That guy who claimed the earth was concave (and we live inside) cited this phenomenon as part of his evidence.
The sky is curved: It’s spherical. And the perpendicular to the Moon’s terminator, if extended in a “straight line” (or rather, a geodesic, the closest thing there is to a straight line) on that spherical surface, does indeed meet the Sun. It just doesn’t look like a straight line to us, because we mistakenly think of the sky as being less curved than it actually is. It’s sort of like how airplane paths look curved on most flat maps, but they really are the shortest path on a globe.
This is a video about the moon terminator illusion. This is the illusion that when both the sun and the moon are visible it does not look like the bright side is facing the sun. I don’t know if it the the one Saint Cad saw.
To most people the sky does not seem to be a hemisphere, but more of a flattened bowl. In other words, the zenith seems to be closer than the horizon. This is perhaps caused, or at least exacerbated, by the fact that clouds usually really are closer when they’re at the zenith than when they’re near the horizon. This is also the only convincing explanation for the Moon Illusion that I’ve heard. And yes I know that The Master addressed this and gives some credence to the relative size theory, but that doesn’t explain why the illusion persists when the moon is over the ocean with no terrestrial objects in view.
–Mark
[quote=“gazpacho, post:23, topic:726961”]
This is a video about the moon terminator illusion. This is the illusion that when both the sun and the moon are visible it does not look like the bright side is facing the sun. I don’t know if it the the one Saint Cad saw.
[/QUOTE]Yep. I was sure it was vsauce but still couldn’t find it. Thanks.
So does the squirrel go “around” the tree?
(Oh, never mind… And, yes, I’ve seen the Cheshire Cat moon, here in southern California, including a lovely one over Disneyland once.)
Excellent explanation, thanks.
It should be noted, by the way, that sometimes the Earth’s shadow does fall on the Moon and darken part of it. But this is a lunar eclipse, and happens only about once a year, not the standard monthly phases.
And the Earth’s shadow during a lunar eclipse is always convex. That’s one of the things that Aristotle used in his argument that the Earth was a sphere, since otherwise, he reasoned, the Earth’s shadow would be different shapes during different eclipses.
–Mark