Lunar nutation effect defines the sign of the Earth rotation rate for now, but this may change soon

Abstract

The Earth slowly decelerates in its rotation due to the energy dissipation caused by the interaction with the Moon. This leads to the continuous increasing in the length of the mean solar day (aka, length-of-day, or, LOD) relatively to 86,400 solar seconds at an average secular rate of +1.8 ms per century. But, on a shorter time scale the process is uneven. A positive leap second is used to be introduced on regular basis to support a consistency between the astronomical and atomic timescales. However, nowadays the LOD is steadily sparking a discussion about the timescale maintenance, in particular, from fears that a negative leap second will have to be introduced for the first time in the foreseeable future. The aim is to show that the LOD is currently dominated by the 18.6 yr lunar nutation signal whereas the long-term trends are essential for extrapolation after 2030. The LOD data since 1962 are used to estimate the long-term variations along the 18.6 yr and other harmonic signals in its spectrum. It is shown that the influence of the lunar nutation impact on the LOD variations was underestimated. At the moment, the LOD changes are completely determined by a signal with a period of 18.6 yrs. More detailed extrapolation reveals that LOD is likely to vary in a range between -1 ms to +1 ms until 2050 or may be longer.