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.
On the terminator lines, we can observe how the lunar craters are lit. The rim of the craters facing away from the light source is dark. And the other side —facing the light source— is lit. All the observation indicates that the Moon is lit by an external light source, which can only be the Sun, and that the Moon is spherical.
This simple observation can quickly rule out many of the ‘theories’ invented by flat-Earthers to explain away the inability of the flat-Earth model to explain the phase of the Moon. Some of which are that the Moon emits light of its own and that the Moon is not a sphere.
- Lunar phase – Wikipedia