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Julian Day Numbers


You may have heard the proverb, 'A person with two watches
doesn't know what time it is.'   It seems to be true for me.

I have been tinkering with making tidal predictions which,
of course, are dependent upon time.  A number of benchmark
are used to establish the phase difference for each harmonic
component calculated. One of the benchmark dates is Noon,
Jan 1, 1900.

IDL> Print, JulDay(1,1,1900,12,0,0)

However, this is just in from a reliable source...

* From the "Explanatory Supplement to the Astronomical
Ephemeris" 1992, p.699
* 1900 JAN 0.5 = JD 2415020.0.

Note the one (1) Julian Day difference.  Ugh!

There are a couple of things I could do I guess:

(1) Assume that RSI is ahead of its time, and just charge
(2) Reduce the Julian Day number by one (I hate to do that
since I don't know why I need to.)
(3) Pass different arguments to JULDAY ...
        IDL> Print, JulDay(1,0.5,1900)
    This item is really just like (2) since JULDAY converts
the input arguments to long integers before processing.
(4) Use the paper tide table published by the local
fishermen's cooperative.
(5) Post a timely (sorry) question to the newsgroup
regarding what to make of the 1 day difference.



The IDL JULDAY code cites the following reference, but I
don't have it handy to check into.
; Translated from "Numerical Recipies in C", by William H.
; Brian P. Flannery, Saul A. Teukolsky, and William T.
; Cambridge University Press, 1988 (second printing).

Ben Tupper
248 Lower Round Pond Road
POB 106
Bristol, ME 04539

Tel: (207) 563-1048
Email: PemaquidRiver@tidewater.net