But this needs a clarification. Standard time replaced local
apparent solar time in several steps. First, clock (mean) time
replaced apparent time for civil purposes. As you can see from the
proliferation of railroad standards, these were both still local to
one place or another. Later,
In fact, leap seconds are simply due to the earth
being slow. How it got to be slow and whether
it is slowing are another issue.
Let me see if I have this right:
1) We have leap seconds because the Earth rotates more slowly
than once every 86,400 SI seconds.
Yes. (and I know what you
If the SI second were properly tuned to the mean solar day, and the
secular slowing were eliminated, there would be no need to mess about with
the civil time scale, because the random accelerations and decelerations
would cancel out in the long run. Of course, we'd have to tolerate larger
UT1 et al are not really measures of time, but of angle (of Terran
To some degree yes, but don't they also include minor
corrections (polar motion, longitude, etc.) and so at one
level they already depart from raw angle measurement
and instead are trying to act like clocks?
I should perhaps explain that I was interested in an internal
representation for durations, which I am now representing as a triple of
months, minutes, and seconds (the number of minutes in a month is not
predictable, nor the number of seconds in a minute given leap seconds,
but all other
Can someone lay out for me exactly what the difference is between
clock precision and clock resolution? I've read the NTP FAQ and
several other pages but am more confused than ever.
(I do understand the distinction between precision and accuracy:
3.1429493 is \pi precise to 8 significant
Michael Sokolov writes:
I cannot make the beautiful analog clock on the tower show 23:59:60.
But it's trivial to make it occasionally take 1.1 SI seconds instead of
1 SI second to turn its hands by 1 civil second.
Yes, this is one of the awkward features of a leap
second (positive leap second,
It should be clear that the gaps and repeats are fictitious, especially
if you think of AEST and AEDT as existing beyond the times when they are
in legal use. Putting it in practical terms, suppose I have a traffic
accident at 0230 on 2006/04/02, what time will the police officer write
Research-quality telescopes, in particular all the ones built in the last
few decades on alt-azimuth mounts, do of course use UT1; a 0.9s error
would be a complex ~10 arcsec error in both axes and give a quite useless
pointing performance. However, UTC is often used as a UT1 delivery
Without further debating the meaning of civil time, consider the
implications of this two stage system. The first stage conveys TAI
or something related to it by a constant offset. The second stage at
any location (correct me if I misunderstand you) would be a secondary
You cannot divide timekeeping, time dissemination,
into neat stages. In the 1960s if ten labs were told
to offset their phase or frequency it affected only a
handful of people or systems. Today when IERS
announces a leap second, millions of machines,
systems, and people are affected.
It is correct that DUT1 changes by +1.0 across a
positive leap second; going from a negative value
(e.g., -0.6) to a positive value (e.g., +0.4).
You would see the inverse in the case of a negative
leap second (DUT1 will, by definition, be positive
before the negative leap second and go negative
The majority of such clocks only run the receiver for some part of
the day to save power.
One particular kind I examined ran the receiver until it had sync,
then powered the receiver down for 23 hours and repeated the cycle.
Yes, but the LS bit stays lit for the entire month (at
A more apt comparison would be to the leap year rules that we
have. We know the rules going
forward a thousand years or so.
Apt indeed. Leap seconds are scheduled at least six months in
advance. That's about one part in 15 million. A thousand year
horizon for scheduling leap days is
With the surge of leap second captures this time
around, are there any concerns over the growing(?)
use of double :59 second or double :00 second
instead of :59:60 for a positive leap second?
Although not technically correct, they do seem a
practical, perhaps even clever, alternative -- in some
Thanks for the link. I see the reference to it is on their
Also note the leap second photos they used in the power
point presentation came from:
How to Watch a Leap Second
BBC News, 9 November 2005, 08:36 GMT [sic]
Consideration of a proposal to redefine everyday timekeeping
by scrapping leap seconds - small changes made to clock
time - has been postponed. A working party weighing the
proposed change to
A cold GPS receiver takes about 20 minutes to get the almanac data
from the GPS constellation. It is intrinsic to GPS that this is the
case. You cannot get around this.
It's easy to solve that if the application requires it.
You could get the almanac from an external source;
such as another
These instances of overflow come from remainders of division
operations overflowing. They all can be derived from a single base
number (say number of seconds since 1970, MJD, etc). However, when
you are deriving that single base number, it can be much harder.
Yeah, I also prefer to
So, dropping leap seconds from UTC would cause these receivers
to, eventually, go back 19 years on cold start? Hardly a major
catastrophe but worth noting.
There are no proposals to drop leap seconds as
such. The proposal, as I understand it, is/was to
hold leap seconds at their current
But a GPS receiver which uses the current leap second
offset (UTC against GPS time) to help guess which 1024
week period it is in will _eventually_ not work quite
I guess that begs the question - which of the hundred
GPS receiver manufacturers actually use the LS field
in the UTC
UTC is a useful approximation to GMT.
Rob, this will always be true, won't it? Whether you
have 100 ms time step adjustments, or 100 x e-10
rate adjustments, leap seconds, or leap hours it
seems to me there has been and will always be an
honest attempt to coordinate the two scales.
It's not a linear curve, it's quadratic. I found some
slides from the torino meeting where this was laid out very
well but I didn't save the URL, sorry.
Ah, yes, I forgot the quadratic term. Steve Allen has
a nice page at:
And his table shows
There have been a number of timing related articles
on Mars recently that got me thinking. Here's my
leap second related question:
Does anyone know if Mars is a better timekeeper
than Earth? Earth's liquid core, polar ice caps,
large moon, oceans, and climate all play a part
in our irregular
The current GPS data format will fail in approximately 2057, 2079, or
2095 for decelerations of 42, 31, or 25.6 s/cy2, respectively.
In terms of deployed systems, that's Real Soon Now.
Not to worry. It won't fail. The solution is simply
to let delta t sub LS in page 18 subframe 4 roll over
Or put it another way, can you think of a single
application where GPS cannot already deliver
the same TAI as Galileo will someday deliver?
Golly, Tom, it's on your own web page
At the whim of the commander in chief, GPS can turn on Selective
The W1K rollover for GPS was in 1999, and all that year was spent
testing various systems to see how they would fail. It would not be
at all surprising if the impending doom of the leap second counter was
noticed during a review of other deficiencies in the GPS system.
Must be a slow news year; here's another one...
At least this article is a lot more accurate than
CNN's and a rare popular article that correctly
distinguishes between rate and deceleration (it
mentions 1.5 milliseconds per century):
And Galileo will operate on TAI.
Could someone explain why this factoid has any
bearing on the issue? I've seen this presented
before and it makes no sense to me. GPS also
operates on TAI: as I'm sure you all know, by
definition, GPS internal time is TAI - 19 s.
Or put it another way, can you
Here's another leap second triva that gives a
historical context you might find interesting.
Can you elaborate on some reports. I have been
asking this question for years and have not yet
found a definitive answer.
Recently I talked with the power company speaker
at the ION GPS conference about synchronization
-- who appeared very familiar with the issues of grid
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