly understood it.  Would you mind confirming my understanding? 
    1) Two stars have the same celestial longitude.
    2) we have a vertical line and a vertical slit, separated by a few meters 
    3) when the stars are observed and they are not on our meridian, only one or the other will be blocked by our equipment referenced in (2).  Why?  The azimuth of the objects are dissimilar when not on our meridian.
    4)  as the earth rotates, the vertical slit is moved, keeping one of the two stars occluded.  The delta azimuth diminishes as the meridian transit nears.
    5)  eventually, both stars are on our meridian and the delta azimuth becomes momentarily zero.  We can then use that as our reference to project other lines on the ground.
    Do I have that right?
    Yes - but more correctly the stars will be below the celestial pole so they will be on the anti-meridian. I suggest you see the original Nature article by Kate Spence, which can be seen at:
    a) two points define a line, consequently, there will be pairs of stars which coincidentally will align vertically when not on the prime meridian.  How did the Egyptians know which two stars had the the same SHA and therefore which pair to use?
    That is a different question and not one Kate Spence (or I for that matter) sought to answer.
    b)  You mentioned that the referenced stars are circumpolar.  This would mean that they would align twice a day, unlike other random pairings. Is this how the star pair was selected?
    Yes, they would align twice a day. Once at night, when they could be seen and once when they could not be seen. This, actually, lends evidence that the AE did use this method to align, their pyramids, but you should read the Spence paper to see how that works out.
    c) How did you keep the vertical slit vertical?  Or from rotating in the slightest of breezes?
    Aligning the slit for verticality against the plumb line is not difficult. Having tried this experiment in Egypt, I can confirm that the air is usually very still at night and swinging of the plumb line due to night breeze is not necessarily a problem.
    Geoffrey Kolbe
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