There are a number of explanations on the Internet, the most plausible
being this and
this, which both say that date was the last co-incidence
of the starts of the following three cycles:
- The Metonic Cycle of 19 is a quirk of two totally unrelated
physical phenomena: the sidereal lunar month, which is the average time
the moon takes to go through a complete cycle of phases, reckoned to have
averaged 29.53058868 days over several centuries; and the solar year,
reckoned to have averaged 365.242190 days. These come together as
frequently as every 235 lunar months: 6939.68834 days which, taking into
account the true length of a solar year, gives 19.0002375 years. The
Metonic Cycle always starts with a New Moon.
- The Solar cycle of 28 is the number of years that dates and days of
the week repeat themselves in the Julian Calendar. The Solar Cycle
always starts with a leap-year whose 1st January is a Monday (curiously,
not Sunday). This proposal was fine in the days of Joseph Justice
Scaliger (who first proposed the Julian Date system in 1583 - the
first complete year of the Gregorian calendar), but now we are well into in
the Gregorian age, this cycle of 28 * 365.25 = 10227 days no longer starts on
1st January, but a bit later in the appropriate leap-year to account for days
omitted by the Gregorian correction. We are currently in the 240th Solar
cycle, which started on Monday 14th January 2008.
- The Indiction cycle of 15 is the number of years in the Roman tax
cycle. It had ramifications throughout Catholic Europe until
Napoléon had the sense to abolish it in 1806 - ’Nuff said!
The last two are both man-made (particularly the latter)
- but then, so are calendars....
Trying to make sense of all this is very difficult:
- If one projects back from the new moon of
1999-Aug-11 CE (the date of the last total solar eclipse
visible from much of Europe), either 83012 or 83013 moons might have passed
since JD zero – these numbers give either slightly more or slightly less
than 29.53058868 days for a lunar month.
- The lunar month changes sinusoidally by plus or minus 6 hours throughout
the year owing to the sun’s gravitational pull (the shortest is at the
June solstice).
- According to moon cycle data throughout the 20th Century, there were 1249
cycles between the new moons of
1900-Jan-01 13:52 and
2000-Dec-25 17:22, which gives a somewhat shorter
average lunar month of 29.5301408 days during the 20th century.
- Add to this, although the average length of a lunar month over the last
two years of the 20th century was only 29.5110833 days, it is increasing
overall because the moon is gradually moving away from the earth.
Who knows? – but the very comprehensive
Celestia
program reckons there was an annular eclipse of the sun at JD -0.343426
(03:45:28 UTC on JDzero)! This would have been visible from 48.8 degrees S and
180 degrees longitude, ie. where is now the South Pacific.
The ‘Julian Cycle’, which is considered to be the product of
19, 28 and 15 (ie. 7980 years) will not end on
3267-Dec-31 CE – as is stated in many places
because of two things:
- There will then be 22 ‘lost days’ introduced by the Gregorian
system that need to be accounted for;
- The Metonic cycle is not exactly 19 years (nor is there any reason
why it should be). The fraction 0.0002375 is 2 hours, 4 minutes and 53.44
seconds, which will add up to 36 days, 10 hours and 14 minutes over 7980
years. We can take out one moon cycle of 29.53 days to get nearer....
(But this still leaves 7 days unaccounted for.)
Thus, the true start date of the next Julian Cycle would work out as being 29
days further into the year 3268 (assuming the average lunar month
remains 29.53058868 days), ie. 3268-Jan-30:
still a Monday!
However, Celestia reckons there will be a new moon 6 days later, on
3268-Feb-05: a Sunday. Oh well....