Comparison of UT2, as determined astronomically at the Neuchâtel Observatory, with the Observatory’s atomic time scale TA. The graph plots the time difference UT2 − TA in milliseconds from January 1957 to February 1958. Circles represent UT2 determinations from observations of ten stars with the photographic zenith tube (PZT), while crosses represent observations of 100 stars with the Danjon astrolabe. The irregularity in February–March 1957 was attributed to unfavorable meteorological conditions. From August 1957 onward, the growing divergence from TA was interpreted as a real change in the rate of UT2 (Bonanomi et al. 1958).