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-17^th 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.

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3. Mechanical Clocks

The next major advancement in time keeping came with the invention of mechanical clocks which brought timing accuracies from minutes to seconds. Rather than using the gradual change in something like water or sand, mechanical clocks involve an escapement mechanism, a way of releasing energy in small, controlled amounts at regular intervals in time. There have been a wide variety of mechanisms invented to serve such a purpose, including cylinder, duplex, lever, and chronometer escapements, all with varying degrees of accuracy.

Perhaps the earliest example of an escapement used in mechanizing water clocks came from the Chinese scientist and statesman Su Song who built a hydro-mechanical, 30-foot astronomical clock tower that not only told time of day but also month and year. The spokes of the large wheel on Song’s clock slowly fill with water at a steady rate, but a mechanical escapement mechanism only releases the wheel once the water in each spoke reaches a certain level. His clock ran from 1092 until 1126 when it was dismantled by political adversaries.

Although Galileo is credited with inventing the pendulum clock, the first to build one was Dutch scientist Christiaan Huygens in 1656. His device achieved unprecedented accuracy of less than one minute per day (later improved to less than 10 seconds per day) by using a weighted pendulum, which swings at a consistent rate, to regulate the speed of turning gears that tick by the time at regular intervals. Pendulums will eventually be overcome by air resistance and friction, so they typically incorporate springs that require regular winding to store up potential energy to keep them swinging.

4. Quartz Clocks

Quartz clocks, developed in the 1920s, moved away from gears and escapements and thus did not require regular winding. Quartz clocks use the fact that quartz crystals will vibrate at a constant frequency when an electric field is applied.

The first quartz clocks, produced in Japan, lost only five seconds over the course of a month. Their accuracy is limited by the fact that a quartz crystal’s size, shape, and temperature will affect its vibrational frequency and so no two are exactly alike. Although their accuracy has been surpassed by atomic clocks, they are still the most common personal time keeping devices based on their affordable price.

5. Atomic Clocks

The standard for time keeping is now an atomic clock which were first introduced in the 1950s. Today’s atomic clocks lose only a second over periods of tens of millions of years. In other words, they have accuracies on the order of hundredths of nanoseconds per day.

Due to their very regularly-spaced energy levels, atoms resonate at specific frequencies and emit electromagnetic waves as they do. The frequency of those emitted light waves—how many arrive per unit of time—can then be measured to very precisely measure the passage of time. The definition of one second is the time it takes for a cesium atom to resonate 9,192,631,770 times.

The most accurate clocks in the world are the cesium clocks at the National Institutes of Standards and Technology in Boulder, Colorado, the United States Naval Observatory in Washington, D.C., the PTB in Germany, and the Paris Observatory in France, although current research continues into atoms other than cesium which could potentially give even higher accuracies.

For more time-keeping fun, check out my colleague Math Dude’s article in Scientific American on how to measure time without a stopwatch.

Until next time, this is Sabrina Stierwalt with Everyday Einstein’s Quick and Dirty Tips for helping you make sense of science. You can become a fan of Everyday Einstein on Facebook or follow me on Twitter, where I’m @QDTeinstein. If you have a question that you’d like to see on a future episode, send me an email at everydayeinstein@quickanddirtytips.com.

Image of time measurement tools © Shutterstock.