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# How Do We Measure Time? 5+ Innovative Ways

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?

By
Sabrina Stierwalt, PhD
Episode #269

Most of us take for granted that we have the power to monitor the passage of time, to know at any moment how many minutes remain until our next appointment, or to be able to agree on the time with someone on the other side of the world. This was, of course, not always the case.

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.

## 5 Tools We Use to Measure Time

1. Sundials
2. Water Clocks
3. Mechanical Clocks
4. Quartz Clocks
5. Atomic Clocks

Let's explore each a little further.

## 1. Sundials

Perhaps the longest standing method for keeping time is the sundial. 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. Time keepers in Ancient Egypt and Sumer as far back as 1500 BC (and likely even earlier) created the first devices to use the sun’s shadow to track the passage of time.

The ancient Romans constructed sundials in their city centers. The wealthy carried pocket versions. The oldest 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. 79.

While there are many variations on the sundial, they all use the sun to mark the passage of time. Many use a narrow, angled object called a gnomon to cast a shadow on markers indicating time of day. Others indicate time by allowing the sun’s light to pass through a small slit. The gnomon is sometimes fixed or it may be moved to account for the different day lengths during different seasons.

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 coarsest sundial measurements were good to within an hour, but sundials were the only commonly used clocks until as late as the mid-17th century and the more modern ones had accuracies as good as 15 to 30 seconds. With some attention to detail, you could even construct your own sundial that would be accurate to within a minute.

The largest sundial in the world is found in Jaipur, Rajasthan, India along with eighteen other astronomical instruments at the Jantar Mantar monument. 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 with an accuracy of two seconds. 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.

## 2. Water Clocks

Of course, sundials had the limitation of only being useful during the daytime without clouds. 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. However, time keepers began to look for non-celestial ways of telling time.

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 Amenhotep I and dates back to 1500 BC. The earlierst water clocks were typically stone pots with sloped sides that allowed water to drip out at a constant rate of a small hole at their base. The insides of the pot contained marks that linked different water levels to the passage of hours.

To increase the accuracy of water clocks, some were mechanized to make the flow of water as constant as possible by regulating the water pressure. However, variations in the temperature of the water could have led to differences in as much as 30 minutes per day.

Some water clocks triggered the movement of figurines on fancy displays while others rang bells. The first water-based alarm clock was built by Plato in 427 BC. In Plato’s clock, water was siphoned off into an additional vessel once it reached a certain level, and the additional vessel contained a tube with a narrow slit. When water passed through the tube, it whistled like the boiling water in a tea kettle. One of the most complex water clocks was designed and built by Muslim engineer Al-Jazari in 1206 and was a weight powered water clock in the form of a large Asian elephant.

Other time-keeping methods that relied on the steady change in some natural medium included incense clocks, sand clocks (otherwise known as hourglasses), and oil lamp clocks. However, similar to the water clock, their accuracies were still limited.