How did ancient civilizations use sundials to tell time?

Sundials began as fairly simple devices, but as time passed they became more complex. Let's take a step back and examine some basic astronomy.

The Earth rotates around the sun, but it does so in an elliptical (oval) orbit, not a circular one. This means that when the sun is closer to the Earth, it appears to move faster across a sundial. In addition to this, the Earth's equator is not in line with its orbital path around the sun; it's tilted about 23.5 degrees off-kilter.

Taking the first of these two facts in hand, we find that the time given by a sundial (known sometimes as solar time or sun time) often differs from the time you'll read on your wristwatch (which can be called clock time or mean solar time). The variance can be up to about 15 minutes at different times throughout the year. This might not have been a huge cause for concern before the invention of mechanical clocks, but in the early days, sundials were often called upon to reset these timepieces when they wound down, so innovators had to alter the design of sundials slightly to accommodate.

Another astronomical factor to keep in mind has to do with the rotation of the Earth as it moves along its orbit. The idea may be hard to picture at first, so consider this: You're standing in your backyard, counting off the days from one day to the next. As the Earth is completing each of these revolutions, every 24 hours in that same period, it's also moving slowly along its orbit. Because of this orbital passage, the view from the nighttime side of the planet will see a slightly different display of stars every evening. Nowadays this phenomenon (called a sidereal day as opposed to a solar day) seems most commonly linked to the signs of the zodiac -- as it often was in ancient times -- but it was also another clue to figuring out how the sky could be used to predict events on Earth.

Two complications arise from the tilt of the Earth when it comes to sundials, having to do with longitude and latitude. In terms of longitude, most sundials need to be set up so they're exactly parallel with the axis of the planet to function properly. For example, people in the Northern Hemisphere need to find the North Pole and aim the gnomon along that line. But don't be too fast whipping out your compass unless you plan to do some quick calculations; the magnetic North Pole is shifty. To find the proper orientation you can also use the alignment of the stars; Polaris, the current North Star, lies at the north celestial pole, and that can show you the way.

Latitude also poses a challenge if a sundial is set up at a different latitude than the one it was created for. Chances are good it will need to be carefully angled to function properly. Even more complicated are the time markers on the dial, which may not be properly spaced to accurately tell time -- that takes trigonometry to figure out.

The next page, we'll take a look at telling time in the ancient world and the evolution of ancient sundials.

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.