Just a ‘Leap Second’ Makes a Perfect Year

WASHINGTON — Because clocks are more precise than Earth’s own rotation, an extra “leap second” in 1990 will adjust for the difference, timekeeping officials said Sunday.

The world keeps official time according to “atomic clocks,” which operate by measuring the vibration of a single atom and are accurate within one-billionth of a second a day. Earth’s rotation, by comparison, is off “by a few tenths of a second per year,” the Time and Frequency Division of the National Institute of Standards and Technology in Boulder, Colo., reported.