Science: 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...

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