[LEAPSECS] the big artillery

Warner Losh imp at bsdimp.com
Thu Nov 6 10:05:00 EST 2014

On Nov 6, 2014, at 5:09 AM, Zefram <zefram at fysh.org> wrote:
> My view is that the UT1 second is a unit, not a variable quantity.
> It's a different unit from the SI second, and can't be described in
> terms of the SI second.  If anything it's a unit of angle, and so can be
> described in radians or an equivalent, but it's philosophically valid
> to treat it as distinct even from the angle units.  The UT1-related
> quantity measurable in SI seconds, which Warner has referred to as "the
> UT1 second", is more accurately described as "the duration of the time
> period in which UT1 increments by one UT1 second", or more succinctly
> "the duration of the UT1 second".

You are correct. The "UT1 second” is a measure of angular motion of
the earth, averaged in a particular way. When I say that it is different
than the SI second by 1e-9 or so, you’re right about what I really mean:
“The typical duration of the time it takes for the earth to rotate a UT1
second differs from the SI second a bit, usually on the order of 1e-9 or so”
but even that’s not quite right since the UT1 second is an average
of a bunch of actual earth rotation seconds to smooth out certain
well known wobbles.

The conversion of the signals to a phase within a cycle, and talking
about differences in phase angles is designed to give a common set
of units. Note this phase angle isn’t measuring the phase angle of the
earth, but rather converting a PPS signal or number to a sine wave
and comparing that. For high frequencies, where you’re trying to find
the difference in frequency between two 5MHz or 1GHz oscillators,
talking about the phase angle difference makes a lot more sense
because it isn’t as artificial a construct. And when comparing atomic
clocks to each other, you almost never use the PPS to make the
comparison, but do measurements and transformations on the high
frequency outputs to measure the difference.

I’ve gotten so used to thinking of things in the short hand terms that
I’d forgotten that I was even using them.

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