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Celestial Events 



Making 

 Time 



by Gail S. Cleere 



By 1370, King Charles V of France had 

 had enough. He was tired of hearing the 

 church bells of Paris ringing at irregular 

 intervals. He ordered a uniform time for 

 the city, making the clocks synchronous 

 with the master clock in the tower of the 

 Palais Royal. Charles's edict estabhshed 

 the first "time service," comparable to the 

 universal system we have today. 



Timekeeping has since become a lot 

 more sophisticated, but uniformity is still 

 the goal. On June 30, in keeping with the 

 decision of the International Earth Rota- 

 tion Service in Paris (a location Charles 

 would consider only fitting), a "leap sec- 

 ond" will be officially inserted into our 

 clock time to keep it precisely matched to 

 solar time, or the rotation of the earth. 

 Only a year after the last adjustment, Co- 

 ordinated Universal Time, usually shown 

 as UTC — and formerly known as Green- 

 wich Mean Time, or GMT — will be re- 

 tarded by one second between the last sec- 

 ond of the day, and the first second of the 

 next. 



Why do we do this? "Because the earth 

 actually rotates irregularly on its axis," ex- 

 plains Bill Klepczynski, an astronomer at 

 the U.S. Naval Observatory's Time Ser- 

 vice in Washington, D.C., where the na- 

 tion's master clock is housed. "Our clocks 

 must be adjusted to stay in pace with the 

 earth's rotation if we want to continue to 

 see the sun in the daytime hours, and the 

 stars at night." 



The mechanics of the earth's motion on 

 its axis are still not completely understood. 

 Tidal friction and the "sloshing" of the 

 earth's fluid core seem to play the largest 

 role in rotation, but atmospheric condi- 



tions may also be a factor. Even El Nino, 

 which pushes warm ocean currents up 

 against the west coast of South America 

 and wreaks havoc with the world's 

 weather, can affect the speed of the earth's 

 spin. 



Over time, such fluctuations add up. 

 Even though our days are roughly twenty- 

 four hours long, that has not always been 

 the case. When dinosaurs roamed the 

 earth, the days were about an hour and a 

 quarter shorter than they are today. Ever 

 since the earth coalesced out of the gases 

 and dust of the primordial cloud and began 

 its orbit around the sun, its spin has been 

 slowing down. 



We used to use the earth's rotation as 

 the standard for our clock time, setting the 

 clocks by observing the regular passage of 

 the stars overhead. The job of determining 

 time was thus up to the astronomer, which 

 is why it remains the bailiwick of the 

 Naval Observatory today. Precise and co- 

 ordinated time is necessary to determine 

 longitude at sea. It was the Naval Obser- 

 vatory's job to adjust the fleet's chronome- 

 ters before placing them aboard ships 

 bound for the high seas. 



As first our pendulum and then our 

 quartz crystal clocks improved, and as our 

 methods of observing distant astronomical 

 objects became more precise, the discrep- 

 ancies between the clocks and the earth's 

 rotation became more and more apparent. 

 Then, in the late 1940s the first atomic 

 clocks were developed; by counting the 

 regular oscillations of atoms, they allow 

 time to be measured with unprecedented 

 accuracy. Since 1967, the international 

 second of time has been defined as 



86 Natural History 6/94 



