The atomic clock is the world's most accurate timekeeping device, measuring time by tracking the ultra-stable vibrations of atoms (usually Cesium-133). The first atomic clock was built at the U.S. National Bureau of Standards in 1949. Before this, time was measured by the rotation of the Earth, which is slightly inconsistent. The precision of atomic clocks is so high that they lose less than one second in millions of years. This invention is critical for modern technology; without the extreme precision of atomic clocks, GPS would not function, and the synchronization of global internet and telecommunication networks would be impossible. In 1967, the international 'Second' was officially redefined based on the frequency of the cesium atom, making the atomic clock the fundamental heartbeat of the modern digital world.