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    Re: Time Sight and Ex-Meridian by Bygrave
    From: Brian Walton
    Date: 2021 Apr 28, 09:11 -0700

    During the early 1980s I happily navigated my boat by Decca, and my 737 by Omega. Then, before scientific calculators and GPS came along, Decca was shut down.

    Fortunately the magic box could still calculate bearing and distance between 2 waypoints, even though there was no signal.  By using almanac data to give GHA and Dec for a body's ground position, and asking the box to calculate track and distance of the body from anywhere, for instance DR, it would come up with bearing (Zn) and distance in nmls. Distance divided by 60 gave degrees, and the remainder gave minutes, to form Zenith distance (ZD). Subtract ZD from 90° and you have Hc. Compare to Hs to get intercept.

    The same trick would work with Omega in flight, and the laser gyros which replaced it. WGS 84 makes little difference if the body is quite high.

    By plotting a St Hilaire as I showed in my post of 6 April, using a Bygrave, I could have got a time sight, Sumner line, or ex-meridian correction.

    The NavList clone on these pages could do the same.  USNO is still silent.  Perhaps this is to fox enemies planning to counter GPS, or maybe to stop cheating in the Golden Globe race.

    Brian Walton

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