Sometimes the Earth spins too fast. Humans then try to keep pace, to adapt their clocks. But adding 1 second in a digitized world is not without consequences.
Like the Terrans, the blue planet was in a hurry to turn the page of the year 2020. It therefore turned faster than usual. Last year, the days lasted an average of 0.05 milliseconds less than usual. Scientists anticipate an identical scenario for the year 2021. However, a day is supposed to last 24 hours, or 86,400 seconds, measured as 9,192,631,770 periods of cesium-133 radiation, according to international atomic time (TAI). But Coordinated Universal Time (UTC), on which our civil time is based, wants the Sun to peak at noon on the spring equinox at Greenwich Observatory.
Some June 30 and December 31, the minute between 11:59 p.m. and midnight in universal time thus lasts… 61 seconds.
When the Earth is too slow, as has been the case in recent years, for various reasons, we add what is called a « leap second » to make astronomical time coincide with atomic time. Since 1972, 37 leap seconds have been added. Some June 30 and December 31, the minute between 11:59 p.m. and midnight in universal time thus lasts… 61 seconds. If the Earth continues to dance vigorously for another five or six years, the reverse could happen, with a 59-second minute. « It is quite possible that a negative leap second becomes necessary if the speed of rotation of the Earth continues to accelerate », explained in early January Peter Whibberley, researcher at the British National Laboratory of Physics. It’s unlikely, but, if it happens,
Network stability
These leap seconds are not without consequences in a computerized or mechanized economy. In the summer of 2012, the addition of a second thus produced malfunctions of many computer servers, including Mozilla. 135 airlines have had their unusable booking system on 1 st July, causing flight delays and registration of passengers by hand!
“You should know that electrical network protection devices sample current and voltage values to the nearest microsecond. A leap second presents for them a discontinuity 1 million times greater than their sampling precision, ”explains Hubert Kirrmann, Swiss member of the International Electrotechnical Commission, for“ HD ”. This is particularly true of the European electricity grid. “All the electrical stations, from the North Cape to Gibraltar, have a receiver which gives them the time to the nearest microsecond,” explains the former professor at the Polytechnic University of Lausanne.
The only purpose of the leap second is to make atomic time mean on average at noon in Greenwich once a year.
This is necessary for the electric frequency to be the same across the continent and to ensure grid stability. “Three years ago, in Germany, we escaped a power cut. Not all devices had introduced the leap second at the same time, ”explains Hubert Kirrmann, who advocates abandoning it. Unlike the meter, or the liter, time is the only one that does not have a continuous scale. The only purpose of the leap second is to make atomic time mean on average at noon in Greenwich once a year. « It serves to perpetuate Greenwich as a temporal reference for longitude », deplores Hubert Kirrmann.
The question of putting an end to this leap second is regularly debated at the International Telecommunication Union (ITU), the master of clocks. Certain economic activities have already seceded and do not respect UTC universal time: GPS satellites exclusively use atomic time without a leap second, because a jamming of the receivers during this second would cause deviations of half a kilometer on the vehicle position.
Source : Humanité
