[LEAPSECS] the big artillery

[Zefram]

michael.deckers via LEAPSECS wrote:
>>>   UT1 is a timescale that ticks 1 SI second when the Earth Rotation Angle
>>>   increases by exactly (2???? rad)/86 636.546 949 141 027 072,
>>
>>Which it rarely does for any length of time.
>
>    On the contrary, the fixed angular speed d(ERA)/d(UT1) is a
>    defining property of UT1,

UT1 always ticks a second for that ERA increase, but Warner's point
is that the second of UT1 isn't an *SI* second.  The time taken for
that ERA increase, and hence the duration of a UT1 second, very rarely
exactly matches an SI second.  The second of UT1 is an angular unit,
defined as 1/86400 circle (= 15 arcseconds), not a unit of physical time.

Of course, due to the history, we alias angular seconds to physical
seconds all over the place, especially in the mathematical expressions
that we use to describe relationships between time scales.  Usually we
gloss over that by just calling them both "second".  But if you're going
to specify which type of second you mean, better pick the right one for
the time scale.

-zefram