Of all the clocks offered by 1-800-4Clocks.com, the atomic clocks are probably the most interesting. Like all clocks, they keep time via an oscillating mass and a spring. The difference is that, in this case, that mass is the nucleus of an atom and its electrons. This is a more accurate way of keeping time, and is said to be more accurate than the rotation of the Earth. We couldn’t have a Global Positioning System or launch spacecraft without them.