Circle of a sphere the Equator the only great circle. By contrast, all meridians of longitude, paired with their opposite meridian in the other hemisphere, form great circles. The diameter of the sphere which passes through the center of the circle is called its axis and the endpoints of this diameter are called its poles. A circle of a sphere can also be defined as the set of points at a given angular distance from a given pole. When the intersection of a sphere and a plane is not empty or a single point, it is a circle. This can be seen as follows:
Half sphere exposure as the number of Cα atoms in the upper half (containing the pseudo-Cβ atom) and analogously HSE-down is defined as the number of Cα atoms in the opposite sphere. If only Cα atoms are available (as is the case for many simplified representations of protein structure), a related measure, called HSEα, can be used. HSEα uses a pseudo-Cβ instead of the real Cβ atom for its calculation. The position of this pseudo-Cβ atom (pCβ) is derived from the positions of preceding Cα and the following Cα. The Cα-pCβ vector is calculated by adding the Cα-Cα and Cα-Cα vectors. HSE is