The orbit of the Earth and that of the Moon are not perfectly aligned in the same plane. The orbital plane of the Moon is inclined 5.145° from the Earth’s orbital plane. This is the reason an eclipse does not happen every month.
Sometimes we see flat-Earthers claim that an eclipse should happen every month. But it does not, and they take that as ‘evidence’ of the failure of modern science to explain the occurrence of an eclipse. Their mistake is that they don’t account for the orbital inclination.
Most diagrams describing the geometry of the Sun-Earth-Moon system are usually drawn seen from the ‘top’, but rarely from the ‘side’.
Because of the Moon’s orbital inclination, during a full moon or a new moon, the Sun, Earth, and Moon are not always forming a straight line, and an eclipse does not occur every month. An eclipse can only happen when the position of the Moon is at the orbital node, or when the Moon crosses the plane of Earth’s orbit.
This misconception is fairly common, and not specific to flat-Earthers. The difference is that flat-Earthers usually take their ignorance as ‘evidence’ of a conspiracy, that they are a victim of a grand scheme or something like that. Even though the simple question of “why does an eclipse not occur every month?” can be asked politely, and it is really not hard to get the answer if anyone wants to know.
- Orbit of the Moon – Wikipedia