Columbus's Method of Determining Longitude: An Analytical View

On 14 September 1494, Christopher Columbus observed a lunar eclipse while at the island of Saona near the eastern tip of Hispaniola. He later recorded in his Libro de las Profecias that, from his timing of the eclipse, he determined his longitude to be ‘five hours and more than one half’west of Cape San Vicente. His actual longitude was three hours and 59 minutes west of Cape San Vicente, so Columbus was off by over an hour and a half, some 23 degrees of longitude.