The most common way of analyzing the global variations in the Earth's magnetic field is to fit the measurements to a set of spherical harmonics. This was first done by Carl Friedrich Gauss. Spherical harmonics are functions that oscillate over the surface of a sphere. They are the product of two functions, one that depends on latitude and one on longitude. The function of longitude is zero along zero or more great circles passing through the North and South Poles; the number of such "nodal lines" is the absolute value of the "order" . The function of latitude is zero along zero or more latitude circles; this plus the order is equal to the "degree" ℓ. Each harmonic is equivalent to a particular arrangement of magnetic charges at the center of the Earth. A "monopole" is an isolated magnetic charge, which has never been observed. A "dipole" is equivalent to two opposing charges brought close together and a "quadrupole" to two dipoles brought together. A quadrupole field is shown in the lower figure on the right.