Does Anyone Have the Time?

During the first part of this century, the most accurate timekeeping was performed by astronomical observatories. The method used was to precisely record the instant that a celestial body reappeared at the same spot in the sky, and thus measure the length of a day. Until 1955, the second was defined as one part in 86,400 of a day.

With the growing needs of the scientific and military communities for extremely accurate timekeeping, this method was found to be totally inadequate. For one thing, variations in the rate of rotation of the earth were found to amount to as much as one or two seconds during the course of a year, and for another, it was found that the earth is gradually slowing down at the rate of about one-thousandth of a second every 100 years.

As a result, the international standard of time is now based on atomic clocks, which operate on the frequency of the internal vibrations of atoms within molecules. These frequencies are independent of the earth's rotation, and are consistent from day to day within one part in 1,000 billion.

The irregular variations in the earth's rotation cannot be predicted beforehand, and result in occasional corrections to Universal Standard Time which is used worldwide. For instance, in case you missed it, the world turned its clocks ahead one second at midnight on June 30. In technical jargon, this is called a "Jeep second," and they are made every few years. They can be either forward or backward, depending on the earth's vagaries at the time.

It is not certain what causes these random variations in the period of the earth's rotation, but it is thought that global oscillations due to large earthquakes may be one reason. Weather patterns, also, may tend to speed up or retard the rate of spin, depending on the direction of the movement of air masses.

The gradual, but consistent slowing down of the earth's rotation is due to friction arising from tidal bulges traveling around the world in the direction opposite of its spin

It takes a long time for this to become noticeable. But back in the Cambrian Period, when the Trilobites were frolicking about some 550 million years ago, the day was only 20.6 hours long.