Atomic clocks measure time by counting the natural vibrations of atoms, making them extraordinarily precise.

The longer version

Electrons in an atom can occupy only certain energy levels, and the gap between two of them corresponds to a fixed frequency. Probing atoms with microwaves tuned to that gap lets a feedback loop lock the radiation exactly to the transition. Counting the cycles of that radiation produces the ticks.

The result is a beat so stable that such clocks would lose well under a second over millions of years. They define the international second, synchronise financial networks, and underpin navigation satellites. Because every atom of a given kind behaves identically, the standard can be reproduced anywhere. Laboratories compare several designs and average them.

Where this entry sits

These facts wander through clocks, calendars, time zones, and the long human effort to measure passing hours. You will find leap years, odd month lengths, ancient sundials, and the reasons modern timekeeping works the way it does. The full list sits on the Time and Calendars page, where every entry is listed in order.