# The view of the sky from an Inverted/Hollow Earth?

Obviously there are plenty of potential physics problems with this scenario, but if people lived along the inner crust of the Earth, and there was a Sun-like core at the center with nothing else in-between other than sky, what would I (as a person living on the inner crust's surface) see when I look up at the "sky"?

Would I still see a blue sky with just a much larger and maybe dimmer Sun-like core in the middle and maybe see just a bit farther down the crust, fading into blue?

Or would I be able to completely see all sides of the inner-crust's surface at all times?

This would be assuming that there was a decent amount of water on the inner crust as well, so the majority of the inner crust is covered by ocean much like Earth's outer crust is.

• The first issue is that a hollow shell has no gravity on the inside. Everything would be sucked up into the little sun (also not possible) and burned to a crisp. Commented Feb 18, 2015 at 0:32
• @Oldcat, The central sun, or indeed any internal mass, would cause there to be an internal gravity toward the centre of the body. The interior of a hollow shell is zero-g only if it contains a vacuum. The mass of the shell effectively cancels itself out gravitationally. Commented Feb 18, 2015 at 0:52
• Are we to assume a full atmosphere inside the planet? Commented Feb 18, 2015 at 1:06
• @MontyWild are you sure about that? Gravity is mass pulling on other mass. So standing on the inside of the crust you have a lot of mass above you pulling you toward the center. I expect it would be about as easy to stand on the roof of a cave. It's not just the mass at the center pulling you it is also the mass of the crust at the other side as well. Commented Feb 18, 2015 at 1:16
• @bowlturner, I'm positive. I know, it seems counter-intuitive, but the gravitic force from the nearby mass is countered by the gravitic force from all the more distant mass: en.wikipedia.org/wiki/Shell_theorem Commented Feb 18, 2015 at 1:28

The first thing we have to think about is atmospheric extinction: that is, how much the atmosphere absorbs and scatters light. Let's look at some cases on Earth to get an idea of how strong this effect is.

1. The sky straight overhead. Even accounting for seeing (atmospheric turbulence) we have an almost totally unimpaired view of the stars at night through the couple of km of air overhead. Satellites also have no trouble looking back down at the Earth. From this perspective the atmosphere is optically thin.

2. The horizon. Looking at the ocean you can barely see a slight curvature. More importantly, objects passing over the horizon (ships and distant land) appear distorted from atmospheric refection and washed-out from scattering. However, although the Sun is dimmed enough to become viewable by the naked eye at sunset, it is still clearly visible. The atmosphere is not optically thick when viewed edge-on.

However, if the Sun (which is extremely bright) is dimmed so much by a couple hundred km of atmosphere, then a couple thousand km would probably be optically thick.

Now we can compare these cases to the situation for the inverted planet. We actually have two cases, one where the atmosphere stretches all the way to the sun, and one where the atmosphere is as thick as normal.

1. Full atmosphere. Objects on the order of 100 km away would look similar. However, as objects recede further into the distance, they would become fainter and fainter, and start moving upward, probably becoming invisible once they were a few degrees above the horizon horizontal. Straight overhead, we would always see a white sky, brighter towards the zenith (straight up). The atmosphere would be too thick to see the sun through, so we would be seeing scattered sunlight. If the sun somehow became dark at 'night,' the sky would be uniformly bright from the airglow of nearby artificial lights, or perhaps the reflected and scattered sunlight from the other side.

2. Thin atmosphere. As before, objects grow fainter as they recede into the distance. However, less than a degree above horizontal, the view starts to become clearer again as your line of sight passes through less and less atmosphere. A few degrees above horizontal, the view is almost undistorted again, and you can see the bowl of the Earth rising up, curving overhead. You would be able to see the other side of the Earth high in the sky, albeit with some amount of desaturation due to the scattered light of the normal blue sky being added to the view (but not twice; since you're not in the atmosphere on the other side, you don't see it's scattered light). If the Sun goes totally dark at night, you would see everything dim uniformly, but not redden since the sun is not setting. If the Sun only shines on half the Earth at a time, then you'd be left with a bright-as-day night sky, with a soft, diffuse illumination equivalent to indoor lighting. The day would look more normal, with the overhead sky being totally blue at noon. At sunon and sunoff you would see half the sky illuminated by earthglow, and you would be able to watch the terminator move across the sky.

This is so cool, I think this is going to be the setting for my next story, even though it's impossible.

• "since you're not in the atmosphere on the other side, you don't see it's scattered light" You don't? Then why does earth look blue from space? Commented Feb 18, 2015 at 7:16
• I think the exotic matter with negative mass in the center is the only way this works. Maybe negative mass matter attracts other negative mass but repels normal mass. That way the sun can hold itself together but also push people out onto the surface of the sphere. Commented Feb 18, 2015 at 9:09
• That could be a whole question by itself though :) Commented Feb 18, 2015 at 9:09
• @celtschk The Earth looks blue because of all the water! Satellite images of Earth have dark shadows, if they were seeing the scattered light they would have sky blue shadows (like we see the moon). Commented Feb 18, 2015 at 11:42

I would guess that if by some miracle you could stand on the inside of such a construct without falling into the center, then what you would see would depend on what is in there. If it is filled with air then things distant will become hazy and eventually fade into the sky. if the atmosphere hangs around the inside of the crust away from the center then things would slowly haze out to the horizon but become clear again closer to straight up as the angle of refraction changes through the atmosphere.

EDT2:As long as you have a mass in the center (producing light) then...

But what you would really see when you look up into the ball of light in the middle is that it is getting closer and closer, faster and faster as you fall into it...

## Not much different

I'll view this from the standpoint that 'magic' or whatever allows for this scenario:

Your horizons will begin to arch up very, very far away. Likely, I'd expect you would not even be able to see much of the following curved walls, unless a couple sparkly reflections from the 'sun'light. Likely, the 'sun' would block out anything that you would see when you look up.

Keep in mind that the sun completely blocks out very shiny things that are behind it, like stars that would appear near it if it went dark. Dust, water, and air pollution would probably let you see things on Earth about 20km away (I'm estimating by looking out my apartment window right now - 63rd floor), so unless something was very bright, I don't think you'd see much of the curved walls and ceiling of the Earth.

Your sky would be blue still, because the same properties are at work.