Wednesday, February 16, 2011

Tools Of The Trade: Getting The Point

The compass is a device that has been in use for hundreds of years. It most probably made its debut at sea, although there is some ongoing debate about that, and has always been a somewhat suspicious item aboard ship. Sailors of old found the mystery of its magnetism akin to magick; they liked their compass stowed neatly in a brass binnacle not just to keep it safe from the elements but to keep them safe from it.

A compass works on the relatively simple principle of the magnetism of Earth’s poles. That may be where “simple” ends as far as a compass, however, at least when one hears it spoken of. Today, let us look at some of the terminology surrounding the compass as it is used for navigation at sea.

The card is the familiar surface of the compass on which the points and degrees are marked as exampled above in the beautifully preserved compass card by Samuel Thaxter and Co. circa 1792. In the center the needle, usually in that familiar arrow shape would rest on the pin. The card itself is free spinning and the needle magnetized to constantly point toward magnetic North.

The points are the 32 divisions on the card. Each one is a standard eleven and one half degrees apart. The degrees, then, are the 360 divisions on a modern compass card. They start at 000 degrees at the North. South is 180 degrees with East being 090 and West 270. A full rotation of 360 degrees brings you back to North.

The cardinal points on a compass are North, South, East and West. The intercardinal points are Northeast, Southeast, Northwest and Southwest. Sometimes these are referred to as the half-cardinal points.

Since the accuracy of a compass varies depending on a number of factors – particularly at sea – variations and deviations have to be taken into account and then corrected for.

Variation is the angle of difference between true and magnetic direction in degrees East or West. Deviation are similarly expressed East or West but are the angle of difference caused by metal objects in the ship itself. The sum, in algebraic terms, of variation and deviation is known as compass error.

The process of determining deviations as they relate to the ship’s course, which needs to be done with some regularity, is known as swinging the compass. A log book, known as a deviation card, is kept to list deviations as determined in this manner.

Once variation and deviation have been taken into account, it’s time for correcting or uncorrecting as the case may be. Correcting is the process of converting the magnetic direction indicated by the compass to the true direction needed to stay on course. Accomplish this by subtracting westerly errors and adding errors to the east.

Uncorrecting, obviously, works the opposite way. In this case one is correcting to magnetic direction from true direction. There is a humorous mnemonic device for this: Timid Virgins Make Dull Company, Add Whiskey (as Jack explained to Stephen on one particular Seafaring Sunday). In this case the westerly errors are added to the true direction to determine magnetic direction.

All of the above, which are relatively simple to the seasoned navigator, seem mind boggling (and perhaps migraine-inducing) to the landsman or new seaman. It is absolutely no wonder that a long apprenticeship was necessary before one could be trusted at the wheel to take correct action in any kind of tense situation. “Come about,” sounds simple, but “Come about two points north-northwest and see to deviation” would take years of practice to accomplish quickly.

Of course now a ship has every kind of imaginable gadget to take care of these issues for it. But some of us are still fond of our magickal, mysterious compass.

Header: As noted above; available for purchase from


Timmy! said...

Ahoy, Pauline! Wow, that is a little mind boggling... or mind bottling if you are a landsman like me...

Pauline said...

Ahoy, Timmy! It's a lot to absorb in writing. That's why on the job training was (and still is) a better way to approach the use of a compass.

Le Loup said...

Very interesting, thank you. There certainly is something very special about an old compass, or at least there is for me.

Marrty said...

I'm with Tim. let us persue this mind boggling with some mind bottling when I return!

Pauline said...

Ahoy, Le Loup! I agree and would extend that to all those old, well used tools that take us back in time if only for a moment.

Pauline said...

Ahoy, Marty! You're on, mate. I'm all for hoisting a tankard with ya when you return to the cold from the bitterly, freezing cold.