Why was JD Zero 1st January 4713?

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 last two are both man-made (particularly the latter) - but then, so are calendars....
Trying to make sense of all this is very difficult:

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:

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....

Steve Glennie-Smith   Dec 29th 2012