One problem in telling time by the stars is that the basic clock (Earth's rotation) is unsteady. Besides a long-term tendency to run slow, it fluctuates over intervals ranging from a few days to several decades.
Most people wouldn't notice these changes, which run from a few ten-thousandths to around a hundredth of a second in day length. But for scientists who need to keep time with precision, they can be a nuisance, and the relative stability of atomic clocks has been welcome. On the other hand, scientists can use the day-length changes to learn more about the interaction between the solid Earth and the earthquakes, winds, ocean currents, shifting continents, tides, and other agents that cause the fluctuations.
For example, meteorologists have long known that some of the shorter-term variations probably reflect exchanges of momentum between the winds and the spinning planet.Lack of good wind data has made this hard to study. Last year, however, in the first worldwide effort of the international Global Atmospheric Research Program, the atmosphere was observed in great detail. There now is a set of high-quality wind data comprehensive enough to be matched against astronomically observed fluctuations in day length.
This has been done by a team of five scientists -- R. Hide, N. T. Birch, and A. A. White of Britain's Meteorological Office; L. V. Morrison of the Royal Greenwich Observatory; and D. J. Shea of the US National Center for Atmospheric Research. Using wind data for January to March and May to June in 1979, they computed the momentum of the atmosphere relative to the spinning solid Earth. Then, comparing changes in this momentum with day-length fluctuations taking place over a few weeks or less, they found the data fit one another well.
Interacting through friction, the winds and the planet are indeed found to be exchanging momentum and thus affecting the length of the day. Westerly winds, blowing in the direction of Earth's spin, lose momentum and speed up the planet. East winds, blowing against the spin, gain momentum and slow Earth down.
The relation between the winds and the planetary spin seems to be so clear cut that Hyde and his colleagues think it can be used to sort out yet another factor influencing day length -- momentum exchange between Earth's mantle and the planet's liquid core.
As the scientists note in reporting their study in Nature, it is the spinning of the solid outer part of the planet (the mantle surrounding the core) that is measured by the day-length observations. For their wind study, they removed the effects of tides (which gradually slow Earth's spin) and other irrelevant changes from the day-length data. It soon may be possible to take out atmospheric effects as well, leaving a residue of day-length variations that should reflect interactions between the mantle and Earth's core.
Thus, subtle changes in the length of the day that have been a nuisance to timekeepers may soon help scientists to track motions in the unseen core of the Earth, as well as to understand better the working of the atmosphere.