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    World Map and Overlays for Great Circle Distances & Zenith Angle
    From: David Iwancio
    Date: 2019 Jul 6, 22:17 -0700

    The former US National Bureau of Standards (NBS, now NIST) published Report 7249, "Technical Considerations in the Selection of Optimum Frequencies for High-Frequency Sky Wave Communication Services," in 1962.  The former Earth Science Services Administration (ESSA, now basically NOAA) reprinted it in 1969, and a copy of the reprint is availabe at Hathi Trust:


    Of possible interest to NavList are the graphical methods in the paper of determining the great circle distance between two points and the sun's zenith distance at a point for a given time-of-day and month.  This is accomplished with a world map and what is basically a series of transparent overlays (the reader is instructed to plot on a blank transparency and transfer the points between pages).

    Page 69 is an equirectangular map of the world, followed by the first transparency on page 70 for determining great circle distance.  The user aligns the equator on the diagram with the one on the map and slides it along until the two points are on or near the same great circle and the distance is read off via numbered curves perpendicular to the great circles.

    Pages 134-145 are tranparencies showing circles of equal altitude, down to nautical twilight, for the sun's declination and equation of time for a given calendar month.  Again the equator of the transparency is aligned to the map, and it is moved to align local mean time to a selected meridian.

    NBS also published a book, "Ionospheric Radio Propagation," that included the world map, great circle overlay and the zenith distance diagram for June.  The GPO's online copy of that book has better scans of those particular diagrams than Hathi.

    https://www.govinfo.gov/con [...].pdf

    These certainly aren't good enough to navigate by, but they make for an interesting visualization and might be good as teaching aids.

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