| Ship's Head | New | Old | By Point |
|---|---|---|---|
| NE | 45° | N 45° E | NE |
| 90° | 90° | N 90° E | EAST |
| SE × E | 123° 45' | S 56° E | SE × E |
| S 20° E | 160° | S 20° E | S x E ¾ E |
| S 2 pts. E | 157° 30' | S 22° 30' E | SSE |
| NW ¾ W | 306° 34' | N 53° W | NW ¾ W |
| 289° 41' | 289° 41' | N 70° W | WNW ¼ W |
I will show you just how each one of these courses is secured from the guide just given you.
Note to Instructor: After explaining these courses in detail, assign for reading in the class room the following articles in Bowditch: Arts. 25-26-27-28-29-30-31-32, 74-75-76-77-78-79-80-81-82.
Every compass, if correct, would have its needle point directly to the real or true North. But practically no compass with which you will become familiar will be correct. It will have an error in it due to the magnetism of the earth. This is called Variation. It will also have an error in it due to the magnetism of the iron in the ship. This is called Deviation. You are undoubtedly familiar with the fact that the earth is a huge magnet and that the magnets in a compass are affected thereby. In other words, the North and South magnetic poles, running through the center of the earth, do not point true North and South. They point at an angle either East or West of the North and South. The amount of this angle in any one spot on the earth is the amount of Variation at that spot. In navigating a ship you must take into account the amount of this Variation. The amount of allowance to be made and the direction (i.e. either East or West) in which it is to be applied are usually indicated on the chart. On large charts, such as those of the North Atlantic, will be found irregular lines running over the chart, and having beside them such notations as 10° W, 15° W, etc. Some lines are marked "No Variation." In such cases no allowance need be made. On harbor charts or other small charts, the Variation is shown by the compass-card printed on the chart. The North point of this card will be found slewed around from the point marking True North and in the compass card will be some such inscription as this: "Variation 9° West in 1914. Increasing 6' per year."
Now let us see how we apply this Variation so that although our compass needle does not point to true North, we can make a correction which will give us our true course in spite of the compass reading. Note these diagrams:
The outer circle represents the sea horizon with the long arrow pointing to true North. The inner circle represents the compass card. In the diagram to the left, the compass needle is pointing three whole points to the left or West of True North. In other words, if your compass said you were heading NE x N, you would not actually be heading NE x N. You would be heading true North.
In other words, standing in the center of the compass and looking toward the circumference, you would find that every true course you sailed would be three points to the left of the compass course. That is called Westerly Variation.
Now look at the diagram to the right. The compass needle is pointing three whole points to the right or East of True North. In other words, standing in the center of the compass and looking toward the circumference, you would find that every true course you sailed would be three points to the right of the compass course. That is called Easterly Variation.