When I write about things like precision orbit determination, I often have to discuss the difference between ephemeris time (ET) and coordinated universal time (UTC). ET is a “clean” time scale: it is essentially the time coordinate of an inertial coordinate frame that is attached to the barycenter of the solar system. On the other hand, UTC is “messy”: it is the time kept by noninertial clocks sitting here on the surface of the Earth. But the fact that terrestrial clocks sit inside the Earth’s gravity well and are subject to acceleration is only part of the picture. There are also those blasted leap seconds. It is because of leap seconds that terrestrial atomic time (TAI) and UTC differ.
Leap seconds arise because we insist on using an inherently wobbly planet as our time standard. The Earth wobbles, sometimes unpredictably (for instance, after a major earthquake) and we mess with our clocks. Quite pointlessly, as a matter of fact. And now, we missed another chance to get rid of this abomination: the International Telecommunication Union failed to achieve consensus, and any decision is postponed until 2015.
For the curious, an approximate formula to convert between TAI and ET is given by ET – TAI = 32.184 + 1.657×10–3 sin E, where E = M + 0.01671 sin M, M = 6.239996 + 1.99096871×10–7 t and t is the time in seconds since J2000 (that is, noon, January 1, 2000, TAI). To convert TAI to UTC, additional leap seconds must be added: 10 seconds for all dates prior to 1972, and then additional leap seconds depending on the date. Most inelegant.
Speaking of leap this and that, I think it’s also high time to get rid of daylight savings time. Its benefits are dubious at best, and I find the practice unnecessarily disruptive.