The U.S. Naval Observatory serves as the official source of time for the Department of Defense and the standard of time for the United States. The atomic clock timescale of the Observatory is based on an ensemble of cesium-beam frequency standards and hydrogen masers. Today, the U.S. Naval Observatory is the preeminent authority in the areas of Precise Time and Astrometry, and distributes Earth Orientation parameters and other Astronomical Data required for accurate navigation and fundamental astronomy. (U.S. Naval Observatory - February 2005)
One nanosecond - one foot
You are in the Directorate of Time. ... You hurry past a glass-paned vault in which the world's number-one clock is soundlessly assembling each second from nine billion parts. It looks more like a rack of computers than a clock. In its core, atoms of cesium vibrate with a goose-stepping pace so sure, so authoritative, so humbling … the director of the Directorate of Time. … Gernot M. R. Winkler … glances across the desk and says, "We have to be fast." The directorate, an agency of the United States military; has scattered dozens of atomic clocks across a calm, manicured hilltop near the Potomac River in Washington. Armed guards stand watch at a security gatehouse down below, mainly because the Vice President's residence occupies the same grounds. ... The Master Clock consults with fifty others in separate climate-controlled vaults-caesium clocks and hydrogen masers powered by diesel generators and backup batteries. They check off the seconds as an ensemble and communicate continuously via fibre-optic cable with counterparts overseas. The clocks monitor one another, and individual devices can come on or off line as their performance warrants. Out-of-sync clocks reveal themselves quickly. Winkler offers an analogy: "It's like a court of law, where you have many slightly different stories and one wildly different story." when the plausible witnesses are chosen and assembled, their output is statistically merged, world-wide, at the Bureau International des Poids et Mesures, outside Paris. The American contribution is the largest.
The result is the exact time. The exact time ... by definition, by worldwide consensus and decree.
Through most of history, time was fixed by astronomical reference points ... the Earth spins once, call it a day. No more. The absolute reference has shifted from the stars to the atomic beams in their vaults. Particles are steadier than planets. ... These anomalies do matter, ... official clocks must every so often ... add an odd second ... "a leap second" ... to the world's calendar. Most often, leap seconds are inserted at the close of December 31 … 11:59:58 p.m., 11:59:59, 11:59:60 (!), 12:00:00 A.M., 12:00:01. …
So here is the real second. Here the technologies of speed reach the ultimate. "Fifty years ago," Winkler says wistfully he was a schoolboy in Austria "we made measurements of a tenth of a second from day to day. That was great. Then more and more applications came in with greater refinements. It is like anywhere in life. When you have a capability, people find a use for that."
Submarines have to surface for communications ... they have atomic clocks, Winkler continues.
"Television transmitters have atomic clocks. If you have two transmitters on the same channel, and you are between two cities, the picture will go up and down unless they are on exactly the same frequency. All good television stations have a rubidium clock."
We have reached the epoch of the nanosecond. ... The finicality of the modern timekeepers departs even further from our everyday experience ... . Particle physicists may freeze a second, open it up, and explore its dappled contents like surgeons pawing through an abdomen, but in real life, when events occur within thousandths of a second, our minds cannot distinguish past from future. What can we grasp in a nanosecond... a billionth of a second? ... Within the millisecond, the bat presses against the ball; a bullet finds time to enter a skull and exit again; a rock plunges into a still pond, where the unexpected geometry of the splash pattern pops into existence. During a nanosecond, balls, bullets, and droplets are motionless.
Inhuman though these compressed time scales may be, many humans crave the precision. Internet users set their computers to update their clocks according to the directorate's time signal. The directorate fields millions of automatic queries each day. By pinging back and forth across the network, software called Nano Second or RightTime or Clockwork or TimeSync or Timeset can correct for propagation delays along the phone lines between the atomic clocks and you. Free connections can be made ... to "time servers" with the whimsical pair of addresses, tick.usno.navy.mil and tock.usno.navy.mil.
Nanosecond precision matters for worldwide communications systems. It matters for navigation by Global Positioning System satellite signals: an error of a billionth of a second means an error of just about a foot, the distance light travels in that time. One nanosecond - one foot. That is a modern equivalence worth memorizing.
Cellular phone networks and broadcasters transmitters need fine timing to squeeze more and more channels of communication into precisely tuned bandwidth. The military, especially, finds ways to use super precise timing. It is no accident that the Directorate of Time belongs to the Department of Defence. Knowing the exact time is an essential feature of delivering airborne explosives to exact locations ... individual buildings, or parts of buildings ... thus minimizing one of the department's standard euphemisms, collateral damage.
Few institutions are so intensely focused on so pure a goal. ... The directorate's astronomers study the most distant quasars ... admiring them for their apparent fixedness in the sky. A favored set of 462 quasars provides as rigid a frame as can be found. Meanwhile, the directorate has a team of earth scientists to study the slowing rotation and the occasional wobble; a problem that comes down to watching the weather, because the planet's spin varies each year with the wind blowing on mountains. In all, the scientists who control the clocks have achieved a surpassing precision. As the eighteenth century mastered the measurement of mass, and the nineteenth, with the establishment of international geodesy, conquered the measurement of distance, the even ghostlier quantity, time, had to wait for the technologies of the twentieth century.
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