The official governing bodies who establish our second, hour, day, date, and year designations are giving you an extra second in June by adding a "leap second" to the time stamp. In most cases, this is no big deal, but there are serious implications which such an apparently small change can induce for others.
What's going on here? Time is perhaps the most confusing of the fundamental physical constants. Everyone knows what it is, yet no one really knows what it is; For a fascinating, sophisticated, and very readable book on the subject, see Sean Carroll's "From Eternity to Here: The Quest for the Ultimate Theory of Time."
Complicating the situation, there are really two metrics of time: interval time, which we now define in terms of atomic vibrations with almost unimaginable precision (an aggregation of hundreds of atomic clocks forming UTC (Universal Coordinated Time); and astronomical time, based on mean solar time and the Earth's motion (characterized by Greenwich Mean Time, or GMT).
For centuries, these two measures of time were synchronized using astronomical time as the baseline, especially because measuring astronomical time to better than a small fraction of a second was very difficult. We now know, however, that these clocks diverge because the Earth's motion is not as consistent as was believed or knowable. The Earth slows down primarily to due tidal friction, and geological events (primarily earthquakes) can shift the mass of the planet and change the Earth's rotational moment of inertia. Basic physics tells us that since angular momentum is conserved, this change can result in slowing down or even speeding up of the Earth's rotation.
The divergence between atomic time and astronomical time isn't trivial, and UTC is a "compromise" between the two clock standards. The UTC has been debating for years whether to gradually decouple UTC from mean solar time, and thus abolish leap seconds, with astronomers on one side and much of the rest of the scientific community on the other.
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