(1) A geodetic datum which specifies that the centre of the ellipsoid shall be located at the Earth's centre of mass. It is customary to specify also that the shortest axis of the ellipsoid be parallel to the Earth's axis of rotation (instantaneous, average or conventional) or to some other similar line of known orientation. Another axis may be required to be parallel to plane of the Greenwich meridian or, for some purposes, to the plane of the zero meridian of the Bureau International de l'Heure. More often, the ellipsoid is placed to provide a satisfactory fit to the geoid. A rectangular coordinate system is often associated with a geocentric geodetic datum by placing the origin of that coordinate system at the centre of the ellipsoid, making the x- and z-axes coincide with the longest and shortest axes, respectively, of the ellipsoid, and requiring that the coordinate system be right handed. (2) A geodetic datum specifying that the centre of the ellipsoid be at the Earth's centre of mass, that the shortest axis be parallel to the Earth's conventional axis of rotation, and that one of the other two axes be parallel to the plane of the meridian of Greenwich.