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    Great Circle Chart - Course & Distance Nomograms
    From: David Iwancio
    Date: 2022 Feb 19, 11:08 -0800

    I recently happened upon this great circle chart used by Charles Lindbergh to plot his path across the Atlantic Ocean:

    https://www.loc.gov/resource/gdcwdl.wdl_06778/

    Other than the historical interest, what caught my eye were the nomograms for determining great circle course and distance by way of some geometric constructions. 

    Course is determined by finding your starting latitude and your latitude after traveling 20° of longitude along your chosen route, and drawing a line between the two latitudes in the nomogram in the bottom right-hand corner of the chart.  A parallel line is then transfered to the compass rose above it.

    Finding distance involves drawing a line perpendicular to the plotted great circle route through the chart's point of tangency (20° N, 30° W in this case).  Measured distances are then transfered to a line constructed parallel to the bottom edge, through special scale points printed on the 30° W line and outside the left and right edges.  Distance is then measured as changes in longitude along this line.

    The table at the bottom is used for determining course after first determining where the great circle crosses the equator.  The method seems better suited than the nomogram when the route is more north-south than east-west.

    There are examples printed on the chart itself, along with LIndbergh's own work (but note he didn't use nautical miles).

       
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