[LEAPSECS] An example

Michael Deckers michael.deckers at yahoo.com
Tue Nov 2 17:04:25 EDT 2010

On 2010-11-02 18:55, Poul-Henning Kamp wrote, first quoting
Steve Allen:

> > The existing international agreement for the meaning

> > of "day" is "mean solar day".


> You mean "one of the existing..." ?


> The astronomical meaning of the word day may indeed be what you say,

> but the equally internationally agreed standard for computer operating

> systems define a day as 86400 SI seconds.


> The question is which one ITU-R adopts.

Isn't a day always exactly 86400 s, in whatever time scale you
are considering? If you consider UT, you get mean solar days,
if you consider TCB you get a day of coordinate time for the solar
system, and if you consider UTC you either get the same as
you would get with TAI, or 1 s more of TAI.

Upon comparison, these intervals of days are different (and the
difference may even depend on the method of comparison), but the
time scales nonetheless use the same units of days, hours, minutes,
seconds, hours. It is the very basis of relativity that two time
scales both measured in SI seconds do not necessarily give the same
intervals of days everywhere. Other time scales with the same
property (which, accordingly, they must have!) add no further
complication, in my opinion.

For height above the geoid (altitude) we have a similar
situation: there are several notions of geodesic height, and
their height diffenrences do not agree, but all can be expressed
in the same unit of SI meters (or feet).

Anyway, thanks for the illuminating exchange of ideas about
the fate of UTC!

Michael Deckers.

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