[time-nuts] HP 5065a Rb ref update and question on time scale

Tom Van Baak tvb at LeapSecond.com
Sat Oct 1 17:26:16 EDT 2011

> I am curious that maybe back in 1974 atomic time was one thing and then a
> correction was done that makes it -250 from what it was. Maybe a historical
> adjustment.

Hi Paul,

In the 1960's when the 5061A and 5065A were designed the
official, legal second was still based on the rotation rate of the

Only a few years earlier the cesium second had first been
calibrated against the rotation rate of the earth in the 1800's
(that's where the 9,192,631,770 number came from). But the
earth rotated faster in the 19th century than it did in the 20th
century. So it is not surprising that the calibrated atomic
second ended up a tiny bit shorter than the legal second.

In addition, the length of the legal second changed every now
and then depending on how the planet was feeling. Depending
on the year, the correction was 130 x10^-10; or 150, or 250,
or 300, etc. A few parts in ten to the 8th.

Anyone doing precision time or frequency back then faced a
real problem: which second do you use? The legal one or the
stable one? Various hp products like the 117A or 5061A and
5065A were designed to create either one, depending on the
customer needs.

By 1972 the problem of different kinds of seconds and randomly
announced changes in the length of the second became such a
nuisance that an alternative solution was implemented. That was,
to make the atomic second the legal second, but keep atomic time
roughly in sync with the planet through the randomly announced
use of a "leap second".


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