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
YIG sphere out of the sphere; the loops are half-turns, positioned at right-angles to each other to prevent direct electromagnetic coupling between them and each is grounded at one end. The field from an electromagnet changes the resonance frequency of the sphere and hence the frequency it will allow to pass. The advantage of this type of filter is that the garnet can be tuned over a very wide frequency range by varying the strength of the magnetic field. Some filters can be tuned from 3 GHz up to 50 GHz. YIG filters usually consist of several coupled stages, each stage consisting