[Date Prev][Date Next][Thread Prev][Thread Next][Date Index][Thread Index]

Re: Julian Day Numbers

Ben Tupper <pemaquidriver@tidewater.net> writes:

> > Thanks, JD, David (I think) and Craig,
> Yes, I agree that the IDL code does calculate what it claims to. My question
> was aimed (poorly) at which kind of Julian Day number IDL calculates.
> It sounds like it comes in many flavors.  I'll punt.

No, there is only one flavor here, as long as we are talking about
simple Julian Days.  If you want January the *0th*, then you'd better
enter it as such.  Then you get the answer you'd expect:

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

The strange thing is that January the 0th is really December 31st.
Everybody I know starts counting calendar days with the number 1, so
the 0th day of the month is actually the last day of the previous
month.  So it's strange that your almanac quoted that day instead of
January the 1st.

There are other conventions, at least in astronomy.  The Modified
Julian Day (MJD) and Truncated Julian Day (TJD) are very similar time
systems, the only difference being the zero-point.  Thankfully these
systems subtract the 0.5 day that makes standard Julian days so
complicated and confusing.  [ A day changeover at *noon* ??? ]


P.S. Hmm, I had no place for sarcasm here. *(:-)

Craig B. Markwardt, Ph.D.         EMAIL:    craigmnet@cow.physics.wisc.edu
Astrophysics, IDL, Finance, Derivatives | Remove "net" for better response