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.