Sundials tell the time by shadow, so they fail at night or on cloudy days.

The longer version

A shadow moves because the sun appears to travel across the sky, and its direction and length change predictably with the hour. A fixed rod casting onto a marked plate therefore turns the sun's position into a reading, as long as the plate is set up for that latitude.

The method fails whenever the sun is hidden, and it also ignores the equation of time: the sun's apparent pace varies slightly through the year, so a dial can differ from a clock by a measurable margin. Before mechanical clocks, people adjusted hour lengths to the season instead.

Where this entry sits

This set looks at ancient civilizations, old inventions, trade routes, calendars, and the ordinary objects people once used every day. Nothing here is advice or opinion, just short notes about how daily life used to be organised long ago. The full list sits on the History page, where every entry is listed in order.