Observations of the sun are the traditional basis for time keeping. Improvements in clock accuracy and stability have entailed changes in our timekeeping practice and the leap second is the most recent and smallest change. Apparent solar time (the time given by sundials) varies predictably through the year for a number of reasons, the most important of which are the Earth's orbital inclination and eccentricity. Good timekeeping averages-out these effects and defines a reference time scale based on the Earth's mean rotation. It has been called variously Universal Time (UT), Zulu Time (Z), and Greenwich Mean Time (GMT). Other 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.
UT0 is the timescale on the prime meridian which results from local, direct measurement of the Earth's rotation angle through observations and timing of the transits of stars and other celestial objects. A mathematical expression is used to convert these observations to mean solar time.
UT1 is the timescale which results if corrections are applied to UT0 to account for the effect on the local observations of the movement of the Earth's axis (polar wander). This is the time scale needed for celestial navigation and surveying.
UT2 is the timescale which results if corrections are applied to UT1 to account for seasonal variations in the Earth's rotational speed.
UTC is the timescale where the rate and time are coordinated through international comparisons organized under the Convention of the Metre . 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 (International Atomic Time, TAI), and leap seconds have been added to UTC as required to keep it aligned with UT1 within 0.9 seconds.
The International Earth Rotation and Reference System 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 any month, but preference is given to the ends of June and December. Clocks which can display the leap second will then have a 61st second at that moment and read 23 hr: 59 min: 60 sec. Since 1972, leap seconds have been required approximately every 18 months. It is expected that they will become more frequent due to a gradual slowing of the Earth's rotation. Negative leap seconds, in which the 60th second of the last minute of the last day of the month is removed, are permitted, but have never been required.
Up-to-date information on leap seconds may be found in our BULLETIN TF-B.