Leap Second to be Added This Year

Jeff Wojnicki

This New Year's Eve brings us our first Leap Second since 1998, this will be the 33rd added since the advent of the Atomic Clock. Leap Seconds were originally introduced in 1971 to reconcile astronomical time (UTC) with atomic time, started in 1958 and referred to as International Atomic Time (TAI).

When TAI was introduced in 1958, TAI and UTC were approximately equal. Over the years, the time expressed by atomic clocks has diverged slightly from time measured by the astronomical approach.

The difference results from changes in the Earth's rotation, due to gravitational forces from the Sun and Moon. Among these forces are tidal friction and various planetary and meteorological effects.

As early as 1695 Sir Edmond Halley suspected an acceleration in the mean motion of the Moon by studying ancient eclipse records. In 1754 Immanuel Kant proposed that the acceleration of the Moon could also be explained by a steady deceleration in the Earth's rotation due to tidal friction.

Because of these effects the length of the mean solar day is about 2.5 milliseconds longer than the standard of 86,400 seconds. This accumulated difference is compensated by the occasional insertion of a Leap Second. A change in the frequency of occurrence of Leap Seconds is an indication of the slowing down or acceleration of the Earth's rotation.

Presently there is a controversy whether the Leap Second should be abandoned in favor of a uniform time based solely on atomic clocks. If adopted, solar time would drift and telescopes would not point in the proper direction.

The insertion of Leap Seconds will become more frequent, but should work adequately for the next 1200 years. However, in GPS satellites the current difference that can be stored is 127 seconds. If Leap Seconds continue to be inserted into UTC the fleet of GPS satellites and all receivers will become obsolete. Since the lifetime of the satellites is around 10 or so years, new ones would have to account for the bigger and bigger time difference.

To create a standard continuous time system the International Telecom-munications Union (ITU) has proposed to discontinue leap seconds in UTC in just a few years time and replace it with International Time (TI). This would result in changing the current standard of keeping atomic and astronomic time within 0.9 seconds with Leap Seconds to up to an hours difference.

The United States delegation supports the abandonment of the Leap Second. There is much information on the Leap Second and the current controversy on the web. Some of the sites about the state of the controversy are:


A good site for the history of the Leap Second is:


Published in the December 2005 issue of the NightTimes