[LEAPSECS] What happened in the late 1990s to slow the rate of leap seconds?

Rob Seaman seaman at noao.edu
Tue Nov 10 13:37:10 EST 2015

On Nov 9, 2015, at 12:18 AM, Poul-Henning Kamp <phk at phk.freebsd.dk> wrote:

> In message <20151109025137.A95A840605C at ip-64-139-1-69.sjc.megapath.net>, Hal Murray writes:
>> Was there a geological incident that explains things?
> We don't know, but the period had other pecularities, for instance almost statistically significant level of vulcanism.

“Almost statistically significant” is like “a little bit not pregnant” ;-)

In addition to what Steve said a geophysicist would have to comment on the likelihood of geological “incidents" to affect Earth’s rotation on the scale seen. Major earthquakes have been reported to have a detectable effect, but only at the level of a microsecond glitch, right? Is there a paper trail of the effect of vulcanism? As dramatic as such phenomena are to surface dwellers, they are quite localized. On the other hand, has there been some assertion of increased vulcanism in one hemisphere and decreased in the other, for instance, as some sort of symptom of titanic processes lower down in the mantle? Presumably geophysics in the inner or outer core or at the bottom of the mantle is very well isolated from minor crustal peccadilloes like volcanoes, quakes or even the entire tectonic mechanism.

A change in the leap second cadence from roughly annually to once or twice a decade means a change in LOD from about 86,400.002 SI-seconds to 86,400.0003 SI-seconds. This is hundreds of times larger – like a magnitude 9 earthquake every day and all the glitches biased for some reason in the same direction. Or is there some reason that every earthquake would result in acceleration, not deceleration? I would be delighted to hear any comments from said geophysicists :-)

However, once the Earth jigs slower again, whether due to the tidal effect or otherwise, and remains permanently above the 1912 historical maximum for LOD, we won’t even formulate the problem the same way. (And independently of whether leap seconds continue.) The issue won’t be viewed as whether the Earth has settled down into its “natural” cadence of 24x60x60 integral SI-seconds, but rather a recognition one way or another that LOD has actually always naturally differed from 86,400 SI-seconds.

At the nominal deceleration of 2 milliseconds / century (depending which slope you pick from Steve’s plots), LOD was 86,399.99 SI-seconds for Magellan, 86,399.95 SI-seconds for Aristotle, and 86,399.90 SI-seconds for Narmer, the first Pharaoh. LOD was 86,399.7 SI-seconds for the cave painters at Lascaux and 86,399.3 SI-seconds when the first drawings were done 35,000 years ago at Chauvet (further distant in time from Lascaux, than Lascaux is from us). When members of the human family were engraving ochre 100,000 years ago in Blombos Cave in South Africa, LOD was 86,398 SI-seconds, and length of day for that happy never-couple Mitochondrial Eve and Y-chromosomal Adam 2000 centuries ago was 86,396 SI-seconds.

Solar and SI time – never the twain shall meet or ever have met.


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