If you replaced the Earth (except the atmosphere) with air and kept the pressure linear to the center (of the now non-existing Earth) while ignoring the effects this would have on gravity and leaving the solar system alone:
- How would it look from the outside (with and without the sun in view)?
- ...and from the inside (at multiple distances from the center)?
I'm guessing that based on Rayleigh scattering that it would look redder the more air is between you and the vaccum in the sun direction. No idea how it looks if the sun is perpendicular or at your back.