least suspected. Among them was the fact that the earth-sun clock • The earth's orbit around the sun is not The earth's axis is tilted to the plane con- • The earth spins at an irregular rate around its axis of rotation. • It also wobbles on its axis. For all of these reasons the earth-sun clock is not an accurate clock. The first two facts alone cause the day, as measured by a sun dial, to differ from time, as we reckon it today, by about 15 minutes a day in February and November. These effects are predictable and cause no serious problem, but there are also significant, unpredictable variations. Gradually, man-made clocks became so much more stable and precise than the earth-sun clock as time scales for measuring short time intervals that solar time had to be "corrected." As mechanical and electrical timepieces became more common and more dependable, as well as easier to use, nearly everyone looked to them for the time and forgot about the earth-sun clock as the master clock. People looked at a clock to see what time the sun rose, instead of looking at the sunrise to see what time it was. 1 METER-STICKS TO MEASURE TIME If we have to weigh a truckload of sand, a bathroom scale is of little use. Nor is it of any use for finding out whether a letter will need one postage stamp or two. A meterstick is all right for measuring centimeters-unless we want to measure a thousand or ten thousand meters-but it won't do for measuring accurately the thickness of an eyeglass lens. Furthermore, if we order a bolt 16 of an inch in diameter and 8-3/16 inches long-and our supplier has only a meter-stick, he will have to use some arithmetic before he can fill our order. His scale is different from ours. Length and mass can be chopped up into any predetermined size bits anyone wishes. Some sizes, of course, are easier to work with than others, and so have come into common use. The important point is that everyone concerned with the measurement agrees on what the scale is to be. Otherwise a liter of tomato juice measured by the juice processor's scale might be quite different from the liter of gasoline measured by the oil company's scale. Time, too, is measured by a scale. For practical reasons, the already existing scale, set by the spinning of the earth on its axis and the rotation of the earth around the sun, provides the basic scale from which others have been derived. WHAT IS A STANDARD? We have noted that the important thing about measurement is that there be general agreement on exactly what the scale is to be, and how the basic unit of that scale is to be defined. In other words, there must be agreement upon the standard against which all other measurements and calculations will be compared. In the United States the standard unit for measuring length is the meter. The basic unit for measurement of mass is the kilogram. The basic unit for measuring time is the second. The second multiplied evenly by 60 gives us minutes, or by 3600 gives us hours. The length of days, and even years, is measured by the basic unit of time, the second. Time intervals of less than a second are measured in 10ths, 100ths, 1000ths-on down to billionths of a second. Each basic unit of measurement is very exactly and explicitly defined by international agreement; and then each nation directs a government agency to make standard units available to anyone who wants them. In our country, the National Bureau of Standards (NBS), a part of the Department of Commerce with headquarters in Gaithersburg, Maryland, provides the primary standard references for ultimate calibration of the many standard weights and measures needed for checking scales in drug and grocery stores, the meters that measure the gasoline we pump into our cars, the octane of that gasoline, the purity of the gold in our jewelry or dental repairs, the strength of the steel used in automobile parts and children's tricycles, and countless other things that have to do with the safety, efficiency, and comfort of our everyday lives. The National Bureau of Standards is also responsible for making the second-the standard unit of time interval-available STANDARD SECOND to many thousands of time users everywhere-not only throughout the land, but to ships at sea, planes in the air, and even vehicles in outer space. This is a tremendous challenge, for the standard second, unlike the standard meter or kilogram, cannot be sent in an envelope or box and put on a shelf for future reference, but must be supplied constantly, on a ceaseless basis, from moment to moment and even counted upon to give the date. HOW TIME TELLS US WHERE IN THE WORLD WE ARE One of the earliest, most vital, and universal needs for precise time information was and still is—as a basis for place location. Navigators of ships at sea, planes in the air, and even small pleasure boats and private aircraft depend constantly and continuously on time information to find out where they are and to chart their course. Many people know this, in a general way, but few understand how it works. Primitive man discovered long ago that the sun and stars could aid him in his travels, especially on water where there are no familiar "signposts." Early explorers and adventurers in the northern hemisphere were particularly fortunate in having a "pole star," the North Star, that appeared to be suspended in the northern night sky; it did not rotate or change its position with respect to Earth as the other stars did. These early travelers also noticed that as they traveled northward, the North Star gradually appeared higher and higher in the sky, until it was directly overhead at the North Pole. By measuring the elevation of the North Star above the horizon, then, a navigator could determine his distance from the North Pole and conversely, his distance from the equator. An instrument called a sextant helped him measure this elevation very accurately. The measurement is usually indicated in degrees of latitude, ranging from 0 degrees latitude at the equator to 90 degrees of latitude at the North Pole. Measuring distance and charting a course east or west, however, presented a more complex problem because of the earth's spin. But the problem also provides the key to its solution. For measurements in the east-west direction, the earth's surface has been divided into lines of longitude, or meridians; one complete circuit around the earth equals 360 degrees of longitude, and all longitude lines intersect at the North and South Poles. By international agreement, the line of longitude that runs through Greenwich, England, has been labeled the zero meridian; and longitude is measured east and west from this meridian to the point where the measurements meet at 180 degrees, on the opposite side of the earth from the zero meridian. At any point on earth, the sun travels across the sky from east to west at the rate of 15 degrees in one hour, or one degree in four minutes. So if a navigator has a very accurate clock aboard his ship one that can tell him very accurately the time at Greenwich or the zero median-he can easily figure his longitude. He simply gets the time where he is from the sun. For every four minutes that his clock, showing Greenwich time, differs from the time determined locally from the sun, he is one degree of longitude away from Greenwich. At night he can get his position by observing the location of two or more stars. The method is similar to obtaining latitude from the North Star. The difference is that whereas the North Star appears suspended in the sky, the other stars appear to move in circular paths around the North Star. Because of this, the navigator must know the time in order to find out where he is. If he does not know the time, he can read his location with respect to the stars, as they "move" around the North Star, but he has no way at all to tell where he is on earth! His navigation charts tell him the positions of the stars at any given time at every season of the year; so if he knows the time, he can find out where he is simply by referring to two or more stars, and reading his charts. The principle of the method is shown in the illustration. For every star in the sky there is a point on the surface of the earth where the star appears directly overhead. This is Point A for Star #1 and Point B for Star #2 in the illustration. The traveler at Point 0 sees Star #1 at some angle from the overhead position. But as the illustration shows, all travelers standing on the black circle will see Star #1 at this same angle. By observing Star #2, the traveler will put himself on another circle of points, the blue circle; so his location will be at one of the two intersection points of the blue and the black circles. He can look at a third star to choose the correct intersection point; or, as is more usually the case, he has at least some idea of his location, so that he can pick the correct intersection point without further observation. The theory is simple. The big problem was that until about 200 years ago, no one was able to make a clock that could keep time accurately at sea. BUILDING A CLOCK THAT WOULDN'T GET SEASICK During the centuries of exploration of the world that lay thousands of miles across uncharted oceans, the need for improved navigation instruments became critical. Ship building improved, and larger, stronger vessels made ocean trade as well as ocean warfare increasingly important. But too often ships laden with priceless merchandise were lost at sea, driven off course by storms, with the crew unable to find out where they were or to chart a course to a safe harbor. Navigators had long been able to read their latitude north of the equator by measuring the angle formed by the horizon and the North Star. But east-west navigation was almost entirely a matter of "dead reckoning." If only they had a clock aboard that could tell them the time at Greenwich, England, then it would be easy to find their position east or west of the zero meridian. It was this crucial need for accurate, dependable clocks aboard ships that pushed inventors into developing better and better timepieces. The pendulum clock had been a real breakthrough, and an enormous improvement over any timekeeping device made before it. But it was no use at all at sea. The rolling and pitching of the ship made the pendulum inoperative. ROLLING SEA SALT SPRAY JOHN In 1713 the British government offered an award of £20,000 to anyone who could build a chronometer that would serve to determine longitude to within 12 degree. Among the many craftsmen who sought to win this handsome award was an English clock maker named John Harrison, who spent more than 40 years trying to meet the specifications. Each model became a bit more promising as he found new ways to cope with the rolling sea, temperature changes that caused intolerable expansion and contraction of delicate metal springs, and salt spray that corroded everything aboard ship. When finally he came up with a chronometer that he considered nearly perfect, the men of the government commission were so afraid that it might be lost at sea that they suspended testing it until Harrison had built a second unit identical with the first, to provide a pattern. Finally, in 1761 Harrison's son William was sent on a voyage to Jamaica to test the instrument. In spite of a severe storm that lasted for days and drove the ship far off course, the chronometer proved to be amazingly accurate, losing |