[LEAPSECS] leap seconds in POSIX

[Brooks Harris]

On 2020-01-27 11:52 AM, Steve Allen wrote:
> On Mon 2020-01-27T19:33:37+0000 Tony Finch hath writ:
>> It looks like we're in another long gap, based on the LOD chart and the
>> UT1-UTC prediction. The current gap is now the second longest...
> In the middle of last year the rotation of the earth accelerated
> enough that LOD went below 86400 for several weeks, and DeltaT
> decreased significantly.
>
>> https://datacenter.iers.org/singlePlot.php?plotname=FinalsDataIAU2000A-UT1-UTC-BULA&id=10
>>
>> https://datacenter.iers.org/singlePlot.php?plotname=FinalsDataIAU2000A-LOD-BULA&id=10
> The LOD effects are easier to see using the plots from the Paris
> bureau of IERS by requesting to remove the predictable tidal
> variations that basically look like noise on the second plot here.
I've been studying DUT1, the 1/10th second "mini leaps", mandated by 
ITU-R Recommendation 460, produced by IERS in Bulletin D 
(ftp://ftp.iers.org/products/eop/bulletind/), and disseminated by the 
time radio broadcasts (WWVB, DCF77, MSF and such). This is another way 
to look at the trends Steve has pointed out.

The Bulletin D publications in the IERS folder go back only to 
1991-06-20. I obtained the full inventory since UTC1972 from an 
authoritative source. I assembled an Excel to examine the distribution 
of intervals (called spans) between of the DUT1 dates.

Worksheet "BulD_Spans" shows the DUT1 span lengths in (UTC) days (Y) 
v.s. the nth DUT1 change (X), and Excel "trend lines". Worksheet 
"UTC_Spans" shows the leap-seconds similarly.

http://edlmax.com/Common_Calendar/BulD_and_UTC_Spans.xls

The trend lines show a continuing increase of the "span" lengths between 
DUT1 changes and leap-seconds.

The most recent Bulletin D DUT1 change, from -0.1s to -0.2s, occurred on 
2019-05-02. Examining UT1 predictions from Bulletin A 
(ftp://ftp.iers.org/products/eop/rapid/bulletina/bulletina-xxxiii-003.txt) 
indicates the next DUT1 change may not happen until 2020-11-10 when 
UT1-UTC crosses -0.25000. This will be a DUT1 span of 558 days. This is 
far longer than the longest earlier DUT1 span of 392 days that occurred 
during the 1999 to 2006 "leap-second drought".

UT1-UTC must reach -0.5s before a leap-second will be announced. If 
these trends hold it could be quite a while, perhaps years, before the 
next leap-second.

(Note that IERS does not use Bulletin A UT1-UTC prediction values for 
determining DUT1 and leap-seconds but rather from EOP C04 
(http://hpiers.obspm.fr/iers/series/opa/eopc04_IAU2000). This is the 
internal data set IERS assembles and is NOT the official 'product'. 
Bulletin A is the official publication. Note Bulletin A gives UT1-UTC to 
5 points of precision while C04 gives 7 points.)

Predicting the future is an inherently risky business.

-Brooks


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