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    Re: Star to Star Distance Sextant Calibration
    From: Paul Hirose
    Date: 2019 Apr 11, 19:52 -0700

    My Lunar program for Windows computes separation angles between any two
    bodies. It won't draw a helpful diagram or suggest suitable stars, but
    if you know the designations of the stars, it computes the data.
    
    The star catalog contains the names of all 57 navigational stars and all
    names standardized by the IAU that are within the magnitude limit. It's
    a subset of the Hipparcos catalog re-reduction (van Leeuwen, 2007)
    complete to magnitude 3.0.
    
    Star data may also be input manually. For example, you can enter data
    from the Gaia catalog.
    
    A useful output is the position angle from each body to the other. This
    aids quick acquisition, especially if the separation is large. For
    example, if the position angle is zero, hold the sextant vertically in
    the usual orientation. If position angle is 45°, rotate the sextant 45°
    counterclockwise from vertical. (This may seem backward, but if you
    think in terms of the celestial sphere viewed from the outside, it's the
    same sense as azimuth, but with the zenith as "north.")
    
    The program is free on request. This page has a hint on how to contact
    me privately.
    
    http://fer3.com/arc/m2.aspx/SofaJpl-page-down-Hirose-apr-2019-g44768
    
    Example: 2019 April 12 03:00 UT1, 40N 100W, 2000 feet, temperature 35 F,
    altimeter setting 29.9 inches, humidity 50%.
    
    16°47.44' Spica unrefracted altitude
    00°03.03' refraction
    16°50.48' refracted altitude
    121°17.81' azimuth
    
    61°08.33' Regulus unrefracted altitude
    00°00.51' refraction
    61°08.85' refracted altitude
    165°02.02' azimuth
    
    54°03.84' separation, unrefracted
    00°02.47' refraction
    54°01.38' separation, refracted
    
    336° position angle, Spica to Regulus. This is a comfortable angle.
    Position angles on the other side of zero are not so convenient since
    the sextant frame must be rotated counterclockwise from vertical, i.e.
    handle up instead of handle down.
    

       
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