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Brief History of the Gregorian Calendar (This
information comes from the original (pre-Motif) DECwindows help file which
accompanied VMS version 5.3. by Marios Cleovoulou http://ourworld.compuserve.com/homepages/cleovoulou/
Copyright 1988, 1989 by Digital Equipment Corporation.)
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 1/4-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 1/4-day 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.
Political Acceptance in Europe
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.
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.
Religious Acceptance Worldwide
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.
The 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:
- 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).
- The 19-year lunar cycle, after which phases of the moon
return to the same dates of the year.
- The 15-year indiction cycle, used in ancient Rome for tax
regulation.
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.
VMS and the Julian Period or:
Why VMS regards November 17, 1858,
as the beginning of time...
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.
Ultrix (Unix) Time Origins
The beginning of time for Ultrix systems is:
Thursday January 1 00:00:00 1970
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.
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.
For further questions, please contact Cliff Kettemborough, TMOD Navigation Program Element System Software Engineer and Y2K Task Manager.
If you have comments or suggestions, email me at crk@jpl.nasa.gov
* Parts of this information is courtesy of Gary Standen of LeRC.
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