[time-nuts] Re: possible negative leap second

[Tom Van Baak]

Hi Jim,

There have been a number of news articles about leap seconds in recent 
years and another batch this week, many with clickbait titles.

 > synchronizing to such would need to know it wasn't a leap second.

Correct, anything synchronizing with UTC already has to know about leap 
seconds so that there are no surprises: no accidental extra seconds, no 
accidental missing seconds. A leap second is a specific event that 
occurs only at the last second of a month. At a minimum you have to know 
which month and what sign (positive or negative). Official notices of 
leap seconds filter down from BIPM to affected timing systems in months 
prior to the event.

 > this is the first I'd seen mentioning removing a second

The modern version of UTC started in 1972 and from the beginning had the 
provision for either positive or negative leap seconds, without favor. 
The goal was simply to keep the stable atomic time scale that we all use 
roughly in sync with unstable earth rotation time. If a second needs to 
be added to UTC it is named 23:59:60 and if a second needs to be deleted 
23:59:59 is skipped. On paper it's very simple. Hardware and software to 
handle either kind of leap second has been around since the beginning.

It's kind of a historical accident that UTC has had only positive leap 
seconds so far. Earth rotation rate is surprisingly erratic [1] and had 
UTC been decades or centuries older we would by now have experienced 
both kinds of leap seconds frequently. The concern is that many timing 
systems today have had relatively little exposure to positive leap 
seconds (since they are rare) and even less exposure to negative leap 
seconds (none needed yet).

The issue of earth temperature, glacial rebound, ocean level, and mass 
distribution is legit, not to mention less well understood effects deep 
within the earth. From a time nut perspective, earth is a poor 
timekeeper. If you saw its ADEV [2] you'd probably just replace it an 
OCXO. ;-)

/tvb

[1] http://leapsecond.com/pages/lod/lod-try-4.png

[2] http://leapsecond.com/museum/earth/