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
Sphere and is called the equation of an imaginary sphere. If formula_5 the only solution of formula_6 is the point formula_7 and the equation is said to be the equation of a point sphere. Finally, in the case formula_8, formula_6 is an equation of a sphere whose center is formula_10 and whose radius is formula_11. If in the above equation is zero then is the equation of a plane. Thus, a plane may be thought of as a sphere of infinite radius whose center is a point at infinity. The points on the sphere with radius formula_12 and center formula_13 can