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From: J_Otto_Tennant@cup.portal.com
Newsgroups: comp.misc
Subject: Julian day numbers
Message-ID: <41601@cup.portal.com>
Date: 24 Apr 91 03:06:43 GMT
Organization: The Portal System (TM)
Lines: 29

In the introductory chapter of _Numerical_Recipies_, programs are 
presented for the calculation of the Julian day number given a 
date in the form of month/day/year, and vice-versa.

The key trick, as described in a recent article in "Dr. Dobb's",
is to renumber months so that February is last.  Given that wonderful
hack (which numbers March as "4" and February as "15"), the next step
is given as:

	JULDAY = INT(365.25*JY)+INT(30.6001*JM)+ID+1720995

The "magic number" 365.25 is easy to understand.  The magic number
1720995 can be figured out (given the renumbering of the months.)
The number 30.6001 is somewhat less easy to understand.

From my point of view, there are two aspects to the incomprehensibility
of 30.6001.

First, why is the constant 30.6001?  JM ranges from 4 to 15.  If we
view the constant 30.6001 as (30.6 + 0.0001), the 0.0001 term is
completely irrelevant to the calculation.  Why, in a book as
careful and accurate as _Numerical_Recipies_ would there be such
an irrelevant part of a constant?

Second, "30.6" works.  Empirically, there are only 12 cases, and
the cases work.  Was this number derived empirically?  Did the
authors, or possibly Zellar, just fiddle with constants until one
worked?  ("30.59_" does not work, by the way.)  Or is there some
derivation of the number?