Observations of the sun and major and minor planets made by transit circle telescopes are used to determine positions of the equinox and the celestial equator and, by repeated observing programs, the motions of these fiducial references. Long series of such absolute observations, when combined into catalogs such as the FK5, yield a fundamental coordinate system which is an observational approximation to an ideal, dynamically defined coordinate system. In such a system the equinox, for example, is defined implicitly by the right ascensions (at mean epoch) and the proper motions of the stars included in the catalog system, together with the adopted constant of precession. It may be noted that independent, highly accurate determinations of the latter quantity thus help to improve the fundamental proper motion system.