https://en.wikipedia.org/wiki/Ole_R%C3%B8merWhen the danish astronomer Ole Romer observed the motions of Jupiter's moons, he noticed that they do not orbit Jupiter perfectly periodically, but that there is a correlation between the time it takes them to orbit Jupiter and the distance between Earth and Jupiter, as calculated in the heliocentric globe-model.
The conclusion was shocking: Light moves at a finite speed!
Eventually, a few years later, it was the astronomer Christiaan Huygens who calculated the speed of light. Huygens took Romer's data and put it into the geocentric globe-model, with Earth being a globe and and Earth and Jupiter both orbiting the Sun. (A formula which, of course, would be radically wrong to begin with if Earth were flat.) And yet, somehow, despite using the heliocentric globe-model, Huygens got the speed of light fairly correct: He was off by just 24%.
Not bad for the first ever measurement of something that people didn't even know exists.
So, every time a Flat-Earther talks about the speed of light: You are welcome. You wouldn't even know that there is such a thing as the speed of light if it weren't for the heliocentric globe-model.