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The earth swings around the sun, and the moon swings around the earth. The earth "swings" around its own axis. These movements can easily be observed and charted, from almost any spot on earth. The observations were and are useful in keeping track of time, even though early observers did not understand the movements and often were completely wrong about the relationships of heavenly bodies to one another. The "swings" happened with dependable regularity, over countless thousands of years, and therefore enabled observers to predict the seasons, eclipses, and other phenomena with great accuracy, many years in advance.

When we observe the earth's swing around its axis, we see only a part of that swing, or an arc, from horizon to horizon, as the sun rises and sets. A big breakthrough in timekeeping came when someone realized that another arc-that of a free-swinging pendulum-could be harnessed and adjusted, and its swings counted, to keep track of passing time. The accuracy of the pendulum clock was far superior to any of the many devices that had preceded it-water clocks, hour glasses, candles, and so on. Furthermore, the pendulum made it possible to "chop up" or refine time into much smaller, measurable bits than had ever been possible before; one could measure-quite roughly, to be sure-seconds and even parts of seconds, and this was a great advancement.

The problem of keeping the pendulum swinging regularly was solved at first by a system of cog wheels and an "escapement" that had the effect of giving the pendulum a slight push with each swing, in much the same way that a child's swing is kept in motion by someone pushing it. A weight on a chain kept the

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escapement lever pushing the pendulum, as it does today in the cuckoo clocks familiar in many homes.

But then someone thought of another way to keep the pendulum swinging—a wound-up spring could supply the needed energy if there were a way to make the "push" from the partially wound spring the same as it was from a tightly wound spring. The "fusee"-a complicated mechanism that was used for only a brief period-was the answer.

From this it was just one more step to apply a spring and "balance wheel" system directly to the pinions or cogs that turned the hands of the clock, and to eliminate the pendulum. The "swings" were all inside the clock, and this saved space and made it possible to keep clocks moving even when they were moved around or laid on their side.

But some scientists who saw a need for much more precise time measurement than could be achieved by conventional mechanical devices began looking at other things that swing-or vibrate or oscillate things that swing much faster than the human senses can count. The vibrations of a tuning fork, for instance, which, if it swings at 440 cycles per second, is "A" above "Middle C" on our music scale. The tiny tuning fork in an electric wrist watch, kept swinging by electric impulses from a battery, hums along at 360 vibrations or "cycles" per second.

As alternating-current electricity became generally available at a reliable 60 swings or cycles per second-or 60 hertz (50 in some areas)—it was fairly simple to gear these swings to the clock face of one of the commonest and most dependable time-pieces we have today. For most day-to-day uses, the inexpensive electric wall or desk clock driven by electricity from the local power line keeps "the time" adequately.

But for some users of precise time these common measuring sticks are as clumsy and unsatisfactory as a liter measuring cup would be for a merchant who sells perfume by the dram. These people need something that cuts time up with swings much faster than 60ths or 100ths of a second. The power company itself, to supply electricity at a constant 60 hertz, must be able to measure swings at a much faster rate.

Power companies, telephone companies, radio and television broadcasters, and many other users of precise time have long depended on the swings or vibrations of quartz crystal oscillators, activated by an electric current, to divide time intervals into megahertz, or millions of cycles per second. The rate at which the crystal oscillates is determined by the thickness or thinness-to which it is ground. Typical frequencies are 2.5 or 5 megahertz (MHz)-22 million or 5 million swings per second.

Incredible as it may seem, it is quite possible to measure swings even much faster than this. What swings faster? Atoms do. One of the properties of each element in the chemistry Periodic Table of Elements is the set of rates at which its atoms swing or resonate. A hydrogen atom, for example, has one of its resonant

frequencies at 1,420,405,752 cycles per second, or hertz. A rubidium atom has one at 6,834,682,608 hertz, and a cesium atom at 9,192,631,770 hertz. These are some of the atoms most commonly used in measuring sticks for precise time-the "atomic clocks" maintained by television network master stations, some scientific laboratories, and others. Primary time standards, such as those maintained by the U.S. Naval Observatory or the National Bureau of Standards, are "atomic clocks."

Everything swings, and anything that swings at a constant rate can be used as a standard for measuring time interval.

GETTING TIME FROM FREQUENCY

The sun as it appears in the sky-or the "apparent sun"crosses the zenith or highest point in its arc with a "frequency" of once a day, and 36514 times a year. A metronome ticks off evenly spaced intervals of time to help a musician maintain the time or tempo of a composition he is studying. By moving the weight on its pendulum he can slow the metronome's "frequency" or speed it up.

Anything that swings evenly can be used to measure time interval simply by counting and keeping track of the number of swings or ticks-provided we know how many swings take place in a recognized unit of time, such as a day, an hour, a minute, or a second. In other words, we can measure time interval if we know the frequency of these swings. A man shut up in a dungeon, where he cannot see the sun, could keep a fairly accurate record of passing time by counting his own heartbeats-if he knew how many times his heart beats in one minute-and if he has nothing to do but count and keep track of the number.

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The term frequency is commonly used to describe swings too fast to be counted by the human ear, and refers to the number of swings or cycles per second-called hertz (Hz), after Heinrich Hertz, who first demonstrated the existence of radio waves.

If we can count and keep track of the cycles of our swinging device, we can construct a time interval at least as accurate as the

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device itself-even to millionths or billionths of a second. And by adding these small, identical bits together, we can measure any "length" of time, from a fraction of a second to an hour-or a week or a month or a century.

Of course, the most precise and accurate measuring device in existence cannot tell us the date-unless we have a source to tell us when to start counting the swings. But if we know this, and if we keep our swinging device "running," we can keep track of both time interval and date by counting the cycles of our device.

WHAT IS A CLOCK?

Time "keeping" is simply a matter of counting cycles or units of time. A clock is what does the counting. In a more strict definition, a clock also keeps track of its count and displays what it has counted. But in a broad sense, the earth and the sun are a clock— the commonest and most ancient clock we have, and the basis of all other clocks.

When ancient peoples put a stick in the ground to observe the movement of its shadow from sunrise to sunset, it was fairly easy and certainly a natural step to mark off "noon" and other points where the shadow lay at other times of day-in other words, to make a sundial. Sundials can tell the time quite reliably when the sun is shining. But, of course, they are of no use at all when the sun is not shining. So people made mechanical devices called clocks to interpolate or keep track of time between checks with the sun. The sun was a sort of "master clock" that served as a primary time scale by which the man-made, secondary clocks were calibrated and adjusted.

Although some early clocks used the flow of water or sand to measure passing time, the most satisfactory clocks were those that counted the swings of a pendulum or of a balance wheel. Quite recently in the history of timekeeping, men have developed extremely accurate clocks that count the vibrations of a quartz crystal activated by an electric current, or the resonances of atoms of selected elements such as rubidium or cesium. Since "reading" such a clock requires counting millions or billions of cycles per second-in contrast to the relatively slow 24-hour cycle of the earthsun clock-an atomic clock requires much more sophisticated equipment for making its count. But given the necessary equipment, one can read an atomic clock with much greater ease, in much less time, and with many thousands of times greater precision than he can read the earth-sun clock.

A mechanism that simply swings or ticks-a clockwork with a pendulum, for example, without hands or face is not, strictly speaking, a clock. The swings or ticks are meaningless, or ambiguous, until we are able to count them and until we establish some base from which to start counting. In other words, until we hook up "hands" to keep track of the count, and put those hands over a face with numbers that help us count the ticks and oscillations and make note of the accumulated count, we don't have a useful device.

The familiar 12-hour clock face is simply a convenient way to keep track of the ticks we wish to count. It serves very well for measuring time interval, in hours, minutes, and seconds, up to a maximum of 12 hours. The less familiar 24-hour clock face serves as a measure of time interval up to 24 hours. But neither will tell us anything about the day, month, or year.

THE EARTH-SUN CLOCK

As we have observed, the spin of the earth on its axis and its rotation around the sun provide the ingredients for a clock-a very fine clock that we can certainly never get along without. It meets many of the most exacting requirements that the scientific community today makes for an acceptable standard:

It is universally available. Anyone, almost
anywhere on earth, can readily read and
use it.

It is reliable. There is no foreseeable
possibility that it may stop or "lose" the
time, as is possible with all man-made
clocks.

It has great over-all stability. On the basis
of its time scale, scientists can predict
such things as the hour, minute, and sec-
ond of sunrise and sunset at any part of
the globe; eclipses of the sun and moon,
and other time-oriented events hundreds
or thousands of years in advance.

In addition, it involves no expense of operation for anyone; there is no possibility of international disagreement as to "whose" sun is the authoritative one, and no responsibility for keeping it running or adjusted.

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AVAILABILITY

RELIABILITY

STABILITY

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THE MISERABLE THING'S
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Nevertheless, this ancient and honored timepiece has some serious limitations. As timekeeping devices were improved and became more common-and as the study of the earth and the universe added facts and figures to those established by earlier observers-it became possible to measure quite precisely some of the phenomena that had long been known in a general way, or at

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