He entered the church becoming a curate in Chiddingstone, Kent, a couple of years after receiving his M.A. from Oxford. Gellibrand succeeded Gunter to the chair of astronomy in Gresham College, London in 1627. It was largely through the influence of Briggs that he received this chair. However he was involved in a religious controversy which led to a court case :-
Gellibrand held puritan meetings in his rooms, and encouraged his servant, William Beale, to publish an almanac for 1631, in which the Catholic saints were superseded by those in Foxe's book of martyrs. Laud, then bishop of London, brought them both into the high commission court. They were acquitted on the ground that similar almanacs had been printed before, and this prosecution was used against Laud at his own trial in 1643.Gellibrand's most famous scientific discovery was the change over the years in magnetic declination. He achieved this by comparing measurement he took in Deptford with similar ones taken by Gunter twelve years earlier. Gellibrand published his findings in A discourse mathematical of the variation of the magneticall needle together with its admirable diminution lately discovered (1635).
He also made mathematical contributions to navigation, in particular working on methods to determine longitude. His methods were based on observing various celestial events and in particular he arranged with Captain Thomas James that they would simultaneously observe the eclipse of the moon on 29 October 1831. James was the leader of an expedition to try to find the north-west passage and at the time of the eclipse was on Charlton Island in James Bay, Canada. Gellibrand, on the other hand, was observing in Gresham College. The time difference allowed Gellibrand to compute the difference in longitude between the two points of observation and he published the results in Appendix concerning Longitude (1633). Gellibrand also published logarithm and trigonometrical tables. After his friend Briggs died in 1630, he worked to complete Briggs's Trigonometria Britannica which he did, publishing the work in 1633.
Although only 39 years of age, Gellibrand retired in 1836 to Mayfield in Sussex. He died of a fever at age 39 but several of his publications were published after he died. Institution Trigonometrical (1638), with an expanded version in 1658, applied trigonometry to navigation and astronomy. Epitome of Navigation first appeared 62 years after his death; a rather remarkable length of time. It was his most popular work.
Article by: J J O'Connor and E F Robertson