[LEAPSECS] LEAPSECS Digest, Vol 48, Issue 13

Finkleman, Dave dfinkleman at agi.com
Thu Dec 16 12:57:32 EST 2010

I learn something with every exchange. Thanks. This is what is in ISO
31-1, which is now ISO 80000-3

"time, time interval

duration t

second s

The second is the duration of 9 192 631 770 periods of the radiation
corresponding to the transition between the two hyperfine levels of the
ground state of the caesium-133 atom

Representations of time of day are defined in ISO 8601.

minute min 1 min = 60 s

hour h 1 h = 60 min = 3 600 s

day d 1 d = 24 h = 86 400 s"

Other ISO standards, for example, Maritime Navigation, define the second
as 1/60th of a minute or 1/86,400 of a day, where a day is from sunrise
to sunrise, a solar day, or from a star passing the local meridian to
its return, a sidereal day. There is ambiguity among ISO standards and
probably those of other organizations.

As space operations grow more complex and the degree of understanding of
operators declines (some corollary of the Peter Principle), the
opportunity for confusion grows as precision in milliseconds or less
becomes more important. A millisecond in Low Earth Orbit is a few
kilometers, and some satellites are regularly in closer conjunction than

Please let me know if this argument seems specious. My involvement
manifests the confusion I have seen and experienced in astrodynamics.
It is true that education in that discipline exposes students to these
matters, but "in one ear and out the other." It is not as important
for them to remember as are the fundamentals of orbital mechanics.

I think we need more widely vetted and easily accessible normative
definitions of the different kinds of seconds and time scales as well as
guidance (at least for satellite operators with little background) in
which to use for a given application and how to use them.

I once had a similar exchange with Yuri Davidov, Deputy Head of
Roskosmos. He said that we should get smarter operators.

Perhaps I am too much into this and not enough educated. I will not be
offended if you opine that my perceptions are incorrect.

Dave Finkleman

Senior Scientist

Center for Space Standards and Innovation

Analytical Graphics, Inc.

7150 Campus Drive

Colorado Springs, CO 80920

Phone: 719-510-8282 or 719-321-4780

Fax: 719-573-9079

Discover CSSI data downloads, technical webinars, publications, and
outreach events at www.CenterForSpace.com.

-----Original Message-----
8601 is a problem.

If you mean their explanations of "time scale", "time point", "time

axis" etc -- well, these are indeed arcane, but they are just taken

from IEC 60 050. (Nowadays, ISO/IEC 80 000 is the international

standard for terminology regarding physical quantites. And the IAU

regulate their own astronomical time scales, of course.)

Michael Deckers.


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