# How big would temperature differences be on a ring world with a wide flat inner surface?

I'm currently working on an idea for a ring world. Thus far I've worked out the following things:

• Width of half the circumference of Earth
• 2000km high walls across the rims to keep the atmosphere in
• Screens move across the top of the walls to give any given place in the ring a 24 hour cycle of day and night akin to what we have on Earth
• The ring's equator runs across the exact center of the wholer ring, so the northern and southern hemispheres are both a quarter of Earth's circumference tall
• The equator is where the ring's star stands the closest to the insides of the ring
• The star is the same size as the Sun
• The ring does in fact orbit the star

Now, what I'm trying to figure out is how the shape of the ring will affect the environment on various distances from the equator. Given the various environmental differences between numerous places on Earth despite being quite close to one another, how big will these differences be on a flat world as described above? Will the environments be comparable to what we have on Earth, will they be more extreme or perhaps more the same?

• The ring does not orbit the star. It spins at whatever rate you want to give cetrifugal weight to the surface. – JDługosz Jul 27 '16 at 0:02
• A ring does not have northern and southern hemispheres. It is difficult to feel confident that I understand what shape you are describing. – A. I. Breveleri Jul 27 '16 at 0:12
• not a ring world, but I would recommend to watch Megastructures 07: Hoopworlds, as also other series from megastructure series at least. @JDługosz not necessary centrifugal, it can be ordinary gravity, on such scale ti's pretty advanced, so might be expected other possible solution.(I meant to say watch too) – MolbOrg Jul 27 '16 at 2:06
• @MolbOrg no, the gravity from a ring will be zero within it. The entire rest of the ring pulls up and cancels the near patch directly under your feet. – JDługosz Jul 27 '16 at 2:26
• @JDługosz totally not true. Would it be so it would be no stability problem, which many pointing with stationary ring. Another examples are dipoles. It have not to be confused with electrostatic field, and sphere. They canceling, in case of ring, only in one point, in center. Any offset from center and it will drift. That's true for static ring, not rotating ring. Maybe most simplest why not true, with some assumptions, is: earth is that local patch, and sun is the rest of ring, even if earth will not orbit (no centrifugal force), we will not fly to sun from earth, if earth is nailed to point – MolbOrg Jul 27 '16 at 7:02

Given your specified dimensions, the diameter of the star is about 70 times the width of the ring, thus the effective insolation will be identical for every point on the inner surface of the ring.

There is no astronomical reason for the climate to vary from place to place on the ring. Weather will be dominated by topography.

• Niven's Ringworld had radiators on the outside of the ring. These would be needed to keep the Ringworld from overheating. So as part of the topography, you would have heatsinks (likely within lakes or oceans) on the inside to give it "cold spots" in its environment. – Michael Richardson Oct 3 '16 at 18:33

I agree with A. I. Breveleri about invariable insolation. Topography would be one of the most important things.

However, I would also expect the ring to flex significantly based on the presence of other planets. If there were a Jupiter-like planet in a Jupiter-like orbit (~5AU), I would expect the ring to deform from a circle into an ellipse with the semi-major axis in the direction of Jupiter. This effect causes eccentricity of in earth's orbit too. (https://en.wikipedia.org/wiki/Milankovitch_cycles)

I can't come close to calculating the magnitude of this deformation, but I feel confident that a race advanced enough to build a ring world would either remove the Jupiter or make it flex with the gravitic effects of other planets.

If Jupiter causes deformation, then the parts of the ring along the semi-major axis would be farther from the sun and in "winter" while the parts nearer the sun would be in "summer". Now assuming that the designers knew about this, they could match the rate of rotation of the ring with the orbital period of the Jupiter to cause summer/winter to cycle at a pre-determined rate...like one earth year, if you will.

The magnitude of difference between winter and summer would be variable depending on the gravity of the Jupiter and the flexibility of the ring, etc, but I suspect the difference in seasons would be mild. The earth's seasons are a result of a BIG insolation difference between summer and winter, for example, from 1.8 kWh/m^2/day to 6.0 in Albany, NY. To get that in an ellipse the inner part of the ring will have to be ~1.7 times closer to the sun than the farthest part. That seems like a lot.

• Having the ring flex is a interesting point. – JDługosz Jul 27 '16 at 2:31

The immediate difference in climate is: the inner part of the ring becomes very hot, permanently, since it is always directed to the star. Some sort of tech is needed to harvest the star's energy and avoid melting.

Assuming that this problem is solved, the climate will be very stable: seasons in Earth are due to axial tilt - Earth's rotation is on an angle with the orbital plane. This cannot happen in a ring.

As an aside: Is your ring a torus? It's not clear, given the "flat inner surface" on the question title, and "northern and southern hemispheres" on the question body.

• And yet, somehow, neither the Earth nor the Moon becomes exceptionally hot and melts, nor do satellites, asteroids, or other objects that the Sun shines on. – Mark Jul 27 '16 at 1:04
• Look at the day side of the moon. It is very hot. True, @mark, what’s left of it does not melt, but any material that can’t stand the heat is long gone. In particular there is no water locke up in the minerals. – JDługosz Jul 27 '16 at 2:30
• Well, this does point out a flaw with the 1 AU distance - as there is no night, the surface effectively gets twice the insolation. Permanent equatorial noon will be rather uninhabitable. A ringworld with a radius closer to Mars' orbit (1.5 AU) would be better. This assumes the illuminated side to be mostly isolated from the dark side. – Chieron Jul 27 '16 at 10:24
• No, it's like Known Space's Ringworld, not like Planescape's Sigil. It's stated to be flat in the title and the question. – Thomas Jacobs Jul 27 '16 at 12:59