31. Time

Abstract

The astronomical observatories which actively cooperate in the network of the system of the Bureau International de l’Heure, are, at present, about 70. Most of the astronomical time determinations are carried out by means of Danjon Astrolabes or transit instruments; 10 observatories dispose of a PZT and 5 others are installing or planning to have it. The results of their observations, ɸ(observed latitude) and Δ T (difference between local U.T.0 and U.T.C), are collected at the BIH and – contrary to the procedure used until 1967 – used to simultaneously obtain the instantaneous coordinates of the pole (x, y), referred to conventional international origin (CIO), and longitudes origin on the pole equator. As a consequence of the adoption of the above-mentioned conventional pole, the BIH had to change all conventional longitudes in order to keep the origin formerly defined (1967); they are now reported in BIH Annual Reports. A further change was operated on the constant of aberration which, at the beginning of 1968, from 20˝.47 was changed to 20˝.496. The BIH had therefore to operate a step of to U.T.1, at the beginning of January 1968, and so did the Greenwich Observatory, with a step of + 0.0069. Global corrections, due to these variations and to others occurred from 1955 to 1968 (in 1958, variation of the polar reference, from Cecchini’s to the mean pole of the date; in 1961, adoption of FK4 and of new longitudes), to the data of the past years (until 1968), are reported on Table 8 of the BIH Annual Report, 1968. The formula supplying the correction U.T.2 —U.T.1 remained unchanged, except for the variable t, which is expressed (since 1967) as a fraction of the Besselian year. Finally, stars coordinates in the catalogs used for time service were revised on the basis of observations made during the latest ten years (Neuchâtel, Greenwich and Mizusawa) and a catalog has been prepared, which includes 807 right ascensions for 1960, drawn from 20 catalogs, with more than 185000 observations of the U.S.S.R. (Pulkova) time service, which is also preparing a new catalog of proper motions. The unification of star positions and proper motions is recommended for the PZT.