A physical clock by itself does not measure time; it only measures the
process. For instance, the rotation of the earth measured in solar
days is a physical clock. Most physical clocks are based on cyclic
physical processes (such as the rotation of the earth). The modifier
``physical'' is used to distinguish this kind of clock from other
kinds of clocks, e.g., the time-line clock (+E9). It is also
descriptive in so far as physical clocks are based on recurring
natural or man-made phenomena (+E8).