The leap second

Observations of the sun are the traditional basis for time keeping. Three improvements in clocks have entailed changes in our timekeeping practice, and the leap second is the most recent and smallest change. Solar days (noon-to-noon) vary predictably through the year, and good timekeeping averages-out these effects and defines a reference time scale, based on earth's mean rotation. It has been called variously Universal Time (UT), Zulu time (Z), and Greenwich Mean Time (GMT). Seasonal and random effects need to be averaged out, and the phrase "mean time" simply implies that an averaging method is being used. Different methods and names have been applied to the average.

The earliest averaging method simply corrected for the seasonal variation due to the Earth's orbital inclination and eccentricity, UT0.

UT0 with the polar wander correction gives UT1, the time scale needed for celestial navigation and surveying.

If the seasonal variation of UT1 is averaged out, UT2 results.

If the rate and time are coordinated through international comparisons organized under the Convention of the Metre, UTC results. UTC, or Coordinated Universal Time is the modern implementation of GMT and is used as the basis for official time around the world.

Until 1972, the duration of the second for each of these time scales varied slightly (but in different ways) to keep in step with variations of Earth's rotation. Since 1972 the duration of the second for UTC has been fixed at the value established by an average of atomic clocks around the world (TAI), and leap seconds have been added as required to keep UTC aligned with UT1 within 0.9 seconds.

The International Earth Rotation Service (IERS) in Paris is charged with predicting when the next leap second will be needed. It then informs national time laboratories, such as the National Research Council, of the impending leap second. The leap second can be inserted in the last second (UTC) of the day, of June 30 or December 31. Clocks which take advantage of the leap second prediction facility, disseminated by the time laboratories, will then have a 60th second at that moment.

Up-to-date information on leap seconds may be found in our BULLETIN TF-B.