Path: utzoo!utgpu!news-server.csri.toronto.edu!rpi!zaphod.mps.ohio-state.edu!samsung!uunet!stanford.edu!agate!sizzlean.berkeley.edu!moews
From: moews@math.berkeley.edu (David Moews)
Newsgroups: comp.misc
Subject: Re: Julian day numbers
Message-ID: <1991Apr24.224138.2985@agate.berkeley.edu>
Date: 24 Apr 91 22:41:38 GMT
References: <41601@cup.portal.com>
Sender: root@agate.berkeley.edu (Charlie Root)
Organization: UC Berkeley Math Department
Lines: 17
Originator: moews@sizzlean.berkeley.edu

In article <41601@cup.portal.com> J_Otto_Tennant@cup.portal.com writes:
|...
|	JULDAY = INT(365.25*JY)+INT(30.6001*JM)+ID+1720995
|...
|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?...

The number 30.6 (= 153/5) does not have a terminating binary representation.
As a result, if you just wrote `30.6' you might get a floating-point number
that was actually a little smaller than 30.6.  Then when you took INT(30.6*5) 
you might get 152 instead of 153, which would be incorrect.  Adding a small
fudge factor like 0.0001 prevents this.
--
David Moews                           moews@math.berkeley.edu