Sentence:  The pulsar PSR B1937+21 has been observed over a period of several years by Kaspi et al. (1994b) and Cognard et al. (1995); although, on a time scale of a few months, this <WSD>pulsar clock</WSD> fits a simple model within the reading accuracy of about 100 nanoseconds, an unpredictable wandering limits its accuracy over a period of a year or more (Figure 5.6). 
Meaning of the word marked with <WSD> tags:  The regular timing of pulses from a pulsar.