The needle or card aligns itself to magnetic north, which does not coincide with true north, so the pilot must also allow for this, called the magnetic variation ( or declination ).
22.
The two iron balls on each side of a magnetic compass, used to balance out and counteract magnetic variations based on a ship's location, are traditionally called " Lamberts ", in his honor.
23.
Beginning in the 1950s, scientists like Victor Vacquier, using magnetic instruments ( magnetometers ) adapted from airborne devices developed during World War II to detect submarines, began recognizing odd magnetic variations across the ocean floor.
24.
Special observations were made by Mr . Darling in 1902 of magnetic variations over that portion of Lake Superior west of Devils Island, including regions of local compass attraction, which had been reported and which endangered navigation by drawing vessels from their courses.
25.
Stukeley concluded the Stonehenge had been set up by the use of a magnetic compass to lay out the works, the needle varying so much, at that time, from true north . He attempted to calculate the change in magnetic variation between the observed and theoretical ( ideal ) Stonehenge sunrise, which he imagined would relate to the date of construction.
26.
Passage charts at smaller scales use the Mercator projection, so that bearings can be directly transferred between charts, although allowances for magnetic variation and magnetic deviation must be considered for accuracy The straight lines drawn on the Mercator projection represent lines of constant bearing; in reality, these lines are not straight but are segments of a three-dimensional loxodromic spiral, or rhumb line.
27.
Aviation sectionals ( maps / charts ) and databases used for air navigation are based on true north rather than magnetic north, and the constant and significant slight changes in the actual location of magnetic north and local irregularities in the planet's magnetic field require that charts and databases be updated at least twice each year to reflect the current magnetic variation correction from true north.
