LESSON E: THE COMPASS

A compass is a magnet. It is suspended in a way that allows it to turn on a horizontal plane when it feels magnetic forces from other sources. In order to work properly, the only magnetic force influencing the compass must be the magnetic field of the Earth itself. Most of us have seen a needle such as a sewing needle that has been magnetized. If we suspend such a needle from a thread, or float it in water, the needle will point to Magnetic North. We use compasses shaped like a pocket watch in school or in scouts. They point North because the Earth itself is a magnet and the magnetic poles of the Earth attract one end of the compass needle and repel the other. This causes the needle to point toward the Earth's magnetic poles. In the Northern Hemisphere, the North Magnet Pole is the reference point. Compass needles point to Magnetic North.

A ship's compass works the same way that any other compass works, but it does not look like a needle. Floating on a fluid inside the compass case is a round disc. The disc has a compass needle built into it. The Disc is free to turn upon the surface of the fluid and it is marked with North, South, East and west. The disc of the ship's compass also has numbers around its perimeter-dividing the circle into 360 different points that correspond to the directions. Each direction forms an angle of a certain size when measured in degrees from Magnetic North. These directions are called "compass bearings." North is both 0 and 360; East is 90; South, 180; and West, 270.

There are many times at sea when a compass comes in handy. If you are ten miles or more from shore, you will not be able to see land. The compass helps you steer a steady course until land comes into sight. Even if you can see land, you might encounter thick fog. When this happens, you can only see a very short distance in any direction. The compass allows you to hold a steady course. Without it you might be going around in circles. There are many times when you can see land, but can't recognize, for sure, what land you are looking at. It just looks like trees and hills and a beach it could be anywhere. If you know your location on the water and the compass bearing to your destination, then you use the compass to help you steer in that direction.

We use compass bearings because they are a more accurate way of specifying a direction than the words "North South East and West", or even words like "Northeast." If we were sailing a bearing of 3 degrees, for instance, it would be going "north" but not exactly north. By keeping that number on the compass card lined up with the mark on the compass case that corresponds the bow of the boat, we hold a straight course across the water in that direction.

It is important to know that Magnetic North and True North usually do not line up exactly. In the region of our voyage, Magnetic North is about 16 degrees west of True North. This difference is called the "variation" and in can change a little in different places and at different times. Therefore, whenever a ship's bearing is given the letter M (standing for Magnetic) or the letter T (standing for True) follows it. For solving navigation problems and for entries in the ship's log, it is better to use True bearings, because they will always be the same no matter when you might choose to travel. The bearing on the ship's compass is converted to a "true" bearing by adding the variation and other deviations that the compass may have under certain circumstances.



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