Ringworld Data

Consider a Ringworld, a ring-shaped world that orbits a star at the center of the ring.

Larry Niven invented this as a compromise between a Dyson Sphere and a planet, and wrote his novel of the same name to describe exploration of such a world. (And if you didn’t know that, go and read that novel. Do it now. It’s one of those classic Sci Fi stories that will get your sense of wonder going, even if the story itself is not particularly strong.

It’s an awesome concept, even with its technical problems (“The Ringworld is unstable!”) and I always wanted to build my own Ringworld. The past few days, the Ringworld concepts have been nagging me in the back of my head and yesterday morning I had an idea I think is interesting to explore. To that end, I need to find out more about the Ringworld’s properties.

First off, I prefer a Banks Orbital to a Niven Ring for three reasons:

  1. Banks Orbitals are marginally more realistic than Niven Rings
  2. Their smaller size makes them more manageable from an author’s perspective
  3. Since they do not orbit a star at their center, they do not suffer the inherent stability issues of Niven Rings

Rotation and Size

The width of the Ring is arbitrary, and I eventually settled on 50000 km.

Rotation is a little more complicated. I wanted it to have a 24h day/night cycle, and since the spin of the Ring determines the “gravity” through centrifugal force, this determines the size the ring can have. I worked on this many years ago, starting in November 2005, when I figured out the dimensions it would have, back when Wikipedia didn’t offer any convenient answer to that problem. I wanted the Ring to have an inner “simulated gravity” of 1G, 9.81m/s^2 (or a close alien equivalent to it, which is why I had 9.78m/s^2 in the old post I think), and a rotation matching 24 hours.

This all works out to:

  • Radius: 1854969,424913 km.
  • Circumference: 11655116.64 km
  • Surface area: 582755831794.04 km^2 or 1142.51 times that of Earth

If you discount Earth’s ocean-covered surface the Ring would have almost four thousand times the land area of Earth. Of course that is neglecting that the Ring will not be entirely covered by land. I haven’t worked out how much is a “good” ocean to land ratio, but I’m guessing 50-70% water. This is purely based on aesthetics; the surface of the Ring should be earth-like, and that means lots of water. This still leaves 1400+ times the Earth’s land surface area.

The Ringworld's Scale
The Ringworld's Scale

Day and Night Cycle

The ringworld orbits slightly tilted towards its primary star. This allows light to reach the inner surface of the more distant side – creating a Ringworld day.

The Ringworld's Day-and-Night Cycle
The Ringworld's Day-and-Night Cycle on the inner side of the ring.

Seasons

You could asjust the climate – or possibly create seasons – by controlling not only the distance from the sun, but by the angle of the Ringworld’s “tilt”. One problem that needs to be solved is that the Ringworld would, according to my understanding, eventually be oriented so that the dipped side is rotated 90° from the sun. That way, the sun’s light would be blocked entirely. The Ringworld days would differ in their length from 12h to 0h in a cycle repeated twice per Orbit. This could be a strange type of season, but I think it is undesirable to the inhabitants and should probably be corrected for by some means.

Angular Size

When you look up into the sky, you see the inner surface of the other side of the ring. At a distance of 3.71 million kilometers, the angular size of that ring is 46.331 minutes, about 50% larger than the Moon appears from Earth. An observer at the center of the ring would see the ring at a size of 92.658 Minutes on all sides – a little more than three times the Moon as seen from Earth.

If the Earth was placed at the center of the Ringworld, the Earth would appear to be of a size of 23.785 minutes of arc, roughly 20% smaller than the moon appears in Earth’s sky. It would be invisible during the night – hidden against the background of the opposite side of the ring – but might be fairly visible during the day, the same way the moon is sometimes visible during a Terran day.

The Moon as seen from Earth during the day
The Moon as seen from Earth during the day

If the Ringworld orbits at a Lagrange point, it will still fill the sky. And even at 150 million kilometers, the distance from the Sun to the Earth, it will be easily visible by naked eye.

A Ring in the Sky: Relative Size of a Banks Orbital
A Ring in the Sky: Relative Size of a Banks Orbital

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