National Geographic : 1947 Sep
Split-second Time Runs Today's \Vorld by careful measurements on the plates, it can be determined whether the star reached the zenith when the master clocks said it should. If there is any difference, the time of sending out the radio time signals is corrected to coin cide with the earth's turning. For instance, if the master clocks say the star should reach the zenith at exactly 11 p. m., but it actually arrives at 1 100th of a second before 11, then the clocks are that much wrong, and the next time signal is sent 1 '100th of a second sooner than it otherwise would be. Of the Observatory's seven master clocks, three are run by pendulums and four by vi brating crystals. The Observatory's time signal is based on crystal clocks only, since they are the most accurate. The master clocks are always "wrong," for once started going they are never reset. To do so would only increase their errors. But it does not matter that they are wrong, since the astronomers always know just how fast or slow they are. Their error is allowed for in sending out the time signals. Even with their constant error, those master clocks are far more nearly accurate than the average good clock or watch that keeps time satisfactorily for you and me. To keep them so exact, the clocks are pro tected from all outside disturbances. The pendulum clocks tick off their time in an in sulated vault 30 feet under ground, away from vibrations and changing temperatures. Each pendulum swings in a vacuum, in a case from which the air has been pumped out, for air resistance would gradually slow down their time of swing. Each pendulum swings in a different direction, too, so that the vibration of one will not affect another. Pendulums Viewed Through Periscope In the vault also temperature is kept con stantly within a fraction of a degree of 85 Fahrenheit, for changes in heat and cold could also vary the pendulums' swing. No one ever enters this vault except to make infrequent repairs, and the swinging pendulums are watched through a periscope from above ground. The pendulum clocks are no longer used in timekeeping. They are now employed in observations for determining positions of stars, since time enters into these calculations. Vibrating crystals that run the other master clocks are only slightly larger than an air-mail stamp. They are sealed inside vacuum tubes like those in your radio, so that they vibrate in a vacuum. Air resistance would slow down their vibration just as it would a pendulum's swing. They too are kept at a temperature that varies no more than 1 100th of a degree. Electric current keeps the crystals vibrating, and once started they vibrate continuously at the same frequency, 100,000 times per second. Clocks run by the vibrating crystals are more nearly accurate than the pendulum clocks, be cause the crystals are not affected by variation in the pull of gravity, which causes slight irregularities in the swinging pendulums, even in the underground vault. Even a change in the level of the water table in the ground will make enough change in gravity's pull to alter the rate of a pendulum's swing. Besides these seven master clocks of its own, the Naval Observatory also uses the master clocks of the Bureau of Standards to help regulate the accuracy of its time signals. These clocks also are crystal-controlled. Mother Earth Slowing Down But absolutely accurate time is a hard thing to achieve, for Mother Earth herself is not a perfect timekeeper. Though the time of the earth's rotation is the basis for our time, even that varies slightly. Records show that its rotation apparently speeds up occasionally, then slows down a little. From 1680 to 1800 the earth lost about 27/100 of a second in that period of 120 years. From 1800 to 1900 it gained slightly more than 30/100 of a second. From 1900 to 1920 it lost a bit again. Since 1920 it has been gaining once more. Nobody knows why. Moreover, despite these periodic gains and losses, the earth clock is gradually running down like any other. In the long run it is turning more slowly all the time, losing about 1 1000 of a second in 100 years. Astronomers know this from records of old eclipses as far back as Babvlonian times. That has no effect on us laymen, but it is important to astrono mers, who think in eons of time. Friction of the tides against the bottoms of the shallower seas is the main force that is gradually slowing the earth down. Another is any shifting of materials from one place on the earth to another, which disturbs its balance slightly. The Mississippi River, transferring millions of tons of silt from the northern United States to the Gulf of Mexico, farther from the center of the earth, slows down the rotation of the earth a little. You could even say that the construction of the Empire State Building, in which ma terials were taken from below the earth's sur face and placed on top of it, caused an in finitesimal slowing down of the earth's rotation. All that makes it at least a little more difficult for the Naval Observatory astronomers to keep accurate time.