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I revisited the Charles Lindbergh’s Great Circle Chart https://www.wdl.org/en/item/6778/ that David Pike had drawn attention to in his post http://fer3.com/arc/m2.aspx/Great-Circle-Sailing-Chart-DavidPike-jan-2016-g34267 Lindbergh’s great circle from New York to Paris is obvious but one can also see the some of the lines that were used in intermediate steps.

A line running from the “Point of Tangency” (30°N 30°W) and meeting the trackline at right angles near the 2100 mile mark (52°N 32°W) is also visible. This line meets the track at right angles and is rotated about the Point of Tangency onto the 30°W meridian from where its latitude is read off. Lindbergh read this as about 52°10’ transferred it to the scales at the bottom edges of the chart where a line is visible running horizontally across the entire width starting and ending near 5°N. Distances in nautical miles (nm) can now be read of the chart directly however Lindbergh has marked  intervals of 100 statute miles (86.8nm) taking 1 Nautical Mile = 1.15156 Statute Miles as noted at the bottom. This would seem to be a rather laborious exercise as the distance scale changes along the track line. He concludes that the total great circle distance is 3616 statute miles. He transfers the 100 mile intermediate points back to the New York to Paris trackline and writes the final distance to Paris as 3610 miles presumably allowing for the fact that Le Bourget Field lies to the northeast of the city.

Also visible are Lindbergh’s workings on the Great Circle Course Diagram to the right. His departure point was Roosevelt Field, Long Island, New York at 40°45.5’N 73°36’W. He drew a line from at 41°N on the left hand scale to just under 49°N on the center scale. The latter value is obtaining by reading off the latitude where his New York – Paris great circle track crosses longitude 53°36’W (=73°36’ - 20°).  The line is transferred parallel onto the compass rose where the initial great circle course is marked as just under 54° True.

Does anyone know why Lindbergh worked in statute miles? It would have been much easier to mark his track off in nautical miles? Was it because aviators of the day typically used land maps or that his instruments were calibrated in miles?

So how well did Lindbergh do in his graphical solution?

Using the following departure and arrival points

Roosevelt Field, Long Island, New York 40°45.5’N 73°36’W

Le Bourget Field, Paris 48°58’N 2°26’W

Distance (miles)                            Initial Course

Lindbergh                         3616                                   < 54°

Ellipsoid                            3618.4                               53.7°

Robin Stuart