# 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 Δ*φ _{AB}* between

*A*and

*B*can be found, as well as the departure, the distance made good east or west. The difference in longitude Δ

*λ*is unknown and has to be calculated using meridional parts as in Mercator sailing.

_{AB}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

- Rhumb line
- Traverse sailing
- Parallel sailing
- Mid-latitude sailing
- Mercator Sailing
- Great-circle navigation