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Topopoli are a brand of circumstellar habitats with some unusual properties. They can be thought of conceptually as a Rama Cylinder with no end points. Specifically, one can imagine them as cylinders which rotate along their long axis for artificial gravity and have been lengthened until they form a complete circle around a star. This seems counter-intuitive at first, as attempting to do this with a small, everyday object like a rubber wheel would result in the object kinking, twisting and possible even breaking. However, a Topopolis is so long that it's inner and outer circumferences are effectively identical. Even rigid materials are elastic enough to withstand the stretching involved in the rotation, which is generally measured in micrometers per meter.

While a single loop around a star qualifies as a Topopolis, the idea as first conceived among Humans by Twilight Era scientist, Patrick Gunkel, involved a far more complicated situation which is the most common among Aquarians and others who have built them. A Topopolis often forms multiple loops around a star, but is still one continuous cylinder. This is a geometry known as a Torus Knot and there are a number of types. Below are some examples, the (2-3), (3-7) and (3-8) Torus Knots respectively from the left.

(2-3) Torus Knot (3-7) Torus Knot (3-8) Torus Knot

The unusual geometry and rotation of a Topopolis carries some great advantages. First off, if the minor radius is kept in the range of only one or two hundred km, it is possible for one to be constructed without the use of magmatter. Combined with a major radius that can be measured in astronomical units, this means it is possible to construct a habitat with thousands or more times the surface area of Terra with conventional matter. They cannot keep this advantage if the weight placed on them is too great however, so if one wishes to not employ magmatter, one cannot make the minor radius too large or mimic a planetary crust as an Eeyocli Ringworld does. To some, this makes them seem more primitive, though many prefer quiet soil recycling by subtle plumbing and variously-scaled bots to mimicry of volcanoes and other geological activity. The other major advantage is the sheer size. From any given location inside a Topopolis, they appear to stretch to infinity in either direction, the curve being too great to be perceived. This tends to make them feel quite a bit roomier than other Closed Type habitats. On the other hand, some find the sense of infinity within a closed space discomforting.

The other disadvantage (to some) is that the orientation of the rotational axis of any given section is perpendicular to the sun at the hub and so they must be Closed Type habitats. Daylight must be simulated. Quite often this is done by channeling light from the sun(s) from the outer hull into a lighting element running along the axis. As many Topopoli are at least 100 km, this normally puts the element above the atmosphere. Other designs opt for something more similar to a Rama Cylinder, channeling the light into an odd number of elements running along the inner surface, illuminating a section opposite them. Yet other designs are more varied.



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