4713 B.C. and All That
Rod Brown
From the 1993 Astronomical Society of Victoria Yearbook, Page 3, the
budding astronomical tyro will read in part:
" Julian Date. To facilitate chronological reckoning, astronomical
days, beginning at Greenwich noon, are numbered consecutively with each
day in this continuous count being assigned its Julian Day Number. The
series began with 0 for the day commencing at 12hours UT on B.C. 4713
January 1."
Norton's amplifies the above remarks by defining the Julian Period as that
"used to calculate the exact interval between dates at long intervals apart.
It started on Jan. 1, 4713 B.C. at noon. The Julian Date is the number of
days that have elapsed since the beginning of the Julian Period."
Well all this seems fine and straightforward but why 4713 B.C.? What's so
special about that particular year and who introduced the idea?
Before answering that some information on calendars is required.
The concept of the calendar is very old having been around since people
started organising themselves into social groups. Calendars have been devised
as convenient means of grouping days to help regulate civil life and religious
observances and to assist in historical and scientific studies. The basic unit
of the calendar is the day which has undergone progressive refinement in its
definition. Grouping of the days was a natural development and such groups
were related mainly to the apparent lunar and solar cycles. It was recognised
quite early that observations of celestial bodies such as the Moon and the
Sun could provide a basis for determining the periods of a workable calendar.
Although the seasons could be linked to the Sun's apparent annual motion or
tropical year by observation, the lunar or synodic month used particularly
for religious observations and of twenty nine and a half days was found to
be incompatible with a year of 365 days. The tropical year was that interval
between successive passages of the vernal equinox and methods for determining
such an event were well established about three to four thousand years ago.
The period of twelve synodic or lunar months is shorter than the tropical
year by about eleven days. Furthermore, neither the tropical year or synodic
month occupy a complete number of days. To overcome this difficulty, the
device of intercalation or insertion of days was used to bring the calendar
into line with the seasons when necessary. Some early observers soon
recognised
the existence of cycles where with the passage of time the calendar and
seasons regularly went into and out of synchronisation. Such cycles offered
the possibility of linking the lunar and solar cycles in an organised way.
One of the earliest cycles proposed was the octaetinis period consisting of
8 years of 365 days equal to 2920 days. This period is close to ninety nine
lunar months equal to 2920.5 days. Another cycle was the saros or eclipse
cycle of 18 year 11.3 +/- 1 days. In 432 B.C. the Metonic cycle of 19 years
consisting of 12 years of 12 lunar months and 7 years of 13 lunar months was
proposed. In all 235 months consisting of 11 months of 29 days and 125 months
of 30 days equate to 6940 days. While this period resulted in 5 days
difference in 19 tropical years, the average year length turns out to be
365.25 days being much closer to the known tropic year period. The Callipic
period of four Metonic periods or 940 lunar months was proposed subsequently.
This cycle consisted of 441 months of 29 days and 499 months of 30 days
equating to 27,759 days. The yearly average became 365.25 days exactly. The
Callipic cycle then fitted 940 lunar months to 76 tropical years of 365.25
days. Despite the refinements, discrepancies continued to occur as the
centuries passed. Accumulating observations enabled Hipparchus to discover
the precession of the equinoxes amounting to 2deg per 150 years. Discovery
of this effect enabled refinement of the length of the tropical year to
365.242 days. With the passage of time various calendars evolved and
stabilised with the Julian Calendar being one in use by the 16th century
and kept in step with the seasons by intercalation of days.
Calendar dating of historical events and the determination of how many days
have elapsed since some astronomical or other event are both difficult tasks
for a number of reasons. In tracking backwards or forwards in time account
must be taken of such things as leap years, month length alteration and the
addition of new months.
To overcome these problems, the 16th century French classical and literary
scholar Joseph Justus Scaliger (1540-1609) made important contributions to
the science of chronology. Born in Agen, France on 5th August 1540, Scaliger
was the son of Julius Caesar Scaliger (1484-1558), an Italian physicist and
philosopher who had emigrated to Agen in 1525. Scaliger's early education
was obtained in Bordeaux and in 1555 he travelled to Paris to study Greek
and Latin. To round things out he began teaching himself Hebrew, Arabic,
Syrian, Persian and modern languages. After converting to Protestantism in
1562 Scaliger toured French and German universities and visited Italy to
study its ancient relics. The mid-16th century saw bitter religious strife
in France and following the St Bartholomew's Day Massacre in August, 1575,
and to avoid the persecution being meted out to Protestants, Scaliger went
to Geneva and taught at an academy there, returning to France in 1574.
Eventually he was called to the University of Leiden, Holland in 1593 and
came to be regarded as one of the most erudite scholars of his time.
Scaliger died there in exile on 21 January 1609.
Scaliger's greatest work "Opus de emendiatane tempore" or "Study on the
improvement of Time" was published in 1583. In this masterpiece he discussed
and compared previous calendars, correcting errors and bringing order into
the chronological system. To help in this respect, Scaliger devised a
consecutive numbering system based on a cyclic period of great length which
he named the Julian Period in honour of his father. The Julian Period was
taken to be a cycle of 7980 years and based on the product of the Metonic
cycle of 19 years, a solar cycle of 28 years and an indiction cycle of 15
years. The Metonic cycle has been described earlier. The solar cycle is the
period after which the days of the seven day week repeat on the same dates.
If no leap year is required, a year of 52 weeks of 7 days each plus 1 day
repeats every 7 years. With the Julian Calendar leap year cycle of 4 years,
the days of the week repeat on the same dates after 4 x 7 = 28 years. The
indiction Cycle is a fiscal cycle of 15 years related to Egyptian tax
receipts and has no astronomical connection.
With the 7980 year period, Scaliger believed he had a period of sufficient
length to encompass most historical and future events. Tracing the Metonic,
solar and indiction cycles backwards in time, Scaliger found that all three
coincided in the year 4713 B.C. when using the Julian Calendar. On the basis
of information available to him, Scaliger believed that date to be before
any historical events and set the beginning of the first Julian Period to
January 1, 4713 B.C. Although the Julian Calendar has been supplanted by
the Gregorian Calendar, Scalagi's legacy of the Julian Period and associated
Julian Date is still in use attesting to its value particularly in
astronomical work.
There you have it. Paraphrased from Encyclopedia Brittanicae
Rod Brown
* Origin: Anti-Matter, The sky's the limit! (03) 752-1171 (3:635/560)
Aaron Schuman
Excellent research! Thanks for posting.
> Although the seasons could be linked to the Sun's apparent
> annual motion or tropical year by observation, the lunar or
> synodic month used particularly for religious observations
> and of twenty nine and a half days was found to be incompatible
> with a year of 365 days.
The sun's motion is linked to seasons, and the importance
of seasons to agricultural people and to nomads can't be beat.
But the moon's phases are also very important to non-urban
people. It's a very practical importance, not just a
religious importance. Farmers need extra time for harvest,
and they can work late when there's a full moon. Hunters
find game under the light of the full moon that they can't
find in dark night. Fishers and shellfishers have known
about the relationship of the moon's phases with tides
since ancient times.
Have you ever studied the Mayan calendar? In a tropical
land like the Yucatan, seasons don't matter much. Their
calendar is centered on the cycles of Venus! Does Venus
have any terrestrial effects? I have pondered this long
and hard, and I can't think of any terrestrial phenomena
that are truly influenced by Venus. Theirs was one
calendar that was primarily of religious significance.
Aaron
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Aaron Schuman 415-390-1901 sch...@sgi.com Silicon Graphics, Inc.