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 Chapter 10 : Circles History of Timekeeping History of Timekeeping Humans have used the relative movement of the sun across Earth's sky as the basis for a system of dividing time intervals for millennia. "Sun time" is based on defining noon as the time each day that the sun reaches its highest point in the sky. The day is then divided into 24 hours based on the time between successive noons. What may surprise you is that using this system gives days, and thus hours, of unequal length. In the middle of September, the length of a day measured in this fashion is only about 23 hours, 59 minutes and 40 seconds long, while around the end of December a day is about 24 hours and 20 seconds long. During the course of a year, these differences add up. Compared to the time given by our clocks, the time given by a sundial, which is based on the sun's position in the sky, can be as much as 16 minutes and 23 seconds fast or up to 14 minutes and 20 seconds slow. There are two reasons for these unequal days. The first has to do with the fact that Earth's axis is tilted with respect to the plane in which Earth revolves around the sun. The second reason is that Earth's orbit is not perfectly circular, but is slightly elliptic. This makes the sun appear to move faster when Earth is closest to the sun. An equation called the Equation of Time was long ago worked out to reconcile the time told by a sundial during the year with the time told by a clock. "Clock time" defines each day as exactly 24 hours long. The 24 hours of one day are exactly the same length as the 24 hours of any other day. This is accomplished by defining mean solar time, which is based on a type of average length day in sun time. This keeps clock time and sun time almost exactly together over each four-year cycle containing a leap year. Still, in three of four turnings of the century, a leap day has to be omitted to keep the times coordinated. And even then, an extra second had to be added to the clock time in recent years. To measure official time, scientists use the fact that each type of atom has particular resonances; that is, each type of atom absorbs and emits light energy at its own characteristic frequencies. These resonances stay constant over time, and don't depend on location. Actually, Earth's rotation isn't quite constant - it is slowing down about one second every 40,000 years. In 1967, the natural frequency of the element cesium was used to define the new international second. A second is now defined as exactly 9,192,631,770 cycles of the cesium atom's resonant frequency. The clocks used to maintain the standard second are now accurate to one one-millionth of a second per year. The National Institute of Standards and Technology, which maintains the standard second, even makes it possible to use the Internet to synchronize your computer's time with their standard time. You can get more information about the history of timekeeping from the Time and Frequency Division .