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    Re: Biruni and the radius of the Earth by dip
    From: John Huth
    Date: 2011 Jan 5, 09:03 -0500
    Marcel - 

    Thanks.   That's a great website and has a good calculator to put in different thermal layering effects.   He uses Wegener's approach, which seems pretty solid, before getting into detailed ray tracing.      

    After looking over the atmospheric effects, it looks like al-Biruni's technique is overwhelmed by the vagaries of refraction.   The paper George named in Wikipedia pays mention to it, and even gets the rough magnitude correct, but for some reason I don't understand downplays it.   In the paper, his figure (figure 4) that shows the effect of refraction is a gross underestimate.   

    Both the dip and the refraction are in the 40 arc minute region for a 500 m peak, but the refractive effects can be shifted around by inversions etc.


    John H. 

    On Tue, Jan 4, 2011 at 8:34 PM, Marcel Tschudin <marcel.e.tschudin@gmail.com> wrote:
    On Wed, Jan 5, 2011 at 2:07 AM, Apache Runner wrote:
    > Someone can check my numbers.
    A typical value for the "Kimmtiefe", i.e. for the (refracted) angle
    between astronomical and geodetical horizon is obtained with:
    "Kimmtiefe" in moa = 2.06 sqrt(H).  For an altitude of 500m thus 46
    moa. This formula is based on k=0.125. See also this page from Andy
    Young http://mintaka.sdsu.edu/GF/explain/atmos_refr/dip.html


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