The Moon-tilt illusion

 I think we are all familiar with the illusion of the moon looking much larger than its true angular size when it is near the horizon, but there's a less familiar moon-related illusion that I only really noticed for myself when I was out running a few years ago. It turns out to be well-documented, and well-understood, but no less weird, because even when you "know" the explanation, your brain still won't readily accept it - or at least, mine won't.

Anyway, today was an ideal day to observe the moon-tilt illusion. It was a crisp clear day and the half-illuminated moon was beautifully clear in the eastern sky well into the afternoon. The Sun was also putting on a jolly good show, roughly in the south at the time I made this short video:

The moon is tiny but visible in this clip taken on my phone. It's in the middle and about two thirds of the way up when the video starts, to the left of the tall bare tree. The view then pans around to the Sun. The illumination angle of the moon, dictated by the moon's apparent tilt, strongly suggests that the illuminating source must be way off to the right and higher in the sky than the apparent elevation of the moon:

But that's very much not where the Sun is! The explanation for the moon-tilt illusion is rather involved, but in essence, it arises from the failure of our brains to possess an innate grasp of spherical coordinate systems. Presumably this is because there would have been no survival benefit in such an understanding, and therefore no selection pressure driving our minds to get better at it. You would think that knowing this would make the moon-tilt illusion evaporate but it remains as stubborn to me as when I first encountered it - just as my brain "insists" that the moon is bigger when it's close to the horizon.

You can read more about the moon-tilt illusion here:

https://www.seas.upenn.edu/~amyers/MoonPaperOnline.pdf

Hoping this is of interest.

Al