Taking compass readings in Deptford, Gellibrand compared his readings to those taken 12 years early he determined that declination (the angle between true, geographic north and magnetic north) had changed. Declination varies at different places, but Gellibrand was the first to observe its variation with time. He published his results in 1635. The change in declination is due to changes in the earth's magnetic field. Gellilbrand also studied ways to improve navigation and find ways to determine longitude from celestial observations.
Although only 39 years of age Gellibrand retired in 1836 moving to Mayfield in Sussex. He died not long after, suffering a fever.
Goodwin, Gorodon; "Henry Gellibrand" in Dictionary of National Biography, 1885-1900, Volume 21; Smith, Elder, and Co.
O'Connor, JJ and Robertson, EF; "Henry Gellibrand"; MacTutor History of Mathematics Archive at www-history.mcs.st-andrews.ac.uk
Henry Gellibrand Wikipedia Entry