Library of Congress Catalog Card Number: 77-600056 National Bureau of Standards Monograph 155 For sale by the Superintendent of Documents, U.S. Government Printing Office, Washington, D.C. 20402 Stock No. 003-003-01650-1 Price: $4 (Add 25 percent additional for other than U.S. mailing). Time and its measurement is, simultaneously, very familiar and very mysterious. I suspect we all believe that the readings of our clocks and watches are somehow related to the sun's position. However, as science and technology developed, this relationship has come to be determined by a very complex system involving-just to name a few-astronomers, physicists, electronic engineers, and statisticians. And because time is both actively and precisely coordinated among all of the technologically advanced nations of the world, international organizations are also involved. The standard time-of-day radio broadcasts of all countries are controlled to at least 1/1000 of a second of each other; most time services, in fact, are controlled within a very few millionths of a second! The National Bureau of Standards (NBS) mounts a major effort in developing and maintaining standards for time and frequency. This effort tends to be highly sophisticated and perhaps even esoteric at times. Of course, most of the publications generated appear in technical journals aimed at specialized, technically sophisticated audiences. I have long been convinced, however, that it is very important to provide a descriptive book, addressed to a much wider audience, on the subject of time. There are many reasons for this, and I will give two. First, it is very simply—a fascinating subject. Again, we often have occasion to explain the NBS time program to interested people who do not have a technical background, and such a book would be an efficient and-hopefully-interesting means of informing them. Finally, this book realizes a long-standing personal desire to see a factual and yet understandable book on the subject of time. James A. Barnes Preparation of this document was supported in part by the 1842nd Electronic Engineering Group, C2/DCS Division, Air Force Communications Service. I. THE RIDDLE OF TIME 1. The Riddle of Time The Nature of Time/What Is Time?/Date, Time Interval, and Synchronization/Ancient 2. Everything Swings Getting Time from Frequency/What Is a Clock?/The Earth-Sun Clock/Meter-Sticks to 3 11 II. MAN-MADE CLOCKS AND WATCHES 3. Early Man-Made Clocks 25 Sand and Water Clocks/Mechanical Clocks/The Pendulum Clock/The Balance-Wheel Clock/ 4. "Q" Is for Quality 31 The Resonance Curve/The Resonance Curve and Decay Time/Accuracy, Stability, and 5. Building Even Better Clocks 39 The Quartz Clock/Atomic Clocks/The Ammonia Resonator/The Cesium Resonator/One 6. The "Correct Time" for the Man in the Street 49 Modern Mechanical Watches/Electric and Electronic Watches/The Quartz-Crystal Watch/ III. FINDING AND KEEPING THE TIME 7. Time Scales 59 The Calendar/The Solar Day/The Stellar or Sidereal Day/Earth Rotation/The Continu- 8. The Clock behind the Clock 71 Flying Clocks/Time on a Radio Beam/Accuracy/Coverage/Reliability/Other Considera- 79 9. The Time Signal on Its Way Choosing a Frequency/Very Low Frequencies/Low Frequencies/Medium Frequencies/ 1 vi THE USES OF TIME 10. Standard Time Standard Time Zones and Daylight-Saving Time/Time as a Standard/Is a Second Really a Second?/Who Cares about the Time? 11. Time, The Great Organizer Electric Power/Modern Communication Systems/Transportation/Navigation by Radio Beacons/Navigation by Satellite/Some Common and Some Far-out Uses of Time and Frequency Technology V TIME, SCIENCE, AND TECHNOLOGY 89 99 12. Time and Mathematics 113 Taking Apart and Putting Together/Slicing up the Past and the Future-Calculus/Conditions and Rules/Getting at the Truth with Differential Calculus/Newton's Law of Gravitation/What's Inside the Differentiating Machine?-An Aside 13. Time and Physics 123 Time is Relative/Time Has Direction/Time Measurement Is Limited/Atomic and Gravitational Clocks/The Struggle to Preserve Symmetry/The Direction of Time and Time Symmetries-An Aside 14. Time and Astronomy 135 Measuring the Age of the Universe/The Expanding Universe/Time Equals Distance/Big 15. Clockwork and Feedback 143 Open-Loop Systems/Closed-Loop Systems/The Response Time/System Magnification or 16. Time as Information Three Kinds of Time Information Revisited/Geological Time/Interchanging Time and 17. The Future of Time Using Time to Increase Space/Time and Frequency Information-Wholesale and Retail/ Time Dissemination/Clocks in the Future/The Atom's Inner Metronome/Time Scales of the Future/The Question of Labeling—-A Second is a Second is a Second/Time through the Ages/What Is Time, Really?/Particles Faster than Light-An Aside 151 159 |