1597-1636

English astronomer who discovered secular variations in magnetic declination—changes over time in the acute angle between magnetic and true north. Edmund Gunter's declination measurements at Limehouse in 1622 were about 5° less than William Borough's 1580 results. Gunter attributed the discrepancy to errors by Borough. Gellibrand and John Marr's 1633 measurements showed a further decrease, which Gellibrand announced in 1635. Gellibrand also made mathematical contributions to navigation including a method for determining longitude at sea.