07/09/2015.
What do you get if you take all of the northern moonrises around the 18.6-year maximum, between the years 10 and 400, and then align them with the calendar?
I see a fall festival.
(But there is an obvious question to ask.)
There is nothing special about the colors; each color just shows a different Major Standstill (around the years 14, 33, 51, 70, 88, 107, 126, 145, 163, 182, 200, 219, 237, 256, 275, 293, 312, 331, 349, 368, 386).
For instance, here's the graph showing the approach to (and retreat from) the Major Standstill in the year 14.
The original graph just overlays a bunch of these.
Here's the question: If it is an 18.6 year cycle, why is the northernmost moonrise always near September?
I think this is a significant discovery. The northernmost moonrise is always near September. That makes it a fall occurrence, a fall festival. Always. Something that can be counted on. By the way, the northernmost minimum moonrise does the same thing. It happens again this September.
And here's the answer to the question. The 18.6 year cycle is so flat near the standstill (which is why it's called a standstill!) that any other cycles on top of it stand out. So we see that cycle that maxes near September which washes out the actual peak of the 18.6 year cycle. I suspect (but and not sure) it's related to the eccentricity of the Earth's orbit.
[Note added 08/26/2025: This suspicion was wrong. However, this was my guess 10 years ago. I now know a lot more. Answer: it's a 177.84 day nutation (wobble) in the Earth's rotation.]
Also note that occasionally the max can (fairly rarely) happen around March. But then that moonrise is always around noon, and not particularly visible. The Hopewell may not have even noticed it, and would still have a fall viewing.
[Note added 08/26/2025: not "fairly rarely" but about half the time. But I was spot on when I said it was a noon moonrise and not particularly visible.]
[Here's the original Facebook post, which wasn't an organized essay but more of a splat onto Brad's Facebook page, followed by explaining what I meant.]
