Are you factoring in the refraction of light caused by the atmosphere?
That is there first excuse but it doesn't hold up to infrared. The thing that they never consider is the limitation of eyesight itself. We can only see so far down a railroad rack until the parallel tracks converge. Pick up some good optics and they separate again.
These guys have all found the excellent optics on the P900 and P1000 cameras and started taking some cool pictures of things that have apparently disappeared over the horizon when in fact they are just out of eyesight range else you couldn't bring them back up. I haven't looked lately to see what they have been doing with Infrared, need to go catch up a little.