> seconds have not been defined as "a convenient fraction of the time it takes a particular large rock to complete a full rotation around its own axis" for quite some time
That is true.
> origin is set to an abstract event in the past
That is also true.
> which is not (as far as I know) subject to retroactive revision as a consequence of the vagarities of planetary or celestial motion
I'm afraid you are wrong on that. The unix time is synced with UTC. UTC has so called "leap seconds" scheduled at irregular intervals by the International Earth Rotation and Reference Systems Service to keep it in sync with the Earth's actual movements. So in effect the unix timestamp is wrangled to sync with the Earth's motion.
Thanks for the correction, this is quite important. I had taken statements that Unix and Posix time excludes or ignores leap seconds to mean that it increases monotonically in a continuous manner in sync with whatever atomic clock is the standard; on the contrary, it recognizes and implements leap-seconds with a discontinuity deviating from the steady tick of that standard.
I also learned that 00:00:00 UTC on 1 January 1970 is a proleptic time, as UTC was not in effect then, though I am not sure that makes it subject to subsequent planetary motions.
"Unix time numbers are repeated in the second immediately following a positive leap second. The Unix time number 1483142400 is thus ambiguous: it can refer either to start of the leap second (2016-12-31 23:59:60) or the end of it, one second later (2017-01-01 00:00:00)."
That is true.
> origin is set to an abstract event in the past
That is also true.
> which is not (as far as I know) subject to retroactive revision as a consequence of the vagarities of planetary or celestial motion
I'm afraid you are wrong on that. The unix time is synced with UTC. UTC has so called "leap seconds" scheduled at irregular intervals by the International Earth Rotation and Reference Systems Service to keep it in sync with the Earth's actual movements. So in effect the unix timestamp is wrangled to sync with the Earth's motion.
> if it is, I would be fascinated to know more
https://en.wikipedia.org/wiki/Unix_time#UTC_basis
https://en.wikipedia.org/wiki/Leap_second