Hourglasses measure time by letting fine sand fall through a narrow opening at a steady rate.

The longer version

Fine dry sand pours through a narrow waist at a rate governed mostly by the size of the hole and the properties of the grains. Because the flow is nearly steady, the time for the upper chamber to empty is predictable and repeatable. Turning the device over restarts the interval exactly.

Humidity and grain shape affect the rate, so these devices are approximate rather than precise. They were prized at sea because they work on a moving deck, unlike a water or pendulum mechanism, and they were used to time watches and measure speed through the water. The familiar three-minute kitchen timer works on the same principle.

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.