I see your point. Why can't you just see the Sun and every other celestrial object 24/7 with a big enough telescope...
The answer is there's a few things which may explain this... Perspective, or the natural tendancy for distant objects to appear lower to a viewer, atmospheric interference - like fog except over 1000's of miles, or refraction which is the bending of light or images off the atmosphere... The below link offer's an example and other ideas/objections to consider.
See "Comprehensive explanation of sunsets" https://forum.tfes.org/index.php?topic=19650.0
Just some potential issues with the explanations:
-
Perspective: The problem with perspective is that objects. , as they gain distance from the observer, decrease visually decrease in size. The Sun does not as it's making its way across the sky toward setting at night time. It does not visually decrease in size.
-
Fog: For fog to be a reason why the Sun disappears, this fog phenomenon would have to be present everywhere on earth, for every viewer, everyday and timed perfectly to expected day and night.
-
Refraction: Refraction is variable. Can be light, can be strong, can be non-existent. Depends on the environment. Again, we bump up against this refraction phenomenon would have to be present everywhere on earth, for every viewer, everyday and timed perfectly to expected day and night.