Everyone on Earth observes the same face and phase of the Moon at the same time, provided the Moon is visible from the location of the observer. This fact is contrary to what the flat Earth model requires. It rules out the possibility of a flat-Earth and that the Moon is a sphere hovering a few thousand miles above Earth’s surface.
It can be surprising that many people believe that the lunar phase is caused by the Earth casting its shadow on the surface of the Moon. Flat-Earthers are no exception. But they take it one step further and use the misconception do “disprove” the fact that Earth is a sphere. They are wrong. The lunar phase is the shape of the sunlit part of the Moon seen from the observer.
On the other hand, the phenomenon where the Earth casting its shadow on the surface of the Moon is called the lunar eclipse. Moon phase occurs all the time and undergoes a monthly cycle, while a lunar eclipse happens only a couple times in a year.
At any time, the Sun illuminates one half of the Moon and the other half dark. The phase of the Moon is the shape of the sunlit portion of the Moon as viewed by observers on Earth.
The phase of the Moon can never be adequately explained using the flat Earth model. To deal with this inconsistency in their theory, some perpetrators of flat Earth invented an ‘explanation’ that the phase of the Moon is not caused by sunlight. They are wrong. We can observe the Moon very carefully using telescopes of high-zoom cameras, and it will be evident how the lunar craters are lit. And this can only happen if the Moon is sunlit.
A lunar eclipse does not happen every month because the orbit of the Moon is tilted 5.145° to Earth’s orbit around the Sun. And thus, the Earth does not always cast its shadow on the Moon during maximum opposition. Instead, a full moon occurs.
Some flat-Earthers take this as ‘evidence’ that a full moon cannot possibly happen because the geometry does not allow for a 100% illuminated Moon. They are wrong. In reality, a full moon is never 100% illuminated.
In the real world, the Moon is spherical. It doesn’t emit its own light; it gets its light from the Sun. The Moon is tidally locked, causing one side of it to be permanently turned towards the Earth. The lunar phase occurs because of the change in the position of Sun-Earth-Moon with respect to each other.
Any flat-Earth model fails to explain many Moon-related phenomena. Flat-Earthers who insist on defending their misguided flat-Earth concept at all costs are forced to invent many “explanations” on how Moon-related events might happen on a flat-Earth.
Simple observation tells us that we always see the same side of the Moon, but the phase changes in a month-long cycle. These facts rule out the majority of the “explanations” invented out of thin air by flat-Earthers, including the “Moon emits its own light”, “half-sphere Moon” and the “transparent Moon” explanations.
The phases of the Moon depends on the position of the Moon relative to the Earth and the Sun. They occur because the Moon is spherical & lit by sunlight and that the Sun is very far away. Both imply that the Earth is spherical too.
The phases of the Moon according to observation cannot be explained in any flat-Earth model. Flat-Earthers had to invent various explanations to explain how the lunar phases can happen in the flat-Earth model. None can explain how a ball has the same phase as the Moon when seen from the same direction as we see the Moon.
Apparently it is a very common misconception that the phases of the Moon is caused by the Earth’s shadow. Not only among flat-Earthers, but the general public as well. The difference is that flat-Earthers will usually jump to conclusion, and have some outlandish interpretation about this.
In reality, the phases of the Moon happen because we see a different sunlit part of the Moon at different time. Not because it is over the shadow of the Earth.