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Does the position of the sun in a hollow earth scenario affect the way light scattering would "color" the sky?

My understanding is the reddish-orange color during sunrise/sunset is caused by the sun being at a more oblique angle in contrast to the standard blue when the sun is fully up.

My intuition states that the sky's color wouldn't change much, or if it did would become a washed out version of whatever it normally would be, ie blue on Earth. The most dramatic coloring that I could imagine would be a gradient from say blue to red as you look from the center of the sky to the horizon, given an Earth colored sun and atmosphere. I doubt the gradient scenario is possible, but it would be neat if it was.

Of course a true hollow earth situation isn't possible. I'm mostly interested in how light scattering works when the light source is placed in the same setup.

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Is the sun a star? (albeit an extraordinary one) Or is it a giant light bulb/magic light orb? – Scott Downey Jan 7 at 16:06
@ScottDowney I don't know.... Does that matter from a light scattering point of view? I'm interested in knowing how the color of the sky is affected by a static central light source vs a normal solar system scenario. I'm picturing in my mind kind of a Journey to the Center of the Earth or classic hollow Earth setup, but if that isn't appropriate then I'm happy to change my mental image to suit. – Erik Jan 7 at 16:12
It might matter - aurora from the solar wind might tint the visible light slightly but green sky is also an interesting answer... – Scott Downey Jan 7 at 16:14
up vote 6 down vote accepted

I wonder if you would even have a "sky", or if you'd just see the other side of the world through a haze?
We can see the moon in the daytime if it's in the right phase after all.
The entire surface of the interior would be lit equally, so if you looked up you'd probably see the shapes of oceans and rivers, fields, and anything else "large enough to be seen from space", depending on how large the interior of your world is of course.

There wouldn't be a horizon, as the horizon is caused by the Earth curving away from us.
Instead, if you looked out, you'd see the ground gradually slope up, like a hill that just keeps going and going.
If there is enough atmosphere and distance for light scattering to make something that looks like sky it would be a constant color (pale blue?) directly above you all the time, like it was noon, and as you looked down it would slowly change to more and more visible geography.

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I'm going with Green. Or Cyan.

Here's the justification, solar wind from the central star will cause constant 'global' aurora which, although dim as seen in daylight, will constitute a large fraction of the atmosphere that is seen when looking any direction other than 'straight up'. Aurora tend to be greeny blue.

There will also be a lot of light reflected from the ground itself inside the sphere, if the interior surface is anything like our Earth. The 'pale blue dot' appears as mostly blue with lots of green and some yellows/browns thrown in.

I wouldn't expect to make out many details on the opposite side of the sphere as the sun would block most of that plus the abundant water vapour in the atmosphere will also reduce visibility.

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Ooh! I like the aurora concept. Not sure that it's possible though, because you can't have a globally strong magnetic field to cause it. (Unless magic.. Oh. Wait.) – Joe Bloggs Jan 7 at 16:43
@JoeBloggs The Earth's magnetic field only concentrates the solar wind at the poles, it isn't required for aurora, that's just the wind interacting with the atmosphere. Inside the hollow planet the wind has nowhere else to go. – Scott Downey Jan 7 at 16:44
That's sort of my point. If we take the particle concentration in the patches where aurora are seen and spread it out over the whole surface, is the aurora going to occur? Bearing in mind that the solar wind must be (by comparison) the same as that hitting Earth. I sense maths is imminent.. – Joe Bloggs Jan 7 at 16:48
Would the light from the sun obscure the affect of the aurora without a magnetic field to pull the solar wind away from the sunlight? – Erik Jan 7 at 17:03
@ToddWilcox It takes a surprisingly small layer of air to block most radiation. Alpha and beta rays are stopped by a foot of air or less. Most of what makes the solar wind are these charged particles or plasma, which would also have it's energy absorbed by atmosphere really fast. – AndyD273 Jan 7 at 20:57

Interesting question.

Assuming in our Hollow Earth that the atmosphere is the same composition and thickness as the atmosphere on Earth , the gap between the atmosphere and the Internal Sun is vacuum, and that the Internal Sun has the same relative brightness and isn't superheating the atmosphere to a plasma:

The other side of the world.

The sun is overhead. So is the rest of the world. Unlike on earth where there is nothing above us but the light scattered off the air, in a hollow world there is also all the light scattered off the other side of the world. The angle of the Sun relative to other parts of the atmosphere turns out not to matter as the light scattered from that bit of the atmosphere also has to scatter off your bit of atmosphere, resulting in a net of very little atmospheric effect when compared to all the light bouncing from the rest of the world.

The more interesting question is where does all the heat go?

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That's what I thought too... Maybe it's just a very very small sun? Then excess heat would be captured by the ground and radiated out into space. Like a lightbulb in a chicken coop. If it is an artificial world you might even want to turn the sun off once in a while... – AndyD273 Jan 7 at 16:12
The same amount of heat that makes it to space also gets radiated back into the centre of the world. The sun would have to be very dim indeed to achieve thermal equilibrium like that.. Given how much handwaving is needed for hollow-earth to work I think we can safely ignore it! :D – Joe Bloggs Jan 7 at 16:16
There's a book series set inside of a bubble world with multiple artificial stars (giant fusion reactors) that turned off at "night" for refueling by robot insects that broke water down into hydrogen. It was really hot/tropical near the sun, and arctic way out toward the edge, but it was a really really big bubble. That's kind of how I picture this one working. I remember it being a fun set of books. – AndyD273 Jan 7 at 16:31

I would expect that the light source would have a huge impact on the colors viewed. But assuming that the light source is similar to our sun, then I'd assume that at or near the light it would be mostly white.

However, unlike earth where from horizon to horizon, there is only air diffusing the light, in a hollow earth you have 'sides' that with the right optics, can be 'visible' all around the light source other than what is 'behind' it hiding behind the corona and mass. So you will have hard surfaces reflecting the light as far as the eye can see. So most likely as your eye moves away from the sun, it will get 'darker' and blueish until the reflections from other surfaces start overriding the view. A huge bright green forest 500 miles away 'uphill' might have a very green tinge, an 'red desert' like in Arizona might have a bit of red.

Sorry not really a hard science answer.

As I commented "I think the sky would look more like the land when you look down out the window of a 747, slowly looking off into the distance. "

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So the sky could be blotchy? That is an interesting thought/image. – Erik Jan 7 at 16:01
@Erik I think the sky would look more like the land when you look down out the window of a 747, slowly looking off into the distance. – bowlturner Jan 7 at 16:22

As long as the atmosphere was the same as on earth, the sky would be blue. The light would still refract the same way as on earth, with no color change.

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(Really a comment but it's too much for one)

Before this question can be addressed we need to figure out what the atmosphere is like inside the world--and that implies we need to understand how gravity works.

The thing is in the real world there's no gravity sticking things to the inside of the world. Everything is going to fall into the sun instead. Since obviously this isn't happening something wacky is going on.

If the effect is localized to the surface you'll end up with much of the atmosphere filling the world. This means you're looking through 4,000 miles of air (much of it a lot denser than ours) to see the sun--it's going to be very red.

If the effect is wide-ranging the atmosphere will fall against the shell and the sun will look like ours at noon.

Note that the sky in this case will be blue-green rather than blue. You'll get the blue from scattering but you'll also be seeing the green of the other side of the world behind it.

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