WASHINGTON. — ''There's a time for some things, and a time for all things; a time for great things, and a time for small things.''
-- Don Quixote, by Miguel de Cervantes
Washington -- Tick-tock-tick. That's one second. One second to be added to the age of our planet Earth. Today, precisely at 23 hours, 59 minutes, and 59 seconds, Coordinated Universal Time, one leap-second will be added to clocks all over the world.
This will be 7:59:59 p.m. Eastern Daylight Time the night before, when the extra second will be inserted in the atomic clock at the U.S. Naval Observatory in Washington.
Our ancient earth is slowing down at an average rate of one second per year as it whirls around its axis. Night and day are becoming infinitesimally longer as we circle around the sun year after year. So we add an extra second to our clocks in order to keep in step.
The planet is slowing down for a number of reasons, astronomers say. One main reason is the gravitational drag caused by the moon, which pulls the ocean tides and ground on the earth's crust toward it, changing the earth's form and slowing us down.
Also the molten material deep inside the planet slowly sloshes around as the globe rotates -- much as a beach ball filled with water is hard to roll around.
Weather also contributes to our slowing down at different seasons, with the drag of the atmosphere's momentum. When the winds speed up, we slow down. Trying to measure the passage of time took a long while to evolve. Take a few moments to ponder how you would mark time if you had no clock. Sun setting? Water dripping? Heart beating? Hair getting greyer?
Thousands of years ago, people's vague sense of time centered round the sun. When the sun rose, it was time to get up; when it set, time for sleep. Centuries later, a stick poked into the ground indicated with its shadow the sun's passage across the sky from east to west.
More elaborately designed sundials were used in Babylon some 4,000 years ago. Water clocks developed by Plato in the fifth century B.C. poured measured amounts of water from one container to another to show the passage of time.
The hourglass worked on the same basic principle, with measured amounts of sand running through a funnel. Candles were marked to show the passage of hours as the flame burned down from one mark to another. It's been said that the Chinese used to put lighted candles between their toes when they went to bed. After a certain number of hours, flame would reach the toes -- literally a hot foot. Time to get moving!
Pendulum clocks were used in the late 16th century when Galileo noticed that the period of a swinging pendulum depended only on the length of the pendulum and not on the extent of the arc. Dutch astronomer Christiaan Huygens applied that principle, and hence the pendulum became contained in a tall, slender box -- the grandfather clock.
Most modern clocks today are run by electricity, using pulsation of alternating current that change the direction of the electric flow 60 times a second.
The invention of atomic clocks that operate by measuring the frequency at which an atom resonates -- cesium, hydrogen, or mercury -- gave mankind the ability to measure time to accuracies of a billionth of a second, says Dennis McCarthy of the Naval Observatory. These atomic clocks became official as the world's timepieces in the late 1960s. By focusing the atoms in a beam, one can count more than 9 billion cycles per second.
In 1972, by international agreement, scientists decided to let atomic clocks run independently of the earth's cycle, keeping the two times separate, and coordinating the two at specific times. As the planet rotates slower, astronomers add a leap-second to the atomic time to keep the difference between earth time and atomic time within nine tenths of a second.
Leap seconds have been added at intervals varying from six months to two years, Mr. McCarthy says. This leap-second is 18 months since the last one. Today will mark the 17th leap-second inserted in world clocks since 1972.
Barbara Tufty is a conservation writer.