Plane sailing

Plane sailing (also, colloquially and historically, spelled plain sailing) is an approximate method of navigation over small ranges of latitude and longitude. With the course and distance known, the difference in latitude ΔφAB between A and B can be found, as well as the departure, the distance made good east or west. The difference in longitude ΔλAB is unknown and has to be calculated using meridional parts as in Mercator sailing.

Departure p and the difference in latitude ΔφAB can be worked out with simple trigonometry.

Both spellings ("plane" and "plain") have been in use for several centuries,[1][2][3]

Plane sailing is based on the assumption that the meridian through the point of departure, the parallel through the destination, and the course line form a right triangle in a plane, called the "plane sailing triangle".

The expressions "plane sailing" (or more commonly "plain sailing") has, by analogy, taken on a more general meaning of any activity that is relatively straightforward.

See also

References
  1. "A Token for Ship-Boys, or Plain Sailing made more plain" (Adam Martindale, 1683, A Collection of Letters for Improvement of Husbandry & Trade)
  2. "The rudiments, which would no sooner be run over, than the rest would become plain sailing." (Fanny Burney, 1796, Camilla)
  3. Oxford English Dictionary
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