Published online by Cambridge University Press: 14 August 2015
For the study of the Earth's rotation and geodetic measurements the longest possible series of homogeneous Universal Time (UT) data is required. For various reasons this remains a difficult problem. Periodically it is necessary to revise past observations and redetermine the UT scale. This was done by the Bureau International de l'Heure (BIH) in 1968 when the FK 4 catalog, revised values of astronomical longitudes, and new techniques were standardized for the reduction of astronomical observations. This system known as the 1968 BIH System has been in use since 1968 and has also been used for the re-reduction of observations back to 1962.0.