The eldest son of a physician, also Henry, Gellibrand was born London. In 1615, he became a commoner at Trinity College, Oxford, and obtained a BA in 1619 and an MA in 1621. After taking holy orders he became curate at Chiddingstone, Kent, but the lectures of Sir Henry Savile inspired him to become a full-time mathematician. He settled in Oxford, where he became friends with Henry Briggs, famed for introducing logarithms to base 10. It was on Briggs’s recommendation that, on the death of Edmund Gunter, Gellibrand succeeded him as Gresham professor of Astronomy in 1627 – a post he held until his death from a fever in 1636. Gellibrand’s principal publications were concerned with mathematics (notably the completion of Briggs’s Trigonometria Britannica after Briggs died in 1630) and navigation.