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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

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.

tome 4y ago
> 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.
- jayknight 4y ago
  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.
  - tome 4y ago
    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.
    - tome 4y ago
      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!
      - jayknight 4y ago
        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.
        tome 4y ago
        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!
        jayknight 4y ago
        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-is-this-sun-and-moon-photographic-anomaly https://earthscience.stackexchange.com/questions/14809/what-... Edit: but ignore the flat-earther in the comments.

tome 4y ago

> 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.

jayknight 4y ago
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.
- tome 4y ago
  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.
  - tome 4y ago
    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!
    - jayknight 4y ago
      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.
      - tome 4y ago
        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!
        jayknight 4y ago
        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-is-this-sun-and-moon-photographic-anomaly https://earthscience.stackexchange.com/questions/14809/what-... Edit: but ignore the flat-earther in the comments.