[LEAPSECS] How you know you're having an effect (was Re: ...)

[Rob Seaman]

Clive D.W. Feather wrote:


> Rob Seaman said:

> 

>> FACT: civil timekeeping is ultimately tied to the synodic day (i.e., "mean solar time")

> 

> You make this claim regularly and label it "fact". However, it is a claim, not a fact.


Evolution is fact.  The expansion of the universe is fact.  Plate tectonics is fact.  The creation of the tortured scablands from the Lake Missoula floods is fact.  That our cultural and technological civilization is tied to the synodic day is similarly a fact.  Patently obvious fact - once you see it.

There are other such facts that people disbelieve.  That humans landed on the Moon.  That the Moon is often visible in the daytime.  And nonsense that people do believe - from a Pink Floyd album, that there is a perpetually dark side to the Moon.

It happens that we are lucky enough to have a Moon.  A result is that over long millennia the tides slow the Earth's synodic day.  This makes civil timekeeping more challenging than otherwise.  It does not remove the requirement to countably match human days to Earth days.

It is New Year's Eve.  Yesterday my wife and I bought our calendar for next year.  Different human cultures rely on many calendars to count the days of our lives.  That the Gregorian and Julian and Chinese and Islamic and Indian and Hebrew calendars can differ in interesting ways (http://astro.nmsu.edu/~lhuber/leaphist.html) is precisely due to their shared recognition of the meaning of the word "day".  Those days are synodic days.  Synodic days accumulate at the (varying) rate of mean solar time.


> It may be true at present, but that's just as likely to be because the people setting civil timekeeping don't understand the issues.


No.  The frivolous suggestion to allow UTC to "perpetually drift" from the synodic day is because "the people setting civil timekeeping don't understand" this fundamental issue.  The report from yesterday's batch of email of their recognition that perpetual drift is unacceptable is a tacit recognition of the requirement for civil timekeeping to ultimately match the synodic day.

Leap seconds are a means to an end.  Disliking leap seconds does not change the requirement placed on civil timekeeping by our living on a spinning ball.  Seek some other mechanism to meet the requirement, but meet the requirement one ultimately must.  It is not the astronomers here who are unwilling to entertain other possibilities, who refuse to participate in the discussion, who have obstinately pursued one and only one option.


> After all, given that the time that summer time starts and ends in a large part of the world depends on what language you speak, can you really claim with a straight face that legislators have thought this through?

> 

> The fact that people happily accept a difference of over two hours between legal and mean solar time means they aren't *that* worried about it. And what little pressure there is on the topic in the UK is in favour of making the discrepency *bigger*.

> 

> (Yes, I know the difference between cyclic and monotonic effects. That's not the point.)


It is precisely the point.  Stop thinking of "mean" as some average over independent determinations of apparent solar position in the sky.  The Earth spins relative to the fixed grid of stars (or quasars).  This is the natural sidereal rate.  The synodic day is simply the sidereal rate adjusted by one day per year to account for lapping the sun.

Apparent time is a periodic divergence from this fundamental rate.  It is a red herring.  Play whatever games you want with DST and timezones.  The underlying rate is set by the calendar, not by the clock.  It is the definition of the word "day" that ties clock to calendar.

Rob