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Comment by isolli | Hacker News Reader

In which I discovered an explanation for an illusion that always surprised me [0]: the fact that a partially illuminated moon does not seem to reliably indicate the direction of the sun.

[0] https://en.wikipedia.org/wiki/Terminator_(solar)#Lunar_termi...

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I find the description of the phenomenon and its explanation incomprehensible. Can anyone explain more clearly what it means?

EDIT: here is a better explanation: https://chrisjones.id.au/MoonIllusion/

OK, the point is that when the moon and the sun appear to be the same height in the sky the sun is actually "above" the moon, because it's further away. Therefore the moon is lit from "above".

It doesn't have anything to do with distance to the objects. Straight lines in the sky are great circles that may appear curved to us because of our biased view standing on the surface of earth. Light rays traveling from the sun to the moon take a great circle path through the sky, and the path might look to us (if we could see it) like it goes "higher" in the sky than either object, but it's a straight line across the dome of the sky.

> It doesn't have anything to do with distance to the objects

Sure it does. If the sun were closer to us than the moon it wouldn't be possible for the sun to appear lower in the sky yet appear to light the moon from above. Indeed the opposite would be true: it would appear to light the moon from below.

So the relative distances to the objects is exactly relevant to determining from which direction the moon appears to be lit.

Their relative distances affect which portion of the moon is illuminated, but not what the line between the two looks like projected against the sky. That line being a great circle is what causes the terminator illusion.

I'm not sure I follow. You said

> It doesn't have anything to do with distance to the objects

yet if the moon and the sun were the same distance from the Earth the phenomenon would not occur, so I'm having trouble understanding what your claim means. It seems the phenomenon can only arise because of the difference in distance to the objects.

Oh no, I take that back. Even if they were at the same distance you could observe this phenomenon. So my explanation in terms of distance only works in a particular case, but the great circle explanation works for all cases. Interesting, thanks!

You had me doubting myself. I'm pretty sure this would happen with a gibbous Mars or Venus that is farther away than the Sun. But we can't see those phases with our naked eye, and the effect would be less because they would be pretty close to the Sun in the sky.

I reckon if we had a gibbous Venus that appeared to be on the same level as the sun, then it would "appear" to be lit from below(ish) (if we could even see it). Does that match what you expect?

EDIT: No, I think that's not even right. This is very mind bending!

It depends on where they are along the ecliptic in the observer's sky, I think. Objects appear to rotate with respect to the ground/horizon as the cross the sky.

This stackexchange question[1] has some interesting answers. The pictures where the lady is holding a globe and the one that shows the sky as a circle (round horizon) vs a flat horizon are my favorite.

[1] https://earthscience.stackexchange.com/questions/14809/what-...

Edit: but ignore the flat-earther in the comments.

This video is also quite interesting:

https://www.youtube.com/watch?v=Y2gTSjoEExc

When you come across this illusion, you can "break" it by getting a piece of string and holding one end over the moon and one over the sun, and you'll more easily see how the moon does after all point at the sun. Lying on the ground can also help.

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