Re: Julian Day Numbers

[thompson(at)orpheus.nascom.nasa.gov (William Thompson)]

Ben Tupper <pemaquidriver@tidewater.net> writes:

>Hello,

>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
>dates
>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)
>       2415021.0

Using completely independent software, I can verify that this is correct.

>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.

This is also correct.  The date-time you used in the first calculation would be
written as "1900 JAN 1.5".  There is no discrepency here, only confusion about
the notation used in the Explanatory Supplement.  I would have written the date
in the above quote as "1899 DEC 31.5"

William Thompson