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I recently got a file from a colleague in Code 553, which had
Aug 16 1997 identified as Julian Day 2450676.  I asked the
colleague about this, and he pointed me to the Forward of a
book: Spacecraft Attitude Determination and Control(ed Wertz),
from which I would like to quote you a couple of salient
paragraphs:

Because calendar time is inconvenient for computations, we would
like an absolute time that is a continuous count of time units
from some arbitrary reference.  The time interval between any
two events may then be found by simnply subtracting the absolute
time of the first event from that of the second event.  The
universally adopted solution for astronomical problems is the
\_Julian\_  \_day\_, a continuous count of the number of days since
noon(12:00 UT) on January 1, 4713 BC.  This strange starting
point was suggested by an Italian scholar of Greek and Hebrew,
Joseph Scaliger, in 1582, as the beginning of the current
\_Julian\_ \_period\_ of 7980 years.  This period is the product of
three numbers: the solar cycle, or the interval at which all
dates recur on the same days of the week(28 years); the lunar
cycle containing an integral number of lunar months(19 years);
and the indiction, or the tax period introduced by the emperor
Constantine in 313 AD(15 years).  The last time that these
started together was 4713 BC and the next time will be 3267 AD.
Scaliger was interested in reducing the astronomical dating
problems associatedc with calendar reforms of his time and his
proposal had the convenient selling point that it predated the
ecclesiastically approved date of creation, October 4, 4004 BC.
The Julian day was named after Scaliger's father, Julius Caesar
Scaliger, and was not associated with the Julian calendar that
had been in use for some centuries.

Tabulations of the current Julian date may be found in nearly
any astronomical ephemeris or almanac.  A particularly clever
procedure for finding the Julian date, or JD, associated with
any current year(I), month(J), and the day of the month(K),
is given by Fliegel and Van Flandern[1968] as a FORTRAN
arithmetic statement using FORTRAN integer arithmetic:

        JD = K - 32075 + 1461*(I + 4800 + (J - 14)/12)/4
     *   + 367*(J - 2 - (J - 14)/12*12)/12 
     *   - 3*((I + 4900 + (J - 14)/12)/100)/4

For example, December 25, 1981(I = 1981, J = 12, K = 25) is
JD  2,444,964.
    (end of quote)

As David Barry might say, I am not making this up.  My apologies
for boring those of you who knew this.  I did not, and I have
rarely read a paragraph as good as the first quoted above for
revealing a great number of interesting and amusing facts.

Best wishes, 
Alan



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