A leap into 1978 by ~ollierSmith The holiday season at NBS/Boulder between clock time and astronomical includes the leap second. always seems to arrive bearing glad time will soon exceed the allowed nine UTC(NBS) is derived from NBS tidings of another leap second. As early tenths of a second, they issue a call for Atomic Time by applying the necessary as October each year for the past 5 another leap second. The first leap leap seconds andverysmall rat3adjust- years theword has come from Paris that second occurred in June 1972, and the ments to maintain coordination with a leap second would be added on next in Decemberthat same year. Since the rest of the world. The present dif- December 31. This year, as usual, the then, one per year has been needed, ference between UTC(NBS) and NBS extra second will be added just before 5 always on December 31. Atomic Time is about 16 seconds, p.m. MST (24:00 Coordinated Universal NBS is one of two government labora- because of the leap seconds and other Time) on New Year's Eve. The leap tories that provide references for most adjustments that have been made since second is inserted into Coordinated public time in the U.S. (The other is the 1958 when NBS Atomic Time had its Universal Time (UTC) at the very end of U.S. Naval Observatory.) Also, since origin. 1977. NBS is charged with keeping standards Local time (Boulder), in turn, is the Leap seconds are needed to keep the for the U.S., it must, by international same as UTC (NBS) except it is 6 or 7 world's clocks closely in step with the agreement, keep its clocks coordinated hours behind, depending on whether earth's spin, so that sunrise, noon and with UTC within about one thousandth daylight saving or standard time is in sunset continue to occur at reasonable of a second. effect. "UTC grew out of the old times according to our clocks. Modern "In actual practice, we do much Greenwich Mean Time and therefore atomic clocks, the basis for worldwide better than that," says James Barnes, today, the 7-hour difference between timekeeping, are used because they are chief of the Time and Frequency local time and UTC still reflects the dif- much more constant than the rotation Division. 'We maintain agreement ference in longitude between London of the earth. But they do keep time at a within a few millionths of a second." (Greenwich) and the local time zone," slightly different rate from that cur- The NBS atomic clock system in- explains Helmut Hellwig, who is in rently indicated by the sun. As a result, cludes nine separate clocks. Eight are charge of the section which operates about once a year we have to make an small commercial cesium clocks and the standards. adjustment to atomic time, and we call the other is a large primary standard How is the leap second actually this a leap second. It is analogous tothe built by NBS. There are only two other inserted into the UTC time scale? Afew extra day added during leap years to laboratories (in Canada and West hours in advance, the system is pro- keep the calendar in step with the Germany) which have similar opera- grammed to add the extra second to the seasons. tional primary cesium standards. The UTC clock readouts. Then, just at the Who decides when a leap second is rest of the NBS clock system consists of right moment, the computer delays the needed? It is a cooperative effort among instrumentation for comparing the 00:00:00 reading on January 1, 1978, many astronomers who monitor the various clocks with the primary stand- by one second. The effect is to make the earth's spin by observing the stars and ard and with each other and a computer last minute of UTC on December 31, physicists who operate the very for compiling weighted averages and 1977, onesecond longer. In Boulder, all accurate atomic clocks. These efforts analyzing the performance of all the this takes place at 5 p.m. MST. are scattered around the world and are clocks. The NBS time and frequency stand- coordinated through the International All these clocks run independently ard is the only U.S. primary standard Time Bureau (BIH) in Paris. The BIH and from them several different time located away from NBS headquarters in collects data from astronomical scales are derived: NBS Atomic Time, Gaithersburg and it is the only one observatories and atomic clock labora- UTC(NBS) and local (Boulder) time. accessible to the public more or less tories in dozens of countries. They NBS Atomic Time is the time scale directly. Public access is provided average the data continuously and produced by the whole system. It is the through the NBS time and frequency when they see that the difference basis for the other scales and never dissemination services which furnish radio, telephone and satellite signals and, on occasion, through direct clock comparisons in Boulder. NBS also has services involving national television networks to provide accurate time and frequency references to most of the country. All these are referenced, with various degrees of accuracy, to the basic standards in Boulder. Several times a year, NBS compares its own clocks' readings with others scattered around the world. . Several methods are used, to do this but the most accurate at present is to physically carry an atomic clock from Boulder to the other clock sites in Paris, at the Naval Observatory and elsewhere. Sometimes portable clocks from some of these laboratories are brought to Boulder. The portable atomic clock, while not as accurate as the primary standards in Boulder, is still good Howard Machlan and Helmut Hellwig (rear) install a working model of a late enough to transfer timewith better than 17th century clock in a display near the NBS atomic clock system. Pendu- millionth-of-a-second accuracy on trips lum clocks of this type were considered excellent if they kept time to a lasting a day or so in each direction. minute a day, a far cry from the three millionths of a second per year Another method, nearly as good, achieved by the modern NBS system. involves the use of the low frequency radio waves of the Loran-C navigation system. In the future, satellites may be used either as relay points which pass on signals from the ground, or to actually carry the atomic clocks into orbit and broadcast their signals to earth. Such methods offer more reliability and accuracy than ground-to- ground transmissions and one satellite can cover up to40percent of theearth's surface at one time. All this effort may seem like a lot of work just to tell time, but of course, there is more to the story than that. Most of the electronics and electrical power industries depend on NBS for accurate frequency standards. Many scientific endeavors, such as environ- mental and seismological sensing net- works require accurate time-of-day information to label their data. Navi- Physicist Dave Glaze was one of the principal builders of the latest NBS time gators need high-quality time informa- and frequency standard NBS-6. He also worked on several of the preceding tion to determine position on the earth five generations of NBS atomic clocks. and oceans, in the air and in space. And the sophistication of most of these ahead of the needs of industry and accurate than you need, but someone needs is growing yearly. technology," says Barnes, "it wouldn't out there appreciates it. As Lewis Carroll's Red Queen doto have NBS in the position of [For thosewhowish totaketheabove observed in Through the Looking- follower in this field." advice, NBS time (UTC) may be heard by Glass,". . .it takes all the running you So feel freetousethe NBS clocktoset dialing (303) 499-71 11, a toll call out- can do, to keep in the same place." your new holiday digital watch; atomic side the Boulder-Denver area.] "We have to keep running to stay time may be a million times more