ELEMENTS, in theoretical astronomy, the six numerical quantities used for the computation of the position of a planet. Two depend on the geometrical properties of the orbit, i.e., its semi-major axis and its eccentricity. Two define the position of the plane of the orbit ; one of these is the angle between the line of nodes, and some fixed line, the other being the inclina tion of the plane of the orbit to the fundamental plane. The fifth is the position of the pericentre ; and the sixth, the position of the planet in the orbit at a given moment. A seventh is the time of the revolution of the body in its orbit, called its period ; but the seven elements are so related that any six only have to be known in order that the seventh may be deduced. (See also ORBIT.) For a definition of a chemical element see ELEMENTS, CHEMICAL. See also TRANSMUTATION OF ELEMENTS. For Euclid's Elements see EUCLID.