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?
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 email@example.com.
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