The plot below shows the variability of the rotation of the earth and the challenge that it poses to time keepers who are tasked with keeping clocks set to agree with the ongoing count of days in a calendar. Note that the earth sometimes spins slower, and sometimes spins faster. The variations are basically due to weather in the atmosphere, oceans, and core of the earth. This is why there cannot be a leap second schedule, for issuing a schedule of leap seconds would be like making a schedule of the weather.
The insertion of leap seconds is
required by international
agreements that define the meaning of the word
The number of leap seconds which must be inserted into UTC is proportional to the shaded area under the curve.
People like to agree on the answer to the question
What time is it?In the 1800s the UK Admiralty recognized that most ships were navigating using chronometers so they switched the nautical almanac to the mean solar time of Greenwich. Then the railroads demanded the use of watches set to standard time to simplify scheduling and avoid crashes. The advent of the trans-Atlantic telegraph cables required the whole world to find a way to agree on the time, and the 1884 International Meridian Conference produced that agreement.
In order to set all clocks to the same time at a nanosecond level there must be some agency in charge of deciding what time it is, and there must be mechanisms for distributing that time from the time producers to the time consumers. Radio broadcasts of time signals have long been the most common mechanism, and consumers do not have much choice about how they get their time.
The plot below shows the kinds of time which have been used internally by physicists and astronomers along with the kinds of time which have been available to the general public.
The descending slide and staircase show the two goals which have been the underlying principles for radio broadcast time signals since 1960:
With leap seconds in UTC one calendar day counts one turn of the earth on its axis with respect to the sun. Without leap seconds one calendar day would count 794 243 384 928 000 hyperfine oscillations of cesium-133. The fifteen year long process during which the ITU-R has not reached a decision indicates that many are not prepared to redefine the day from the sun to the cesium.
Prior to the era of coordination every agency responsible for broadcasts of time signals reset its clock as needed in order to keep the broadcasts in agreement with earth rotation. From the beginning it had been inconceivable that the radio broadcasts could be allowed to differ significantly from earth rotation. The following plot shows the behavior of time signals broadcast by the US NBS which made 29 leaps during 3 years. In constrast with the plot above where flat means uniform, the following plot shows UT2 as flat, and atomic time as slanted curves with leaps.
The US and UK agreed to coordinate their broadcasts of time signals in 1959 August, and coordination began early in 1960. Prior to that date these sorts of steps or leaps were inserted regularly into all time scales, and each national agency responsible for broadcasts of time signals added steps of different sizes at different times. Prior to atomic chronometers there was no easy way to make a record of the steps or a plot like this because there was no chronometer more stable than the earth.