Philosophical Society of Washington

Minutes of the 2043rd Meeting

Speaker: Donald B. Sullivan, Time and Frequency Division, National Institute of Standards and Technology
Topic: “Timekeeping: Synch or Swim”

The President Mr. Ohlmacher called the 2043rd meeting to order at 8:27 p.m. on April 7, 1995. The Treasurer read the minutes of the 2042nd meeting and they were approved. The President then read a portion of the minutes of the 435th meeting March 30, 1895.

The President introduced Mr. Donald Sullivan of the Time and Frequency Division, National Institute of Standards and Technology to discuss “Worlds Apart: the Inner Planets”.

Mr. Sullivan indicated that timekeeping is not usually discussed in the history of astronomy. The first clock was made in the 1600's by Christian Huygens. The clock was based on the pendulum, which depends on frequency as its standard. Galileo had suggested the pendulum in the 16th Century. The accuracy of the clock was one second per month and was considered excellent.

The drawback of a pendulum clock is that it works only on bedrock, and not a moving platform like a ship. At that time, navigation was accomplished using a sextant to measure latitude, and star charts and timekeeping to measure longitude. However, accurate star charts and timekeeping are necessary; a one hour error is good enough to navigate to something the size of Texas. Inability to accurately determine longitude resulted in many shipwrecks. The loss of four ships with 2000 men at the mouth of the Thames River channel in 1707 propelled the British government to pass an act to reward the discovery of an accurate method of determining longitude.

A carpenter turned clockmaker named John Harrison heard of the prize 11 years later and developed a clock that used two counter-oscillating pendulums to compensate for boat rocking. The clock tested in 1735 on a trip from England to Portugal proved itself good enough for Mr. Harrison to receive a prize of 500 pounds. Mr. Harrison developed a new model three years, and a third model 20 years later. Then he began to work on pocket watch technology. In 1750, he tested his fourth model based on pocket watch technology. The test involved keeping time on a round trip between England and the West Indies. The clock kept time to within 5.1 seconds. However, the prize board did not award him the prize, claiming the test was not controlled and requesting a new test on a trip to Barbados.

The clock was nine miles within the prize requirement. However, the Board only awarded half the prize because it had not been shown that the clock could be accurately reproduced.

These clocks opened trade and navigation, but never approached the accuracy of a pendulum clock. Quartz piezoelectric clocks surpassed the accuracy of the pendulum clock in 1943. In 1950, an ammonium atomic clock provided accuracy that surpassed that of quartz clocks. The ammonium atomic clock used the resonance of the ammonium molecule to set the frequency standard. The second was redefined in 1967 to be based on the cesium-133 atom. However, cesium transition is not good to measure because it has a very long wavelength.

The National Bureau of Standards built a series of clocks based on the cesium atom of decreasing time measurement uncertainty. The accuracy of the current cesium clock, NIST-7, is limited by the thermal Doppler shift. Atomic clocks are currently used in global positioning satellites, NAVSTAR, which are used to provide the latitude, longitude, and elevation of an object. The market for this is growing although the atomic clock was developed for purely academic reasons, and could not be justified to Federal budget analysts. Some new developments in atomic clocks include a hydrogen MASER clock that uses a tuned cavity to get a build up of microwave photons that resonate. Another clock under development uses stored ions that are cooled to extremely low temperatures using LASERs.

Mr. Sullivan closed his talk by describing NBS's role in developing closed-captioning. Originally, NBS developed a method to send time on a TV signal. ABC and PBS helped in the development. All three received an Emmy for this effort.

Mr. Sullivan then kindly answered questions from the audience. The President thanked the speaker on behalf of the Society. The President then made the usual parking announcement, announced the next meeting, and adjourned the 2043rd meeting at 9:41 p.m.


Respectfully submitted,
Alan Russell
Treasurer, for the Recording Secretary

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