![]() In particular, the Ancient Greeks and Romans were fans of the water clock, although one of the oldest examples was found in the tomb of the Egyptian pharaoh opens in a new windowAmenhotep I and dates back to 1500 BC. However, time keepers began to look for non-celestial ways of telling time. Some built moondials, although their accuracy could vary with the phase of the moon, and the merkhet used different stars to track the passage of time throughout the night. Of course, sundials had the limitation of only being useful during the daytime without clouds. Taipei 101, which was the tallest building in the world until the Burj Khalifa was built in 2010, also acts as a huge sundial, as does the Luxor Obelisk in Paris. ![]() The sundial stands 27 meters (or 88 feet) high, has a cupola on top for announcing eclipses and the arrival of monsoons, and can tell the time opens in a new windowwith an accuracy of two seconds. The largest sundial in the world is found in Jaipur, Rajasthan, India along with eighteen other astronomical instruments at the Jantar Mantar monument. With some attention to detail, you could even opens in a new windowconstruct your own sundial that would be accurate to within a minute. The coarsest sundial measurements were good to within an hour, but sundials were the only commonly used clocks until as late as the mid-17 th century and the more modern ones had accuracies as good as 15 to 30 seconds. The accuracy of a sundial depends on its calibration, the precision of the marker it uses (in other words, is the shadow it casts wide and fuzzy?), and the size of its gnomon. The gnomon is sometimes fixed or it may be moved to account for the different day lengths during different seasons. Others indicate time by allowing the sun’s light to pass through a small slit. Many use a narrow, angled object called a gnomon to cast a shadow on markers indicating time of day. While there are many variations on the sundial, they all use the sun to mark the passage of time. The opens in a new windowoldest known example of a portable sundial, described as a piece of metal in the shape of an Italian ham that could fit within a coffee mug, was found in an Italian villa buried under volcanic ash in the eruption of Mount Vesuvius in A.D. The ancient Romans constructed sundials in their city centers. Time keepers in Ancient Egypt and Sumer as far back as opens in a new window1500 BC (and likely even earlier) created the first devices to use the sun’s shadow to track the passage of time. Given no other resources, you can step outside-on a non-cloudy day, of course-and get an approximate sense of the time of day by observing the sun’s position in the sky. Perhaps the longest standing method for keeping time is the sundial. ![]() How do we measure time? How accurate are today’s clocks relative to the first clocks of ancient times? And what is the definition of a second? Let’s take a walk through the evolution of time measurement. ![]()
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