time, calendar, datetime, etc

John Roth newsgroups at jhrothjr.com
Mon Aug 4 01:12:45 CEST 2003


"Dennis Lee Bieber" <wlfraed at ix.netcom.com> wrote in message
news:954001-p55.ln1 at beastie.ix.netcom.com...
> John Roth fed this fish to the penguins on Sunday 03 August 2003 03:47
> am:
>
> >
> > AFIK, it hasn't changed. I *think* you're talking about the epoch,
> > which is an astronomical thing that's rightly ignored outside of
>
>         There is a subtle change in the length of the "year" (besides the
> redefinition of where "0/first point of Aries" is with respect to the
> stars).
>
>         J2000 defines a year as 365.25 days, B1900 used 365.242198781
>
> From "Spherical Astronomy" (Robin M. Green; 1985 Cambridge University
> Press)
>
> "... The starting epoch for JD has the same formal definition in every
> time-scale, but will not correspond to the same instant of time. ..."
>
> While both use 4713BC Jan 1.5 as the "0" point, the moment that point
> occurred differs in the two calculations.

Undoubtedly true, but it makes absolutely no difference for what
we're talking about. All we need in this conversation is a mapping
of days to integers that goes back far enough. Converting from
GMT / UT / whatever to ET is something that can safely be
left to the astronomers, and that's what the detail you're talking
about is concerned with.

John Roth
>






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