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How did ancient civilizations use sundials to tell time?


The Evolution of Sundials
This Roman sundial is very similar to the hemicycles created by the Greeks.
This Roman sundial is very similar to the hemicycles created by the Greeks.
© iStockphoto/Giovanni Rinaldi

Sundials may evoke impressions of primitive, outdated technologies or beautiful backyard ornaments, b­ut that's not the full story. Even with the invention of mechanical clocks, sundials were still used as reliable time devices into the modern era. This continued use stemmed at least in part from the fact that mechanical watches still needed to be accurately reset. Yet, it's the fascination and admiration of people around the world that has really helped ensure this ancient technology's longevity.

But now let's look back. Early sundials showed hours called seasonal hours. The day was divided into 12 hours, but in the wintertime, those hours were shorter than in the summer because summer days are longer. Near the equator, this difference wasn't highly pronounced, but toward the poles timekeeping fluctuated significantly for people in more extreme climes. This shows us an interesting alternative to the regimented timekeeping people in this day and age adhere to.

Sundials served a number of important functions for ancient civilizations as they became used alternatively or conjunctively to track the seasons, solstices and equinoxes. First attributed to the Greeks, sundials such as these (commonly known as hemispherical sundials or a hemispherium) used a hollowed out bowl with a pointed gnomon to tell not only time, but also seasonal information. This was possible because the point of the gnomon specified the time of day, while the size of the shadow was indicative of the time of year. Sometimes one half of the bowl would be cut away (at which point it might be called a hemicyclium or hemicycle), but in theory, the two models worked basically the same way.

If you remember back to the last page, the tilt of the Earth leads to some pretty tricky complications when it comes to sundial design and placement. It does, however, help determine seasonal information if you know how to harness it. By placing the gnomon across a curved surface, it's possible to trace lines through the dish that correlate with the summer solstice, winter solstice and the equinoxes (which share the same path).

Many different models of sundials were made throughout the centuries in a variety of cultures, and for many, the imagination was the limit. For example, during the stagnation of the European Dark Ages, Muslims used trigonometry principles to make the flat circular sundials that are arguably the most frequently seen today. They're also commonly credited as the first to propose hours of equal length, and Muslim sundials were often marked with the hours at which they prayed. These equal-length hours gradually caught on, but despite the début of mechanical clocks in the 1300s, seasonal hours were still frequently used for many years until they were gradually phased out by mean solar time and eventually time zones.

With innovations like these, ancient civilizations were able to keep records of past events and plan for future ones. They could formalize governmental, religious and societal activities with a unified schedule -- a legacy we have inherited and increasingly restructured to the precision accuracy of cesium atomic clocks.

On the next page, you'll find links to lots more information about astronomy and clocks, along with interesting questions on the ancient world answered.