Sailor Mouth Saturday: Plain/Plane

Plane and plain when used at sea mainly reference navigation. Both words derive from a common Latin root: planus meaning level or flat. This connotation serves them well at sea but also makes their usages a bit confusing when you consider that our world is a globe.


Plain, of course, is a form of terrain and the opposite of mountain or hill. It may not be perfectly flat, but it is generally an open area with no more than fauna, berms or divets to obstruct forward progress. There is no “climbing” a plain.

By comparison plane does indicate a flat, level surface.

In ancient times, navigation often depended on what is now known as a plane-chart. This form of mapping supposes that the world is a level plane; in essence, flat. Needless to say such charts, though still produced, were out of favor with seamen by the dawn of the medieval period. Contrary to popular belief, it was hardly Columbus that “proved” the world round.

More progressive navigation was later done based on the all important meridian. This is an imaginary line at certain points on a chart or globe that bisects the Equator as it travels from one pole, through another and back around. Noon at any point on a map occurred at its meridian, and therefore in sailing points directly north and south of that point shared the same time of day. Since “calling noon” began a ship’s day, knowing the meridian was a powerful tool not only in navigation but in keeping precious order, especially in blue water. The meridian transits of planets and stars were also an aid to navigation. It is from the marine use of meridians that we get our abbreviation for before and after noon: A.M. being ante meridiem and P.M. being post meridiem.

Plane sailing is a term that those who have read nautical fiction may be familiar with. In the 17th, 18th and 19th centuries it was a euphemism for anything simple, easy to understand or impossible to mess up. Plane sailing was – and to some degree still is – a form of navigation that Admiral Smyth describes as:

That part of navigation which treats a ship’s course as an angle, and the distance, difference of latitude, and easting or westing, as the sides of a right-angled triangle. The easting and westing are called departure.

This is a general way to gage a ship’s direction that will suffer in accuracy with even the slightest bit of foul weather or wind. It was by and large the go-to form of navigation, along with dead reckoning, for centuries of sailing.

I hope that’s plain to one and all. Far winds and safe to your port of call, Brethren; I’ll raise a mug o’ grog to you sooner than later.

Header: 18th century Spanish map of the Americas, showing the established meridian lines