Harrison's first marine chronometer was a 72 pound monster which used complex devices to compensate for temperature changes. John Harrison (1693-1776) was a self-educated English clockmaker. He invented the marine chronometer. A marine chronometer is a clock that is precise and accurate enough to be used as a portable time standard used to determine longitude by means of celestial navigation. When first developed in the eighteenth century it was a major technical achievement, as accurate knowledge of the time over a long sea voyage is necessary for navigation, lacking electronic or communications aids. The first true chronometer was the life work of one man, John Harrison, spanning 31 years of persistent experimentation and test that revolutionized naval (and later aerial) navigation enabling the Age of Discovery and Colonialism to accelerate.