No Simple Answer

Surprisingly, the answer to this question is not a simple “Sixty”. While nearly every minute measured has sixty seconds, since 1972 there have been twenty-three minutes which each had sixty-one seconds, and there is the possibility of more in the future. To add to the complexity, it is possible that, at some point in the future, specific minutes will have only fifty-nine seconds.

Before explaining why (and when) this will happen, some information on the history of the science of measuring time is required.

Longitude

In 1675 Greenwich Observatory was set up by King Charles II, and became the recognised authority on the subject of longitude. Subsequent to an agreement reached in 1884 ¹ to set a specific location as the "prime meridian" (i.e. 0 degrees longitude), it was only a matter of time before Greenwich was accepted as this location ².

Greenwich And Greenwich Mean Time

As the ability to determine the precise longitude of a ship at sea was linked to the requirement to accurately measure time, Greenwich also became the focal point for this branch of science.

Definitions - Days, Seconds And Minutes

Although most people would consider a "day" to be the amount of time taken for the Earth to rotate once about its own axis, the length of each day varies slightly with the seasons and from year to year. Because of this scientists at Greenwich defined an invariable day, called a "mean solar day", based on the average of the Sun's apparent speed across the sky. ³

A second was then defined as 1/86400th of the mean solar day, with a minute being defined as 60 seconds.

Redefinition Of A Day

Because of the observed variance in the length of the mean solar day over time, and the requirement for "a day" to be a constant, in 1960 the definition for a mean solar day was changed, and became based on the average of the length of each day in 1900.

GMT, UT, UT0, UT1 And UT2

In 1928 the International Astronomical Union recommended that Greenwich Mean Time (GMT) be referred to as Universal Time (UT). However since UT is affected by the motion of the Earth rotational pole, UT as measured in one location would differ from UT as measured somewhere else. Consequently UT as measured at a particular location is known as UT0, while the measurement "corrected" for the effect of the motion of the rotational pole is called UT1.

However UT1 is not a uniform time scale, since the earth does not spin at a constant rate.

After UT1, UT2 was established. UT2 is obtained from applying a formula which attempts to "average out" the variations in the earth's rotation over time. However as the earth's spin is continually slowing down, the formula needs to be continually changed, so UT2 is also not a uniform time scale.

UTC

Then, in 1967, a new measurement of time which did not use the Sun as reference was established, called UTC.

UTC (Universal Coordinated Time) became the most commonly used time scale in the world. It is a compromise between the highly stable atomic time and the time of day generated by the irregular rotation of the Earth (UT1). The definition of a second is now not related to the length of a day, but to that of the radiation emitted from a caesium-133 atom. ⁴

Unfortunately over time the difference between UTC and UT1 increases ⁵. If this difference were not addressed, it would result in the Sun not being directly overhead on the Greenwich Meridian at noon and, eventually, the sun not being visible from Greenwich at any time during the period defined by the clocks as "daytime".⁶

The Leap Second

In order to maintain UTC in approximate synch with the rotation of the Earth, while still being based on the invariable "atomic second", the International Earth Rotation Service (IERS) has the authority to add or remove a second from UTC, which they will do as required to ensure that the absolute difference between UT1 and UTC remains less than one second. ⁷.

Although this adjustment can be done at the end of March, June, September or December,⁸ since the system was introduced in 1972 only dates in June and December have been used. As at January, 2007, there have been twenty-three leap seconds added to UTC ⁹. On these days, the last minute of the day had 61 seconds, with 23:59 and 59 seconds being followed by 23:59 and 60 seconds, then by the more usual 00:00 and 00 seconds of the following day.

When Is The Next Leap Second?

The IERS announces whether the option to add a leap second will be used about six months in advance.

As the leap second is inserted at the same time all over the world, the specific minute which would contain 61 seconds therefore depends on the local time zone. Only locations which use UTC as "local time" would add the second just before midnight, time zones west of UTC would add their second sometime before midnight, while those east of UTC would add the second sometime during the next day.

The announcement ¹⁰ of the leap second which was added to the last minute of December 31^st., 2005 (UTC) can be found on the IERS web site .

With the reduction in the use of celestial navigation as a result of the increased availability of satellite navigation systems, whether or not there is still a need to keep the difference between solar time and atomic time less than one second is currently the subject of scientific debate, so it may be that there will be no more leap seconds ¹¹.

Will The Insertion Of The Next Leap Second Cause Any Problems?

For those very accurate timepieces which are set by atomic clocks which are synchronised to UTC, the next application of a leap second will simply cause them to display the extra second¹². Clocks which are driven by GPS signals may be one second fast for a few seconds, then again show the correct local time¹³. For clocks which need to be set by hand, the application of the leap second will merely cause them to become one second "faster" than they were beforehand.

However complex systems which rely on continuous and highly accurate time measurements could be affected by the non-uniform sequence of seconds resulting from a leap second. As a consequence of this requirement, the potential insertion of that extra second into a particular minute of UTC time, even if the intent to do this is announced well in advance, could result in the reduction of the acceptance of UTC as the international standard measurement of time.

More Information

Markus Kuhn, lecturer with the University of Cambridge Computer Laboratory and a fellow of Wolfson College, provides an interesting article on leap seconds at http://www.cl.cam.ac.uk/~mgk25/time/leap/ .

www.leapsecond.com has extensive detail on “leap seconds”, including advice as to where you might be able to “see” a leap second occurring. The Greenwich Mean Time Web Site is another good reference point¹⁴.