Why is the sun not always visible?

Why is it the sun is not always visible?  If it's 3,000 miles above us, and has no where to hide, why can we not always see it?

From my understanding there are 3 main factors.

1. The density of the atmosphere over large distances is thick enough to block the light.

2. Perspective give the sun an angle of 0 degrees.

3. Bendy light could also play a factor.

From my understanding there are 3 main factors.

1. The density of the atmosphere over large distances is thick enough to block the light.

2. Perspective give the sun an angle of 0 degrees.

3. Bendy light could also play a factor.

Why is it then while visible the sun is always a circle?

Not sure. I can't answer that. I'm not keen on the idea myself. There's a different theory on perspective in 'earth not a globe'.