[LEAPSECS] the leap second in the media

[Magnus Danielson]

John Hawkinson skrev:

> (chiming in a bit late...)

> 

> Tony Finch <dot at dotat.at> wrote on Thu,  1 Jan 2009

> at 06:10:59 +0000 in <alpine.LSU.2.00.0901010602100.2395 at hermes-1.csi.cam.ac.uk>:

> 

>> Wednesday's PM on Radio 4 included an item about the way the keepers of

>> the Big Ben clock handled the leap second. They slowed it down for several

>> hours by removing a stack of old coins from a plate near the top of the

>> pendulum to lower its centre of gravity. They replaced them again when it

>> had lost a seond. Rubber seconds, 19th century style!

> 

> This is exactly how Unix systems handle changing the clock!

>>From the Solaris adjtime(2) manpage:

> 

> DESCRIPTION

>      The adjtime() function adjusts the system's  notion  of  the

>      current  time  as returned by gettimeofday(3C), advancing or

>      retarding it by the amount of time specified in  the  struct

>      timeval pointed to by delta.

> 

>      The adjustment is effected by speeding up (if that amount of

>      time is positive) or slowing down (if that amount of time is

>      negative) the system's clock by some small percentage,  gen-

>      erally a fraction of one percent. The time is always a mono-

>      tonically increasing function. A  time  correction  from  an

>      earlier call to adjtime() may not be finished when adjtime()

>      is called again.


Not exactly, this is the old interface. Many uses a new interface when 
doing NTP to achieve better performance. Instead ntp_adjtime() is used.

The idea is similar thought. What happend was the the control loop was 
moved into the kernel for better performance. Implemented on many 
systems. For more info see:
http://www.cis.udel.edu/~mills/resource.html#micro

You could of course to some degree steer the clock on a machine by 
changing the computing load on it, as the tempco of most systems crystal 
is pretty bad. It would kinda work except when it is out of 
controlrange. I would not recommend such a tuning method, but it would 
be on the levels of those coins.

It is noteworthy that it is an old set of coins. Choosing a new set 
would require retuning of the set of coins being used. I would not mind 
seeing a measurement log from Big Ben...

Cheers,
Magnus