Henry Gellibrand (1597–1637) was an English mathematician. He is known for his work on the Earth's magnetic field. He discovered that magnetic declination - the angle of dip of a compass needle - is not constant but changes over time. He announced this in 1635, relying on previous observations by others, which had not yet been correctly interpreted.
He also devised a method for measuring longitude, based on eclipses. The mathematical tables of Henry Briggs, consisting of logarithms of trigonometric functions, were published by Gellibrand in 1633 as Trigonometria Britannica.