[Dynam0]

With regards to your "Idea 3", this is a decent enough explanation I dug up:

Have you ever looked at your shadow when it is bright and sunny? Move your hand further away from the ground... what will happen is that the shadow will look blurry. Why? As the above stated, diffraction comes into effect BUT more importantly, the smaller the object, the smaller the shadow. The smaller the shadow, the less diffraction is needed to hide or remove the shadow. So, simply, in a pinch, you could say that Air molecules are too small and light doesn't tend to hit them. Also note that light reflects off of objects too! So that means that light hits a single spot from multiple angles. That is why you can see the ground even though there is a shadow.

Reference https://www.physicsforums.com/thread...hadows.761571/


The atmosphere thus has no real effect on shadows, however it does play a role in a number of optical phenomena (it's why the sky is blue FYI):
https://en.wikipedia.org/wiki/Atmosp...ial_refraction