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Or magnetic declination, is the angle at any place between the magnetic meridian and the geographical, or true, meridian. It is named east if the north end of the compass needle points to the right and west if it points to the left of the true meridian. With other corrections, notably deviation, variation is applied to the compass course or compass bearing. At any location, the variation may increase or decrease from year to year according to the movement of the north magnetic pole, and such differences will be indicated on the chart. The first chart to show lines of equal variation was that by the English astronomer Edmund Halley (1656–1742), which was published in 1701. This was also the first printed isoline map of any kind. For illus. see compass error.

Subjects: Maritime History.

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