On Mon, 14 Aug 2000, Nick Ing-Simmons wrote:
> Jonathan Scott Duff <[EMAIL PROTECTED]> writes:
> >On Mon, Aug 14, 2000 at 06:13:13PM -0000, Perl6 RFC Librarian wrote:
> >> =head1 TITLE
> >>
> >> Maintain internal time in Modified Julian (not epoch)
>
> There has to be _an_ epoch - even Caesar started somewhere...
>
Julian Date is measured (roughly) from earliest astronomical observations
that we have available to us. MJD is JD with the slowly varying bit
removed (and correcting for the 0.5 day so that MJD starts at midnight
rather than midday).
> >
> >How would this be stored? As a floating point number? What about
> >sub-second accuracy? To get seconds you'd need about 5.15 decimal
> >places (let's just call that 6)
>
> IEEE 'float' (the short one) has about 6 places (24 bit mantissa).
> double has many more. But even I don't think using years as the
> "unit" is right thing to do.
>
> Seconds is my favourite...
>
Just to clarify, MJD is days not years. A 32-bit double preicision number
is usually adequate -- although have not thought about nano seconds!
--
Tim Jenness
JCMT software engineer/Support scientist
http://www.jach.hawaii.edu/~timj
