An extra second in 2012 

With this atomic definition of time, we are faced with two times: we still have astronomical time, the one we experience every day if you like, which is the legacy of the definition based on the rotation of the earth. And then there is atomic time, which is completely independent of this rotation. We therefore have what is known as universal time (UT), based on the rotation of the earth, and international atomic time (TAI), provided by the average of a set of several hundred atomic clocks distributed worldwide. This average is calculated by the International Bureau of Weights and Measures in the suburbs of Paris. The two times will drift over time since the first one is based on a phenomenon that slows down on average while the second one is far more stable and regular. If we measure exactly 24 hours on an atomic clock, you will note that during this lapse of time, the earth hasn’t quite completed its rotation. This is the reason why a ‘leap’ second has to be added from time to time (or removed if, for instance, the distribution of masses on the surface of the earth were to momentarily accelerate its rotation!). This is what will happen this year, in 2012: the last minute of the last day in June will have an extra second: after 23:59:59, 30 June, GMT, it will be 23:59:60, and only 00:00:00, 1 July, one second later. Here, the difference between the two time scales will once again fall below a 0.9 second. Since 1972, 24 leap seconds have been added to maintain the coincidence between the two types of time. The correct TAI time is thus called Coordinated Universal Time (UTC). It has the dual advantage of having the accuracy of TAI while remaining synchronised with the earth’s rotation, i.e., with UT.

Why seek this coincidence when the difference is so minimal that it wouldn’t be noticed for a very long time? We would indeed have to wait several thousand years for there to be a difference of one hour between the two time scales. There would still be time at this point to carry out a correction in one go... It was actually seamen who asked for synchronisation at the beginning of the 1970s, when atomic time had just been defined. They needed time signals that were as much in line with the earth’s rotation as possible because they were still making many of their measurements using a sextant. This need disappeared with the advent of GPS and eliminating the leap second is looking evermore likely since it is no easy matter to add a second to all the atomic clocks at the same time. “However, the English aren’t keen on this”, Thierry Bastin smiles, “because this would mean that the Greenwich Meridian would lose even more of its importance! Through the misuse of language, Greenwich Mean Time (GMT) is now often used as a synonym for Coordinated Universal Time (UTC), although GMT in the strict sense simply means the average solar time at the Greenwich Meridian and therefore normally corresponds to Universal Time (UT). So, if UTC were to disappear in favour of TAI, Greenwich would be well and truly relegated to history..., at least as far as its special role in establishing the official time scale is concerned.”

The Liège clock

The first atomic clocks were created at the end of the 1940s, but these trials weren’t conclusive. This was to change in the mid-1950s, with the recourse to caesium atoms, or isotope 133 to be exact, which isn't radioactive. “In principle, any kind of atom can be used in atomic clocks”, Thierry Bastin points out, “but when people started making them, they selected atoms (caesium, rubidium and hydrogen) that emitted in a range of well-mastered frequencies, i.e., microwaves.” While caesium atoms were the first to be used and undoubtedly remain the most frequent, hydrogen clocks (called hydrogen masers [Microwave Amplification by Stimulated Emission of Radiation]) appeared at the beginning of the 1960s, offering greater stability. The clock developed by the University of Liège belongs to this category.