[LEAPSECS] LEAPSECS Digest, Vol 48, Issue 13

[Warner Losh]

On 12/16/2010 10:57, Finkleman, Dave wrote:

>

> 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"

>

In the absence of leap seconds, this is correct :)

>

>

> Other ISO standards, for example, Maritime Navigation, define the 

> second as 1/60^th 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.

>

There are many different kinds of seconds, alas.  The SI second differs, 
as you know, from the sidereal second or the "ut1" second.  It all 
depends on the context.

Warner

>

> 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 that.

>

> 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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