[HorstFue]

There's another ongoing thread for Full Moon Impossible on Flat Earth?.
I think I better do my explanation in a separate thread, avoiding to disturb the discussion about the "Math.".

A question came up or more, it was refuted, that this is possible on RE, with the sun already beyond the horizon.

s.a. Moon Tilt Illision

First try to explain it:

• Observers view of the world is geocentric ("observer centric"). So assume a moon orbiting the earth, with the earth and thus the observer in the center of the orbit, the moon lit from a far away sun, also orbiting the earth some way far out. All, observer, moon's orbit and sun are in the same plane (approximately). Sun rays will reach all points of moon's orbit parallel (nearly).
  And: we put a small pointer on the lit side of the moon pointing to the sun
• Now lets start with a half moon: The rays from the sun will reach this half moon so, that they are tangential to moon's orbit at that point and perpendicular to observers viewing line to the moon. So that pointer mentioned before, will not only point to the sun, but tangential to moon's orbit, indicating the path where the moon will go next (trajectory?). And it will do so during the whole orbit.
• Now a waxing gibbous moon: Sun rays will now reach the moon only after crossing moon's orbit and come from "inside" the orbit ("last miles"). Compared to the half moon the "pointer" will be 'tilted' somewhat to the center of the orbit, but it will still be in the same plane as sun, moon's orbit and observer.
• Now as observer, moon's orbit and the pointer are in the same plane, the observer cannot detect that tilt of the pointer. He will only see the projection of the pointer onto moon's orbit. So again same as with the half moon, for the observer this pointer indicates the movement (trajectory) of the moon.
  As after moon rise the moon is going up for his course across the night sky, also this pointer appears to point up, away from the horizon.

Second Try:

• Lets assume - could be, I see nothing that could exclude this generally, in any case - sun and moon are in such positions, that a light ray from the sun goes nearly straight above observers head to the moon. And I claim this is also working when the light ray does not go directly over the head of the observer, but in some distance.
• This reminds me of something analog ... "going in a straight line across the sky high above the observer". This was discussed in EnaG and many threads in this forum: Sun's apparent path from rise to set. You could also use a big aircraft, there are also numerous examples on this forum
• Let's follow the path of the ray in reverse direction, from the moon to the sun: This ray appears - as the path of the sun/airplane - to go up from the horizon or the moon (still rather close to the horizon) in an steep angle until it reaches the point above the observer, than it will appear to go down with a similar steep angle to the opposite horizon.

This all is caused by the observer, who changes his orientation, when he follows the "course" of this ray. He is turning around facing both, opposite horizons. A direct straight line can only be viewed if you do it in a "one shot", with both moon and sun within the field of vision, if orientation of observer does not change.
You could try with a fish eye lens, but end up with the horizon appearing to be bended dramatically.

[ICanScienceThat]

I would argue that the simple way is to hold up a ball in front of the moon and compare. As long as both the sun and moon are up together, this should prove beyond a shadow of a doubt that the moon is lit by the same sun as the ball.

"What if the sun is down?" Just do this while the sun is up. Do it several times during the day. Make a little chart of your results. What is the tilt angle of the moon at 5pm? 6pm? 7pm? Now that the sun has set at 8pm, just extend your chart.

And we should be done here. That should be all it takes.

[Bobby Shafto]

Through all the discussion so far about the apparent "moon tilt," I don't think it's been mentioned that if it's not an illusion and the sun really ISN'T responsible for the moon casting light, then that's not a flat earth vs. round earth issue because if it's a problem for "round earth theory" to explain, the it is also for "flat earth theory."

The phenomenon was raised (in the Full Moon Impossible on Flat Earth topic) in conjunction with the hypothesis that maybe the moon produces it's own ligh and doesn't reflect the light of the sun. That's a "maybe" that's independent of whether or not the earth is round or flat. Conventional round earth doesn't turn to that moon self-luminescence "maybe" since there's nothing compelling to do so. The explanations for moon phases and lunar eclipses being arrangements of spherical sun/earth/moon are sufficient. Flat earth theory has some questions still tackling the phenomenon of moon light and so the moon emanating it's own light is a "maybe" that some propose as a solution.

Maybe the moon is self-luminating.
Maybe the moon is being lit by focused light from the shadow object.
Maybe the moon is a hologram.
Maybe the moon surface is reactive and self-luminescent but in response to some unknown stimulus.

Maybe...

You can "maybe" your way to almost anything. But why? Because the moon light of a waxing half moon just after the sun has set doesn't appear to be pointing toward the set sun?  Because its geometrically impossible to observe anything more than a 99.99% fully illuminated sun from the surface of round earth. (Less if on a flat earth?). Are those the kind of fabricated problems that drive you to "maybe" theories and resist the simple answers?