If you’re going to accurately rely on your compass, you should be aware of something called “Variation”, which affects its accuracy. Variation is the difference between “true north” and “magnetic north.” The difference between true and magnetic is a result of the earth’s magnetic poles not being located where its geographic poles are. The magnetic poles are what affect your compass. The geographic poles are the points where the meridians of longitude (those vertical lines on your chart) connect at the North and South Poles.
The difference occurs for a few reasons. First, the magnetic attraction is not exactly at the North Pole. Second, the magnetic attraction is not uniform in its shape and intensity. And third, the magnetic attraction is always slightly moving beneath the earth’s surface. For these reasons, your compass usually provides a reading that is at variance with true north. The resulting difference is called “variation.”
Nautical charts provide printed information on variation in the compass rose. The compass rose is made up of two rings, graduated in 360 degrees. The outer ring represents true north and the inner ring represents magnetic north. The difference, or the variation, is printed in “degrees east” or “degrees west,” depending on which direction magnetic north varies from true north. In the center of the compass rose, you will see the abbreviation “VAR” followed by the number of degrees between true and magnetic north.
Variation is not uniform around the globe since the proximity of true north, magnetic north and your vessel change as you travel. At one point you may be due south of both true north and magnetic north. At other times, you may be south of true north and southeast of magnetic north. Since variation is the angular difference between the two, as your relative position changes, the amount of variation changes.
Your chart will generally have several compass roses. In most cases, the variation printed in the center of the compass roses on the same chart is slightly different for each, due to their differing locations. For this reason, to ensure accuracy, you should always refer to the compass rose nearest to your actual position.
In addition to the existence of variation, its value is always in a state of change; since magnetic north is always slightly moving (that geological phenomenon is a topic for another day). You will notice that the “VAR” in the center of the compass rose is followed a number of degrees (east or west) and by a year in parenthesis. That year indicates the year in which the chart was printed, and the year for which the “VAR” is accurate. Below the “VAR” and its year, is the “annual increase” or “annual decrease.” The annual increase or decrease is the adjustment to be made to the printed variation for each year since the chart was last revised.
Until next time, we wish you clear skies, fair winds and calm seas!