[LEAPSECS] Bulletin C and all that

[G Ashton]

Rob Seaman wrote:

>Interesting if UTC can indeed be said to have implemented two different
mechanisms whose entire point was to keep Universal Time synchronized with
Mean Solar Time.

The point of the multiple mechanisms was to keep UTC close to UT which is
mean solar time at Greenwich, or wherever zero degrees longitude is deemed
to be.

Rob Seaman also wrote:
> Leap seconds are introduced at midnight UTC, not when TAI modulo 86400
equals zero.

 I would think that midnight UTC is the instant when the UTC time becomes
00:00:00. I would call the introduced second the one that began at 11:59:60
and ended one second later at 00:00:00.

Gerard Ashton

-----Original Message-----
From: LEAPSECS [mailto:leapsecs-bounces at leapsecond.com] On Behalf Of Rob
Seaman
Sent: Sunday, January 25, 2015 19:26
To: Leap Second Discussion List
Subject: Re: [LEAPSECS] Bulletin C and all that

On Jan 25, 2015, at 1:03 PM, Stephen Scott <stephenscott at videotron.ca>
wrote:

> Since UTC is defined by the IERS before 1972-01-01 "beginning_of_utc" is
not appropriate.
> This is the beginning of integer leap seconds, not UTC.

Contributors to this list can always count on prompt fact checking ;-)  That
said, the IERS came later than that, didn't it?

Interesting if UTC can indeed be said to have implemented two different
mechanisms whose entire point was to keep Universal Time synchronized with
Mean Solar Time.

> How about "leap_second_epoch" or if the term epoch is undesirable
"leap_seconds_origin" labelled as "leap00"

Ok, I'll re-index to leap0 and have a new cname called origin.leapsec.com.
How's that?


On Jan 25, 2015, at 2:01 PM, Brooks Harris <brooks at edlmax.com> wrote:

> TAI is often also represented as a date-time but there is rarely a clear
distinction made about what it means.

TAI is most naturally expressed as an unending tally of SI-seconds.  UTC as
a sexigesimal fraction of a solar (synodic) day.  It is conversion between
the two concepts that get us into trouble.  This DNS scheme might provide a
small step toward letting them live together in greater harmony, and the
tzdist standard a larger step addressing additional use cases.

> And, Rob, what, exactly, does "1972  1" mean in your Leap Seconds table?
1972-01-01T00:00:00 (TAI) or 1972-01-01T00:00:00Z (UTC)?

As PHK has observed, the essential concept of the IPv4 DNS leap second
coding is to express the equivalent of Bulletin C.  I have always
interpreted the independent variable of the IERS table as UTC.  Leap seconds
are introduced at midnight UTC, not when TAI modulo 86400 equals zero.

Rob

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