The globe itself rotates within a metallic meridian to which its axis is attached.
Van't Hoff and others have established, corresponds to the presence of one or more asymmetric atoms of carbon - that is, atoms directly united to elements or radicles all different from one another - and in every case there exists an isomer that rotates the plane of polarization to the same degree in the opposite direction.
The former consists of two semicircular plates of quartz, perpendicular to the optic axis and of opposite rotations, placed so as to have a common diameter and having such a thickness that each rotates the plane of polarization of mean yellow light through the same multiple of 90°.
The vertical line at any place being the fundamental axis of the apparent system of co-ordinates, this system rotates with the earth, and so seems to us as fixed.
If a planet rotates on its axis so rapidly as to have a considerable ellipticity, and if it has satellites revolving very near the plane of the equator, the combined actions of the sun and of the equatorial protuberances may be such that the whole system will rotate almost as if the planes of revolution of the satellites were solidly fixed to the plane of the equator.