Sphere two poles is called the axis of rotation. Circles on the sphere that are parallel to the equator are lines of latitude. This terminology is also used for such approximately spheroidal astronomical bodies as the planet Earth (see geoid). Any plane that includes the center of a sphere divides it into two equal hemispheres. Any two intersecting planes that include the center of a sphere subdivide the sphere into four lunes or biangles, the vertices of which all coincide with the antipodal points lying on the line of intersection of the planes. The antipodal quotient of the sphere is the
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.