Time .. Julian Day
Nancy Soreide (nns@noaapmel.gov)
Fri, 14 Jan 94 13:04:04 PST
gumley@climate.gsfc.nasa.gov (Liam E. Gumley) writes:
> I agree that using Julian Day, and number of milliseconds since midnight
of
> the Julian day is a good way to store time data.
>
> However, does it introduce some possible confusion? The following is
from
> "Numerical Recipes in C", 2nd Ed., p. 12.
>
> "Astronomers number each 24-hour period, starting and ending at noon,
with
> a unique integer, the Julian Day Number."
>
> That is, Julian Day Number 2440000 starts at 1200 GMT on 23 May 1968,
not
> at 0000 GMT.
>
> So do you
>
> (a) Assume that the Julian Day number is to be strictly interpreted,
and
> start the millisecond count at 0 at 1200 GMT on the calendar date (e.g.
23
> May 1968) in question,
>
> or
>
> (b) Assume that the calendar date holds precedence, start the millisecond
> count at 0 at 0000 GMT on the calendar day in question, and interpret
the
> Julian Day as starting and ending at the same times as the calendar
day.
>
We use (b)... In the EPS two-integer time array, word 1 is the true
Julian Day (GMT), and word 2 is the number of milliseconds since the
start of that day. For convenience, we start the day at 0000 GMT and
we do not use the astronomer's convention of starting the Julian Day at
noon.
-------------------------------------------------------------
Nancy N. Soreide Phone: 206-526-6728
NOAA/PMEL/OCRD FAX: 206-526-6744
7600 Sand Point Way NE OMNET: TAO.PMEL
Seattle, WA 98115 Internet: nns@pmel.noaa.gov
