Variation

Variation (also called magnetic declination) is the angle at any point on the earth's surface between the direction of true geographic north and the direction of magnetic north indicated by a freely suspended compass needle. It arises because the earth's magnetic poles do not coincide with its geographic poles. Variation is measured in degrees East or West and changes slowly over time due to the secular movement of the magnetic poles; charts show isogonic lines of equal variation, with the annual rate of change printed alongside. Bowditch (American Practical Navigator) provides tables and formulas for interpolating variation at any position and applying the secular change. For the OOW, variation is the first correction applied when converting compass headings or bearings to true, or vice versa. Using an outdated variation figure—because a passage chart has not been updated—can introduce significant errors on high-latitude routes where variation changes rapidly. The Admiralty Manual of Navigation advises officers to extract variation from the largest-scale chart in use and to note the year of the chart to assess how much secular change has occurred since printing. Errors in applying variation have contributed to grounding accidents, reinforcing the importance of this correction as a fundamental navigation discipline.