Knowledge of time at sea allows determination of longitude. The first practical seagoing clock was John Harrison's timekeeper No. 4. The historic 1759 instrument gained a 20,000 pound reward for the English inventor by Parliament 250 years ago. On January 1, 1959, the U.S. Naval Observatory introduced a system of uniform atomic time using cesium beam atomic oscillators. This measurement was adopted as standard by the International Committee on Weights and Measures. (Photo taken by Robert DeGast for Proceedings magazine, June 1964)