Sometimes when I look at the Moon and the Sun, the Moon's crescent isn't properly aligned with the Sun. Why is this?

My guess is that it's an optical illusion caused by the fact that to follow the rays of the SUn in 3-d space to show that they strike perpendicularly to the horns of the Moon, we are trying to see this on a 2-d screen formed by the sky because the brain has no 'distance' information with which to add the third dimension to resolve the seeming paradox.

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