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
Ewald's sphere falling on the crystal has a wave vector formula_2 whose length is formula_3. The diffracted plane wave has a wave vector formula_4. If no energy is gained or lost in the diffraction process (it is elastic) then formula_4 has the same length as formula_2. The difference between the wave-vectors of diffracted and incident wave is defined as scattering vector formula_7. Since formula_2 and formula_4 have the same length the scattering vector must lie on the surface of a sphere of radius formula_3. This sphere is called the Ewald sphere. The reciprocal lattice points are the values of momentum transfer where