Re: Where the responsibility lies

From: Rob Seaman <seaman_at_NOAO.EDU>
Date: Tue, 3 Jan 2006 11:44:32 -0700

John Hawkinson replies:

>> Time handling bugs typically appear in the interfaces between
>> systems that make contradictory assumptions.

> I think phk's point ("text book example") was that these problems
> were more likely to have been detected in a world where everyone's
> default timescale (UTC) was not subject to leap seconds -- rare
> events that may not come up in the course of normal testing unless
> special care is given.

I think PHK has demonstrated the ability (and willingness :-) to hold
up his own end of an argument. Should we ever find ourselves at the
same conference, I'll buy him a beer in anticipation of a rousing
discussion.

There are several issues confounded here. First, an untested
assertion that eliminating leap seconds will simplify time handling.
DUT1 looms large in astronomical software and one would have to be
convinced that this is not an issue with other disciplines.

Second, that UTC is indeed "everyone's default". It is (rather
loosely) the current civil time standard, but I fail to see why this
makes it a default choice for a precision timing application. "Civil
measurement" in the USA still revolves around twelve inches to a
foot, but SI units are the default for American scientists as is true
elsewhere in the world. I won't belabor my assertion that mean solar
time is the obvious civil time standard. But why do we expect that
an X-ray spacecraft would of necessity adopt such a standard?

Third, leap seconds are a mechanism to realize mean solar time in
practice. The underlying issues are those of solar time, not how
this is achieved. One can assert that leap seconds should cease, but
the fundamental civil timing requirements will remain in force. Some
other mechanism for synchronizing our clocks to our home planet must
needs be identified. The acknowledgment of a contingent need for
leap hours shows that the authors of the ITU proposal understand this.

Fourth, the need for leap seconds is growing quadratically as the
Earth continues to slow. We have no business making ad hoc policies
based on the rarity of events that are becoming more frequent. The
need for "leap hours" will grow just as rapidly - and much more
dramatically. A solution that ignores real world constraints is no
solution.

Fifth, normal testing should involve special care - or what is its
value?

In short, leap hours are - well - dumb. A proposal that relies on
their use, naive.

Rob Seaman
National Optical Astronomy Observatory
Received on Tue Jan 03 2006 - 10:45:41 PST