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I'm trying to wrap my head around how phases of the moon(s) would operate in a hollow earth. For reference, I am picturing a situation like this:

concave hollow earth

In this system, the center of the hollow earth has a dark body that blocks most, if not all light, in the center, while the sun is stationary off to the side. The Earth-shell rotates around to create a day-night cycle, and the axis on which it spins rotates, creating seasonal variation.

Now, the picture shows the moon and planets rotating around the center darkness, but I don't think that this would produce moon phases. For the planets, I'm fine with handwaving this as them emitting their own light.

But is there any way that a moon could be reflecting light from a sun that's on the other side of the central darkness? And how would it move in order to have different portions lit according to a cycle?

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2 Answers 2

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If you look at a model of our Solar System, you will notice that we observe phases only for the bodies (Mercury, Venus and Moon) orbiting between the Sun and Earth, due to the relative positions they get during their orbit.

In your situation, from the point of view of an observer on the surface of the hollow Earth, the moon would be between them and the Sun, thus replicating the configuration of the inner planets and exhibiting phases.

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  • $\begingroup$ So if I'm understanding correctly, like Venus, these "moons" would only come out during the early morning/early evening hours but be invisible close to midnight? $\endgroup$
    – A Herrera
    Commented Nov 18, 2023 at 22:20
  • $\begingroup$ I too think that the moon in this configuration would exhibit phases, however the phases would be different. For example I can think of no configuration where you would experience a full moon (except during day time). New moon on the other hand would be simply while sun and moon are on opposing sides of the center darkness. $\endgroup$
    – datacube
    Commented Nov 21, 2023 at 15:53
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This is profoundly silly, but I love your illustration so I am going to have a go at this.

As drawn, the ball with all the stars on is small compared to the void in the sphere. Either the sky would be filled with the view of the other side of the earth, or light does not travel in straight lines. Let's say light does not travel in straight lines.

Let us take our universe, centre it on the centre of the Earth, and transform it so R maps onto 1/R with R=1 being the average surface of the Earth. The centre of the earth is now infinitely distant, and the distant stars are now a very small cloud of objects close to the centre of the sphere/universe. If you look up, your light of sight bends to point towards this centre.

The Moon and the Sun are still spheres with this projection: a sphere maps onto a sphere. They are not quite the same spheres as the features on the near side will be slightly larger than the features on the far side, but this distortion would be small for anything that has little visual depth for the observer.

The Moon would go in a circle. The Sun would go in a smaller circle. The planets will orbit the sun in circles that aren't centred on the sun. The physics behind this will be hard to formulate in simple laws, because we have a lot of visual depth. Take Jupiter, for example. That is somewhere between 4.2 to 6.2 AU, where the Sun is close to 1.0, so Jupiter will move in a circle that is about a fifth the distance from the centre of the universe as the sun. We do not see these circles from the surface of Hollow Earth - we see the sun and the planets as we see them on Sensible Earth.

The one thing I cannot manage is a Lunar eclipse. We can't see the shadow of the Earth on the moon.

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