(1) A right-handed, rectangular Cartesian coordinate system which has the z-axis perpen-dicular to the plane of the Earth's orbit and the x axis parallel to the line of intersection of the plane of the Earth's orbit with the plane of the celestial equator. Only the system's orientation is defined. The origin may be anywhere in the Solar system but is usually placed in the Earth or in the Sun. (2) A coordinate system whole geometric elements are a straight line (called the polar axis or z-axis), a straight line (called the x axis) intersecting the z-axis at right angles, and the point (called the origin) at which the z axis intersects the x,y-plane. The coordinates of a point are the angle (called the longitude) from the plane of the z and x-axes to the plane through the z axis and the point in question; the angle (called the latitude) which is the complement of the angle from the z-axis to a line joining the origin to the point in question; and the distance from the origin to the point in question.