Henry Gellibrand was born on November 17, 1597 in Aldersgate, London, England. He was the first of five sons of Henry Gellibrand, a fellow of All Souls College, Oxford. The year after the death of his father he began studying at Trinity College, Oxford, where he earned two arts degrees, a BA in 1619 and a MA in 1623. Gellibrand took holy orders and served as a curate in Chiddingstone, Kent. In 1626 he became professor of astronomy at Gresham College, London upon the recommendation of Henry Briggs. When Briggs died in 1630 he left his unfinished Trigometria Britanica to Gellibrand. Gellibrand finished the manuscript and published in 1633.
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.