Timeline of time measurement technology 270 BC - Ctesibius[?] builds a popular water clock[?] 46 BC - Julius Caesar and Sosigenes develop a solar calendar with leap years 1502 - Peter Henlein[?] builds the first pocketwatch[?] 1582 - Pope Gregory XIII, Aloysius Lilius, and Christopher Clavius[?] introduce a Gregorian calendar with an improved leap year system 1656 - Christian Huygens builds the first accurate pendulum clock 1676 - Motion works and minute hand introduced by Daniel Quare[?] 1680 - Second hand introduced 1737 - John Harrison presents the first stable nautical chronometer, thereby allowing for precise longitude determination while at sea 1928 - Joseph Horton[?] and Warren Morrison[?] build the first quartz crystal oscillator clock[?] 1946 - Felix Bloch and Edward Purcell[?] develop nuclear magnetic resonance 1949 - Harold Lyons[?] develops an atomic clock based on the quantum mechanical vibrations of the ammonia molecule