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    Re: Sunset Intercepts
    From: Antoine Couëtte
    Date: 2012 Aug 3, 23:03 -0700

    Yes, Greg, thanks for your last note.

    Given the importance of the Refraction Correction for h=0°, any small change in either Temperature and/or pressure might yield a significant modification in the computed intercept value.

    For example, if I now enter your values with 72°F and 1016 mb, which are not "immensely" far from standard values (50 °F and 1013.25 mb) and the effects of which also partially cancel each out (if I am right), the intercept already jumps from +0.5 NM to a new value of +1.8 NM.

    But we all know, don't we, that Refractions at low altitudes are subject to strong and unpredictable variations. Therefore our Intercepts published with 0'1 figures cannot be taken at face value for such low heights.

    Best Regards and Thanks




    Refraction values seem to vary with NAs, apps, and on-line computers when using hs 0* as an entry.

    Naval Observatory NA -33.8'
    Tamaya NC-77 -34.8'
    Palm Pilot -34.4'
    Umland (on-line) -34.0'
    Wise (on-line) -33.8'

    The NA standard- temp 10*C press. 1013 mb (29.93 in.). Any deviation from standard changes values noticeably.

    Greg Rudzinski

    [NavList] Re: Sunset Intercepts
    From: Antoine Couëtte
    Date: 3 Aug 2012 08:46
    Thanks Greg.

    Under standard conditions, I find I/Az = +0.5 NM T /291°8

    The intercept difference (you found +1.1 T) seems mostly due to refraction, then.

    I am aware that the Nautical US and French Refraction tables differ by 0.5' or 0'6 for Altitude 0°, and that above 5° they differ by less than 0'1.

    Thanks again, Greg, and

    Have a Great Day


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