NavList:

I wrote:  However, I was able to confirm something that’s been worrying me for a while.  We all know that light travels in straight lines, and we read that because of this the perpendicular bisector of the Moon’s horns points towards the Sun.  Except it doesn’t; it tweaks up a touch towards my zenith.  So, I ask myself, should I try and imagine a great circle drawn in the sky from the Moon to the Sun instead; then the perpendicular bisector will point towards the Sun.  In fact, might the same be true for all these imaginary lines in the sky published to help star recognition.  DaveP

After some thought, I don’t think this has anything to do with great circles and rhumb lines.  I think it’s more to do with the fact that neither the Sun nor the Moon is on the celestial sphere.  Celestially speaking, the Sun is considerably closer to the Earth, and the Moon is very close to the Earth.  I’ve started looking at how there’s a viewing plane difference between the Sun, the Moon, and the centre of the Earth and the Sun, the Moon and Lincolnshire.  At the moment, using declinations from 15.30 UTC 18th February 2021 (11.24’S & 14.50’N) I’m making the horns twist down from Lincolnshire (53N) when I’d like to be able to show they twist up.  Incidentally, if you blow up the Moon sufficiently for an observer at 53N on ‘Stellarium’ with ‘Azimuth Grid’ as a background, you can also see this tweak-up.  Usual caveat, I'm just thinking aloud, I could be completely wrong. DaveP