And Now, the Leap Second

Every leap year, modern calendars are expanded to include an extra day. There is a valid astronomical reason for the adjustment: it takes almost six hours more than 365 days for the earth to complete its annual trip around the sun. Thus, to keep the calendar in time with the earth, a 366th day--Feb. 29 --is added every fourth year. Now, as leap year 1972 approaches, scientists are preparing to insert a new and considerably smaller correction into the calendar: the leap second.

The leap second grows out of science's pressing need for extremely accurate clocks. In 1967, an international agreement redefined the basic unit of time --the second--in terms of the precise tuning-fork-like vibrations of the cesium atom (9,192,631,770 cycles per sec.). But while cesium, or atomic, clocks are the most accurate timepieces ever built by man (they lose no more than one ten-millionth of a second in a day), other measures of time--hours, days, months --are still geared to the earth's rotation. Unfortunately, as clocks go, the earth is less than perfect. It is slowing down by as much as a second a year.

Painstaking Correction. The slowdown is something of a puzzle to scientists: some suspect that it may be due to the slippage of the earth's mantle over its underlying core. Whatever the cause, the slowdown is a major nuisance to the National Bureau of Standards--which watches over the national time standard with its cesium clocks at Boulder, Colo.--and other institutions and laboratories that operate atomic timepieces. To keep these clocks in step with the earth's less-than-regular rotation, they must be reset periodically by a small and painstakingly calculated amount. Because the corrections are done independently and at different times, one lab's atomic clock may not read the same as another's.

The leap second should eliminate such discrepancies. The International Time Bureau in Paris will now simply issue a directive, probably once a year beginning in 1972, based on worldwide astronomical observations of the earth's rate of rotation. If the accumulated slowdown requires it, the bureau will advise participating countries to reset their clocks by the addition of a second (or subtraction of a second if the earth's rotation should speed up). Thus atomic clocks in all parts of the world should always be ticking off the same seconds. Why wasn't the leap second created sooner? Explains James A. Barnes, chief of the time and frequency division of the National Bureau of Standards: "It takes time to agree on time."

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