Lats, longs, knots, points and cables

Life on a ship has a lexicon all of its own. Many might know the bow or the stern. You might know that starboard is the right-hand side of the ship facing the bow and that the port side is the left. On board, you don't talk of walls and floors–you're in the world of bulk heads and decks. Some of the terminology has it's basis in the old setup of a ship. The fo'c's'le/forecastle/foredeck was traditionally where the castle towards the bow on a ship was. And while heads are located all around the ship these days, in times past they were also on the bow. These days the forecastle deck is a more pleasant place. But I digress, what I wanted to talk about was the nautical terminology for distances, speeds and directions.

A picture of the usually peaceful foredeck on Discovery taken from our rival blog: Memories of Discovery

Latitude and longitude is a good place to start. These are the two measures used to break globe into north, south, east and west. Latitude is zero at the equator and ±90 at the poles. Longitude goes from 0 to 360º. The zero of longitude is arbitrary but nowadays is accepted to be at the Greenwich Meridian near London. The same place that gives us the Greenwich Mean Time–GMT–that Darren described. The prime meridian wasn't always at Greenwich and, if you're a keen fan of Jules Verne, you'll have to take Paris as your prime meridian to figure out where he's talking about. Paris Mean Time–PMT–didn't last long and, primarily because it was very grumpy and irritable, was eventually superseded by GMT, which is now universally accepted.

One degree of latitude is 60 nautical miles. Or one minute of latitude, 1/60 of a degree, is one nautical mile. Longitude isn't spatially consistent*. 1º of longitude is equal to 60 nautical miles at the equator but elsewhere you have to take a cosine of latitude to get the distance. For example, at 52ºN, the latitude of Cork, one degree of longitude is 37 nautical miles. It is possible, depending on your location to change longitude very quickly. For example, in the picture of Morrissey with a cat on his head (below), should the cat spin around, he/she will go through 360º of Morrissey. The same would be true of a human at the north pole.

Morrissey with a cat on his head. The cat is demonstrating how it is possible to go through 360º of longitude by spinning while located at the poles... or in this case Morrissey's head

Another measure of distance is the cable. It is one tenth of a nautical mile. After eagerly learning this new nautical term, I was determined to use it as often as I could. And, like every scientist who learns a new nautical term, I eventually use it incorrectly. Like one time when we asked our American colleagues to set up two cables from a mooring site and were greeted with blank expressions: Americans don't do cables. 

Is there a point in all this? Well, I'm glad you asked. The point is a unit of direction. While a point as a unit for direction seems a little arbitrary at 11º to begin with, the explanation has some sense to it. It is a point of a compass. If north is at 0º, west is at 90º. Then NW is 45º, NNW is 22.5º. Half of this again is 11.25º and that is one point.

Hard to fathom? Not really: a fathom is 0.01 of a cable and is used as a unit for depth.


Gerard


*Shrewd readers may now be thinking that latitude isn't exactly spatially consistent either due to the earth being an imperfect spheroid.