Tuesday, December 9, 2008

Seismic time - change your watches

Earthquake information is given in UCT, coordinated universal time, which is, more or less, the same as Greenwich Mean Time. (The letters are backwards thanks to the French, which is why the abbreviation for the metric International System of units is also backwards, SI). You can read about time systems at the excellent web pages of the United States' official timekeeper, the U.S. Naval Observatory.

The graph above shows how many milliseconds the length of day varies from 86,400 seconds, or 24.00 hours. The length of day is now about 2 milliseconds less than it was since I graduated from college in 1972 - no wonder I always seem behind the times. What about those shorter variations in the LOD (length of day): fodder for another blog.

The Earth, like me, is slowing down, due to the braking action of the tides, by about 1.4 milliseconds per day per century on the average. This is equivalent to 1 second every 714 days. Periodically, a leap second is added to account for this. The last leap second was added at the end of 2005.

So, if 2008 was a good year for you, you're lucky, because an extra leap second will be inserted once more on December 31, 2008.


  1. Charlie ScharnbergerDecember 9, 2008 at 3:03 PM

    The long-term trend is explained by tidal friction, and the very short-term fluctuations I assume are due to seasonal shifts of air masses. But what is the cause of the intermediate cycle (looks like a period of about 15 years)?

  2. An online article, Earth rotation and global change, by Clark R. Wilson at the University of Texas, Austin (U.S. National Report to IUGG, 1991-1994 Rev. Geophys. Vol. 33 Suppl., 1995 American Geophysical Union) discusses this at http://www.agu.org/revgeophys/wilson01/wilson01.html. In the section "LOD [length of day] and PM [polar motion] at Longer Periods", he says "it has become common to invoke the core as the major cause of decadal LOD changes. Although some climatic forcing of long period LOD has been recognized, (Salstein and Rosen, 1986; Eubanks, 1993), it is uncertain at what time scale air and water become less important than the core. Unfortunately, the role of the core remains largely unquantified because it is too remote to be easily observed."

    And, the web page for the Royal Observatory of Belgium says "On decadal time scales, the variations in the LOD have been shown to be correlated with the secular variation of the magnetic field. This suggests that the core plays an important role in these LOD changes. These LOD variations are thus believed to be associated with the changes in the core angular momentum" (http://www.astro.oma.be/SBC/earthrot.html).


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