The cesium fountain clock at the United Kingdom's National Physical Laboratory is considered to be among the most accurate timekeepers in the world. Kurt Gibble from Pennsylvania State University worked with Krzysztof Szymaniec, who built the clock, to help assess and improve upon its accuracy to make it an even more accurate atomic clock.

Credit: National Physical Laboratory, United Kingdom

Kurt Gibble and then graduate student Chad Fertig designed and built an atomic clock that uses rubidium instead of cesium. The clock (shown here) is in his lab at Pennsylvania State University.

Credit: James Collins, Penn State University

According to Clayton Simien, an NSF-funded physicist from the University of Alabama at Birmingham, the importance of accurate timekeeping goes back to the early days of sailors struggling to determine longitude to improve navigation. Today's atomic clocks still help with navigation as they are set inside GPS satellites to synch signals.

Credit: Clayton E. Simien, University of Alabama-Birmingham