Visibility and Characters Flying Far Enough to Disappear from Sight

[Lightbuster30]

So with an unobstructed view, we can see up to 20 kilometers right? Well I've been noticing quite a bit, characters who travel so far that they completely fade into the sky, no longer able to be seen due to how far they are. I was wondering if it would be possible to calculate, and apply a speed statistic for such characters while using this distance for them?

 

[DontTalkDT]

Maybe, though there are other limiting factors to that, if the objects are rather small.

For a video there is the limited resolution, for example.

 

[Lightbuster30]

What if the object is human sized?