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% A brief history of calendars and such
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% "Borrowed" from Stan Rabinowitz's response to questions regarding time 
% systems, now found as part of the DECwindows Calendar Application.
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\begin{document}

\begin{center}
{\huge \sf A Brief History of Calendars}
\end{center}
\vskip 1in

\def\xyzzysection#1{\bigskip\noindent{\large \sf #1 \medskip}}
\xyzzysection{Brief History of the Gregorian Calendar}

As decreed by Pope Gregory XIII, October 4, 1582, was
followed by October 15, 1582. Thus ended the 1600-year
reign of the Julian calendar upon which the Gregorian
calendar is based, and thus began the calendar which
DECwindows Calendar uses to measure time. 

Calendars based on sun and moon movement were used even
by the ancients, but the first reasonably accurate one
was the $365{1\over 4}$ -day cycle calculated by the Greek
Sosigenes. This was the calendar authorized by Julius
Caesar in 46 BC. The Julian calendar (not to be
confused with the Julian period; see below) had 3 years
of 365 days each, followed by a fourth year of 366
days. 

The $365{1\over 4}$ cycle was more accurately defined in 730
AD by the Venerable Bede, an Anglo-Saxon monk, who
shortened the time by 11 minutes, 14 seconds. This
accumulates to a whole day's error every 128 years, or
a little more than 3 days every 400 years. This being
the Dark Ages, nothing was done to adjust the Calendar,
despite Roger Bacon sending a note to Pope Clement IV,
informing him of the drifting of the date for the
vernal equinox. Later, Pope Sixtus IV did become
convinced that another reform was needed and called the
German astronomer Regiomontanus to Rome to advise him.
Unfortunately, Regiomontanus died of the plague shortly
thereafter and the plans died with him. 

Thursday, October 4, 1582 was the next time the
calendar was adjusted. This last day of the Julian
calendar was followed by Friday, October 15. So began
the Gregorian calendar that we use today, named after
Pope Gregory XIII. He commissioned the mathematician
Father Christopher Clavius, S.J., to do the necessary
calculations, having been authorized to reform the
calendar by the Council of Trent in 1545. 

The Vatican librarian Aloysius Giglio provided a
formula for long-range accuracy. He suggested that
every fourth year be a leap year, except for century
years that are not divisible by 400. Thus 1700, 1800,
and 1900 would not be leap years, but 2000 would be,
because 2000 is divisible by 400. This rule eliminated
3 leap years every 4 centuries, making the calendar
sufficiently correct for most ordinary purposes. 

\newpage
\xyzzysection{Political Acceptance in Europe}

The new calendar was adopted at once by France, Spain,
Italy, Portugal, and Luxembourg. By 1584, Belgium, parts
of the Netherlands, Switzerland, and most Catholic
German states had joined, and by 1587, so had Hungary.
It was not until 1699--1700 that these countries were
joined by the rest of the Netherlands, Denmark, and the
Protestant German countries. 

By the time the British imposed the calendar on all its
possessions, in 1752, 11 days needed to be lost.
September 2, 1752, was thus decreed to be followed by
September 14. In addition, New Year's day was moved
back from March 25 to January 1. (For example, before,
March 24, 1700 had been followed by March 25, 1701).
Among other repercussions, this moved Washington's
birth date from February 11, 1731, to February 22,
1732. The following year, 1753, Sweden too adopted the
calendar. 

In 1793, the French Revolutionary government adopted a
calendar of 12 months of 30 days each, with 5 extra
days in September (6 on leap years). The Gregorian
calendar was reinstated in 1806 by Napoleon. 


\xyzzysection{Political Acceptance World Wide}

Adoption of the calendar in countries outside Europe
and its Crown possessions occurred much later, and
often in conjunction with political upheaval: Japan in
1873, Egypt in 1875, China in 1912, and Turkey in 1917. 

In 1918, Russia's revolutionary government decreed that
January 31, 1918, would be followed by February 14,
1918. 


\xyzzysection{Religious Acceptance Worldwide}

Religious institutions in general seemed to be more
reluctant to take to the new system. 

German Protestants used the old calendar until 1776,
three quarters of a century after their countries had
adopted the Gregorian system. 

Sweden retained the old Easter rules for 90 years after
switching to the Gregorian calendar, and many Middle
Eastern Christian sects still retain the Julian
calendar. 

The Russian Orthodox Church still follows the Julian
system. 


\xyzzysection{Julian Period}

Astronomers use the Julian period because it is
convenient to express long time intervals in days
rather than months, weeks and years. It was devised by
Joseph Scaliger, in 1582, who named it after his father
Julius, thus creating the confusion between the Julian
(Caesar) calendar and the Julian (Scaliger) period. 

Julian Day 1 began at 12:00 noon, January 1, 4713 BC.
This date was thought by some to correspond
approximately to the beginning of the universe.
Certainly it predated any known astronomical events
known in the 16th century without resorting to negative
times. Scaliger decided on the actual date on the
grounds that it was the most recent coincidence of
three major chronological cycles: 
\begin{itemize}
\item[$\cdot$] The 28-year solar cycle, after which dates in the
  Julian calendar (for example September 27) return to
  the same days of the week (for example Tuesday).
\item[$\cdot$] The 19-year lunar cycle, after which phases of the
  moon return to the same dates of the year.
\item[$\cdot$] The 15-year indiction cycle, used in ancient Rome for
  tax regulation. 
\end{itemize} 
It takes 7980 years to complete the cycle. Noon of
January 1, 1988, marks the beginning of Julian Day
2447161. 

The Julian period is also of interest because of its
use as a time base by the VMS operating system. 


\xyzzysection{VMS and the Julian Period}

\noindent
{\sf Or: Why VMS regards November 17, 1858, as the beginning of
time\dots} 

The modified Julian date adopted by SAO (Smithsonian
Astrophysical Observatory) for satellite tracking is
Julian Day 2400000, which turns out to be November 17,
1858. 

SAO started tracking satellites with an 8K (nonvirtual)
36-bit IBM 704 in 1957, when Sputnik went into orbit.
The Julian day was 2435839 on January 1, 1957. This is
11225377 octal, which was too big to fit into an 18-bit
field. With only 8K of memory, the 14 bits left over by
keeping the Julian date in its own 36-bit word would
have been wasted. They also needed the fraction of the
current day (for which 18 bits gave enough accuracy),
so it was decided to keep the number of days in the
left 18 bits and the fraction of a day in the right 18
bits of one word. 

Eighteen bits allows the truncated Julian day (the SAO
day) to grow as large as 262143, which from November
17, 1858, allowed for 7 centuries. Possibly, the date
could only grow as large as 131071 (using 17 bits), but
this still covers 3 centuries and leaves the
possibility of representing negative time. The 1858
date preceded the oldest star catalogue in use at SAO,
which also avoided having to use negative time in any
of the satellite tracking calculations. 

\newpage
\xyzzysection{Ultrix Time Origins}

The beginning of time for Ultrix systems is:
\begin{center}
{\tt Thursday January 1 00:00:00 1970}
\end{center}
The reason for this date being chosen is that this was
the year that UNIX, the ``father'' of Ultrix, was first
released.

Thus dates prior to 1970 are BU; 1970 and later dates
are AU.


\xyzzysection{History and DECwindows Calendar}

If you read the topics concerning the political and
religious acceptance of the Gregorian calendar, you
will see that there is a problem: there are many dates
for the conversion from the Julian to the Gregorian
calendar. For example, the British (and therefore the
Americans) converted in September 1752, so British and
American Calendar users might expect Calendar to show
September 2, 1752, to be followed by September 14,
1752. However, a Russian user would expect to see this
jump between January 31, 1918, and February 14, 1918. 

DECwindows Calendar conforms to the date of the
original decree, therefore no days have been lost since
Friday, October 15, 1582, nor since the beginning of
DECwindows Calendar time: January 1, 1600. Thus for
everyone except for users from the majority of the
Catholic European countries, which all converted before
this date, there will appear to be an ``error'' in
Calendar, where the conversion actually took place.
This generalization was felt to be acceptable for an
application not specifically designed for historians. 

\end{document}