I'd initially started on a detailed proof of one aspect of moon phases, when I realized the entire cosmology around the moon is a complete mathematical mess. Rather than just hit one of the issues, I put together a graphic and will walk through a number of problems as we visualize what must happen for the FET position to be correct.
TL;DR - While this can't (by itself) disprove the FET, it exposes some serious problems with the currently accepted explanations regarding the moon. The aim is that this disproof of the current cosmology can stand without any more evidence than simply looking up at the moon. We've all seen the phases, right? (If you
really want evidence, I'd start here:
https://wiki.tfes.org/Moon)
To begin with, a quick summary of the FET cosmology:
- The sun & moon are 30-mile diameter spheres.
- They rotate around the north pole at an altitude of roughly 3000 miles, while 1) moving back and forth across the equator to make seasons and 2) "wobbling" (https://wiki.tfes.org/The_Phases_of_the_Moon) up and down to make the moon phases
- A "perspective effect" maintains their apparent size throughout the day and as they rise/set.
- Like lights in fog, eventually the light from the sun & moon gets too far to see, and they actually set.
Okay, now to the graphic I put together:

For purposes of easing the explanation, let's walk through these phases as though I'm in the northern hemisphere, and seeing the moon in the same position every night, looking east. Assuming the FET cosmology is correct:
- For this first quarter, sun and moon are at the same altitude and so I see a quarter moon. The sun would be on the other side of the earth. Nothing wrong yet.
- In a full moon, the sun needs to illuminate most of the moon (at least enough as to be imperceptible). Where is the sun at this point? Either a) it's on the ground incinerating me or b) it's still at 3000mi and the moon (basic trigonometry) is a crazy number of miles above that. My math says at least 100,000 miles (and more like 300,000) for the trig to work. (more on this below)
- With a waning moon, the moon can stay at 3000mi while the sun has to (again, basic trig) have a much higher altitude. Even a generous 60degree angle (a nice crescent moon) puts the sun over 8000 miles. I'm not sure how big the angle can be before we'd see a "new moon," but it likely puts the sun at similar altitudes of the moon when it's full. So, 100-300k miles.
- With a waxing moon, the sun has to now be positioned on the same side of the earth as me. The math doesn't allow for anything else. It's not an issue that the sun and moon are both visible, but is clearly a problem that this is the only way for it to be waxing.
Let's walk through the problems:
- The sun (or moon) needs to be a lot further away to produce the kind of angles necessary for moon phases, not just a little "wobbling" around the 3000 mile mark.
- Even if a "perspective effect" could maintain the apparent size a) the vanishing point for a 30-mile object is about 90,000 miles and b) the object that's further away (than ~3,000mi) would be at all sorts of wrong angles relative to the ground.
- We clearly see both waxing and waning moons at night, and yet the requisite position of the sun doesn't support that possibility.
- I described the view as though I'm looking east, with the sun to my left on the other side of the world. On the same night as the moon heads towards setting, I'd be looking west with the sun to my right. This means we'd get both crescents in a single night.
Again, not enough to disprove FET on its own, but demonstrates the mathematical impossibility that is the current understanding of how the sun and moon relate to each other.
QED