  Weyl notes that what we seem to observe as the rotation of the stars, is in reality not the rotation of the stars themselves but the  rotation of the “star compass” [Sternenkompass] which  consists of light signals from the stars that meet our eyes at our  present location from a certain direction. It is crucial, Weyl  reminds us, to be cognisant of the existence of the metric field  between the stars and our eyes. This metric field determines the  propagation of light, and, like the electromagnetic field, it is  capable of change and variation. Weyl (1924b) says that “the  metric field is no less important for the direction in which I see  the star then is the location of the star itself.” How is it  possible, Weyl asks, to compare within the context of the general  theory of relativity, the state of motions of two separate bodies? Of course, Weyl notes, prior to the general theory of relativity, during Mach’s time, one could rely on a rigid frame of reference such as the earth, and indefinitely extend such a frame throughout space. One  could then postulate the relative motion of the stars with respect to this frame. However, under the hands of Einstein the coordinate  system has lost its rigidity to such a degree, that it can always  “cling to the motion of all bodies simultaneously”; that  is, whatever the motions of the bodies are, there exists a coordinate system such that all bodies are at rest with respect to that  coordinate system. Weyl then clarifies and illustrates the above with the plasticine example, which Weyl (1949a, 105) elsewhere describes  as follows: 