3 ms·

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

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

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.