Circle of a sphere the Equator the only great circle. By contrast, all meridians of longitude, paired with their opposite meridian in the other hemisphere, form great circles. The diameter of the sphere which passes through the center of the circle is called its axis and the endpoints of this diameter are called its poles. A circle of a sphere can also be defined as the set of points at a given angular distance from a given pole. When the intersection of a sphere and a plane is not empty or a single point, it is a circle. This can be seen as follows:
Bernal sphere optimum mass-efficiency at providing radiation shielding. O'Neill envisioned the next generation of space habitat as a larger version of "Island One". "Island Two" would be approximately 1800 meters in diameter, yielding an equatorial circumference of nearly six and a half kilometers (four miles). The size was driven by economics; the habitat was to be small enough to allow for efficient transportation within the habitat and large enough to support an efficient industrial base. Bernal sphere A Bernal sphere is a type of space habitat intended as a long-term home for permanent residents, first proposed in 1929 by John Desmond Bernal.