The length of day since 1656

Observed values of the difference between the time determined using the rotation of the Earth and a uniform time scale are available since 1627, with useful observations becoming available in 1656. These early data were recorded with low precision and must be smoothed numerically in order to be useful for the derivation of estimates of the difference between the actual length of the day and the standard 24 h, known as excess length of day, or the Earth's rotational speed. Modern data requires no smoothing. The observational data have been adjusted to be consistent with one estimate of the lunar acceleration and smoothed so that the internal and external precision of the observations are approximately equal. The final values along with the derived excess length of day are presented and their spectra are discussed.