Never mind the wind, waves, storms, shoals and countless other factors that once made sailing across an ocean an incredibly complex task fraught with danger.

Perhaps the most confounding aspect of a navigator’s life at sea was trying to account for the phenomenon of magnetic variation (also known as magnetic declination): the angle between magnetic north and true north that changes with location and over time due to fluctuations in the Earth’s magnetic field.

The map pictured here, published by French hydrographer Jacques-Nicolas Bellin in 1765, shows curved isogones — imaginary lines that connect points on the Earth’s surface having equal magnetic declination — and was used by mariners to keep their ships on course.

Each isogone is marked with a number that represents the degree of variation between magnetic north and true north. If there was a negative declination (where magnetic north is west of true north), navigators would add the number to the compass bearing; if there was a positive declination (where magnetic north is east of true north), they’d subtract it.

*with files from Isabelle Charron, early cartographic archivist, Library and Archives Canada

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