[fitsbits] leap second alert

[Paul Schlyter]

In article <199912160128.MAA16488 at grus.atnf.CSIRO.AU>,
Mark Calabretta  <Mark.Calabretta at atnf.csiro.au> wrote:
 
> I'm surprised at the apparent willingness of the astronomical community
> (or perhaps just the FITS community) to accept a change in the
> fundamental definition of UTC.
> 
> For a start, it seems to me that although the astronomical community may
> be responsibile for the implementation of civil timekeeping it is not
> their prerogative to change the definition of the system in general use.
> 
> Surely civil timekeeping is a matter of government regulation.  It is
> perhaps nothing short of a miracle that all countries (are there any
> exceptions?)
 
Yes there are: some islamic countries are still using mean solar time,
i.e. "every longitude is its own time zone".
 
> agree to use a consistent timekeeping system, UTC.  There would need
> to be general agreement between international governments on any changes
> made to its definition.
> 
> The basic definition of UTC ties it closely to the Earth's rate of
> rotation.  Surely "Ephemeris Time" (the precursor of TDT/TT) was available
> if a "uniform" timescale had been desired for international use.
 
Ephemeris Time has only been available since 1960.  Also: the exact
difference ET-UT could only be determined a few years after the fact,
mostly by analysis of observations of the Moon's motion.  Therefore
ET was impractical for everyday use.  The time had to first be
determined in UT, and only a few years later, when the ET-UT difference
was accurately known, could the precise time in ET be determined.
 
Atomic clocks changed that of course.
 
> Certainly TAI is available now.
> 
> When I originally responded to Demetrios Matsakis' call for opinions on
> the matter I pointed out that it is often convenient that computer system
> time (synchronized to UTC via NTP) is a close approximation to UT1.  In
> applications where millisecond accuracy is not critical, LAST (local
> apparent sidereal time) can be computed to within a second without having
> to resort to table-lookups.  The particular application I had in mind
> involved a graphical, time-changing, representation of the sky used as a
> user interface in real-time telescope control.  Sub-second accuracy is
> not required but errors of tens of seconds would be noticable here.
> There must be a lot of other astronomical software which falls into this
> category.
> 
> On questioning, Demetrios said that one of the protagonists for the
> change was the NTP community itself since it "...takes a while to
> propagate leap secs thru the hierarchy. Leap seconds are a tremendous
> headache in the NTP world because they cannot be predicted."
> 
> While I sympathise with their difficulties (I've been there myself), it
> is invariably true that IERS gives *plenty* of notice (several months
> at least) of an upcoming leap second.  I can only conclude that NTP was
> not designed with leap seconds in mind.  This would have to rank as a
> Y2K-type flaw if you consider that leap seconds have been around for
> nearly thirty years now.
> 
> In any case, my feeling is that anti-leap-secondists should be lobbying
> for civil time to be changed from UTC to TAI (+ offset?).  The proposal
> to change the definition of UTC appears to me to be a back-door route to
> achieving this.
 
The anti-leap-secondists should leave civil time alone, and instead
adopt some time scale without leap seconds!  For instance TT, or TAI,
or TAI+some_offset.  Astronomers have done this for decades already:
their ET (Ephemeris Time) since 1960, and TT (Terrestial Time) since
1984 serves this purpose well -- however astronomers need to keep
track of both ET/TT and UT, since UT *is* a measure of the Earth's
rotation, which matters astronomically.  Computer folks have it
easier: they need not worry much about the Earth's rotation, so they
could forget about UT and simply adopt TAI.
 
> But if, in the end, civil time was to be converted to some uniform time
> scale then why not go the whole hog and dispense with timezones as well.
> Then at least civil time would correspond with solar time along one of
> the Earth's meridians.  Move over Greenwich!
 
Don't you think the French would want us to use the Paris meridian instead? :-)
 
However, if you want a uniform time scale, the Greenwich time won't
do, since it's connected to the rotation of the Earth.  Here you can
borrow another concept of astronomy though: the Ephemeris Longitude,
which was a longitude system of an imaginary Earth with perfectly
uniform rotation.  The Mean Solar Time at Ephemeris Longitude zero
was by definition equal to ET, Ephemeris Time.  Of course Ephemeris
Longitude is slowly drifting away from physical longitude: since
ET-UT currently is some 66 seconds, Ephemeris Longitude zero
currently resides at about 0.275 deg E (physical) longitude.
 
-- 
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Paul Schlyter,  Swedish Amateur Astronomer's Society (SAAF)
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