Sundials tell time by tracking the shadow cast by the Sun across a marked surface.

The longer version

A vertical stick or angled blade throws a shadow that sweeps across a marked plate as the Sun appears to move. The engraved lines convert the shadow's position into hours. Accuracy depends on correct orientation and on the tilt matching the latitude where the dial stands. The shadow moves roughly fifteen degrees each hour.

Because the Sun's apparent path changes through the year, a simple dial reads true only with correction curves drawn on its face. Such devices were the standard public timekeeper for centuries and were used to check mechanical clocks, which in early days drifted badly. They fail at night and in cloud, which drove the search for other methods.

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.