If the celestials were physical then the altitude angle to them would change with the observer’s height above sea level.

Flat Earther ignorance knows no bounds and is often wilful. To them, stars and planets are just “lights in the sky”. Any suggestion that a great deal is known about celestial objects is met with denial; because we cannot touch them, they believe we can know nothing about them. Astronomy, cosmology and the exploration of space are all a big lie to them.

From one well known (in the small community that is flat earth) particularly ignorant commenter. You know who you are.

Sound is an electromagnetic wave.