History of Navigation: Using a Compass

Today large ships use a gyrocompass, aligned with the true north pole. But seafarers still use the reliable thousand year old magnetic compass, which points to the magnetic north pole. The difference between these two north poles is called the variation. In Maine, the variation is about 18° West, so that if you want to go due east, true, you would add 18° to 90° then proceed on a course of 108° magnetic.

The course or bearing from one spot or ship’s position to another is determined by drawing a line between the points on a chart. Using a parallel ruler or other line transferring tool, the line is “moved” to the nearest compass rose. The bearing is then read on the compass rose. The outer ring reads true, and the inner ring reads magnetic. Using dividers or another distance measuring tool, the distance between the two points is transferred to the latitude scale on the side of the chart, and the distance is read. When sailing the course, bearings can be taken to marks to help determine how far a vessel has traveled or where it is.

Using Parallel Rules

Reading the Compass Course

Measuring the Distance

For example, using the Penobscot River chart, one can set a course by identifying where the vessel is, say, at a green gong buoy off Sears Island, and placing parallel rules on a line from there to where you next want to go. If we want to proceed southwest toward a red and white bell that has a Morse code “A” light characteristic (long on, short off, short on, long off), we set the parallel rule along the line between the two buoys. Then we “walk” the parallel rules to the nearest compass rose and find that we want to proceed on a course of 221° true (using the outer ring of the compass rose) or 239° magnetic (using the inner ring of the compass rose). The distance can be measured with a pair of dividers, which are then set along the latitude scale on the side of the chart, to see that the distance is 3.15 nautical miles.

The Plotted Course

The Visual Bearing

Along the way, one could determine how far the vessel has traveled by taking a bearing. If we looked over the magnetic compass and saw the red bell buoy off of Steel’s Ledge in Belfast at a bearing of 295° magnetic, we could draw a 295° magnetic line from the red bell buoy toward our course line. We would then see that the bearing line crossed the track line .65 nautical miles from our starting green gong buoy off Sears Island.