The transition from June to July will be delayed today by circumstances beyond everyone's control. Time will stand still for one second on this evening (June 30) because a "leap second" will be added to let a lagging Earth catch up to super-accurate atomic clocks.
International Atomic Time is a very accurate and stable time scale. It is a weighted average of the time kept by about 200 atomic clocks in over 50 national laboratories worldwide. Atomic time is measured through vibrations of atoms in a metal isotope that resembles mercury and can keep time to within a tenth of a billionth of a second per day. The result is extremely accurate time that can be used to improve synchronization in precision navigation and positioning systems, telecommunications networks and deep-space communications.
But from their careful observations of the positions of the stars, astronomers have deduced that Earth's rotation is ever so slightly slowing down at a non-uniform rate, probably attributable to its sloshing molten core, the rolling of the oceans, the melting of polar ice and the effects of solar and lunar gravity.
Today's atomic clocks are accurate to approximately one second in 200 million years. On average, our planet has been falling behind atomic time at a rate of about two milliseconds per day. As a result, it now trails the "official" clock by about six-tenths of a second.
As a result of this difference, atomic clocks, which are used to set all other clocks, can get out of sync with the Earth and periodically have to be adjusted. A leap second has to be added from time to time to make up the difference.
The next time will be this evening, when the master clock at the United States Naval Observatory will be adjusted at 7:59:60 p.m. Eastern Daylight Time, or 23:59:60 Coordinated Universal Time (UTC). This will put Mother Earth about four-tenths of a second ahead of the clock, giving her a bit of a head start as we transition into the new month of July.
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