Flat-Earthers like to dismiss evidence of spherical Earth if it is in the form of an observation of the sky. In reality, many of the observed motions of celestial objects can only be explained if Earth is a sphere. These observations are sufficient to prove that Earth is a sphere and rule out a flat Earth.
Apparent motion is the appearance that celestial objects move due to the motion of Earth. If all the objects that appear in the sky are moving similarly, then it is far more likely that the movement is only apparent and caused by the motion of the observer.
Flat-Earthers claim that the motion of celestial objects “proves” that Earth is stationary, and the celestial objects are moving. By the same “reasoning,” we can conclude that the train we are in is stationary, it is just all the objects outside, as luck would have it, are moving in a very similar fashion.