Magnetic Declination Chart

Timeline

1. 1581London · EnglandInstrument maker Robert Norman published The Newe Attractive, the first precise description of magnetic needle dip (magnetic inclination); the same year naval officer William Borough published A Discourse of the Variation of the Compas, providing systematic measured declination data for the London area (about 11° east), an important benchmark for later comparative studies. Together the two works established the empirical tradition of scientific declination research.
2. 1600London · EnglandEnglish physician William Gilbert published De Magnete, systematically expounding the theory that the Earth itself is a giant magnet, explaining magnetic declination and inclination phenomena from a physical mechanism, and compiling declination observations from multiple regions; the book became the authoritative foundation of 17th-century magnetism research, inspiring scholars everywhere to systematically measure and record local declination values.
3. 1620–1626London · EnglandEdmund Gunter, astronomy professor at Gresham College, precisely measured magnetic declination in London in 1622, finding its value (about 6° east) significantly different from Borough's 11° east in 1580, thereby proving that declination is not fixed but undergoes slow long-term drift (later called 'secular variation'); Gunter also designed a compass with declination correction and nautical measuring instruments, directly converting theoretical research into practical navigational aids.
4. 1622–1630Amsterdam and London · Western EuropeThe Dutch East India Company (VOC) and the English East India Company systematically collected measured records of compass deviation from ocean fleets; Dutch cartographer Willem Blaeu integrated declination data gathered during voyages into sea charts; new-style English and Dutch charts began marking declination values or early isogonic lines for specific sea areas, allowing sailors to compute true north from known magnetic north, significantly improving ocean positioning accuracy.
5. 1630Global oceans · GlobalHenry Gellibrand formally published a paper in 1635, systematically demonstrating with data from Gunter, Borough, and others the long-term centennial variation of declination, establishing the concept of 'geomagnetic secular variation'; Edmund Halley drew the first global isogonic chart in 1701; declination charts thereby became standard ocean navigation equipment, and the modern International Geomagnetic Reference Field (IGRF) continues this tradition, providing precise orientation for global aviation and navigation through declination correction.