After studying at Oxford Gellibrand was first pastor in Chiddingstone, Kent. Later, he studied mathematics at Oxford, among others, Henry Savile, and was professor of astronomy at Gresham 1627 College London. It was after the death of Henry Briggs (1630 ) out the second volume of the Trigonometrica Britannica ( later called Briggi'sche logarithms ).
From agreed with Thomas James, simultaneous observation of a lunar eclipse on October 29, 1631 in Charlton Iceland, James Bay, Canada and in London and the time difference thus determined the occurrence of the event Gellibrand was later able to determine the distance of the places where the observation of each was made. Gellibrand was particularly known for his discovery that the Earth's magnetic field is not - as William Gilbert have said - is constant. One indication is that this will change the declination of a compass needle in the course of decades. Gellibrand conducted in 1634 in London carefully controlled measurements. His test results he compared with those of William Borough from 1580 and those of Edmund Gunter 1622 and found that - at least in London - the declination had been steadily decreasing over the last decades (by about 7 degrees of arc ). This Gellibrand pulled the - true - concluded that the declination (and magnetism ) on the whole earth is variable (technical term: secular variation ).
About his results he reported in 1635 under the title: "A discourse mathematical on the variation of the magnetical needle, together with its admirable diminution lately Discovered ".
Gellibrand died at the age of 39 years after a fever disease. Like its predecessor, at Gresham College, Edmund Gunter, Gellibrand abandoned in the late 19th century London church of St. Peter le Poor was buried.