NavList:

Earlier, I wrote:

"I remember someone here posting an image which nicely explained why the times [of the various definitions of the equinox] are different. But I can't remember who it was."

Well ... after tediously searching through the images and files, I found it. And it was from a much older post than I thought. Apparently, it was originally posted by Herbert Prinz back in 2006 and then quoted by Bill B in 2012.

I didn't really understand this image when I first saw it. The idea that the apparent center of the Sun's disk could be any distance away from the ecliptic made no sense to me. But yesterday I read Herbert's original post, in which he wrote: "How come the Sun has latitude? Hint: The latitude of the Sun is always very roughly (give or take 0.2") that of the Moon, divided by 32000."

So, I did some searching and my "google-fu" led me to ... [drumroll] ... Wikipedia. Of course. There, I learned that the Moon in its orbit causes the Earth to "wobble" around the ecliptic. And since the celestial equator is simply a projection of the Earth's equator, it now makes sense to me. I originally thought that it might be due to nutation, but some cursory investigation didn't show any obvious correlation there.

But is that really the reason? Or at least part of it? I'm honestly not sure. At any rate, I'm glad my spreadsheet at least generally agrees with USNO's MICA on this matter. (Not that MICA is special, it's just the only other source I bothered to check.)

Cheers!

Sean C.