Navigation satellites depend on extremely accurate onboard clocks to calculate positions on Earth's surface.

The longer version

A receiver measures how long signals took to arrive from several satellites and converts each delay into a distance. Light travels about 30 centimetres in a nanosecond, so a timing error of a microsecond would put the calculated position hundreds of metres away. Four or more ranges pin down a location in three dimensions.

Satellites therefore carry atomic clocks, and the system constantly corrects for their drift from ground stations. Relativity also enters: the satellites' speed and their weaker gravity make their clocks run at slightly different rates from those on the surface, and the corrections are built into the signal. Without them, positions would drift by kilometres each day.

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.