Slowing Rotation

An article in the December 13, 1984 Washington Post report that the Earth had experienced a sudden unexpected slowdown in rotation. Later, on July 15, 1988, an article in the Wall Street Journal stated "Why earth should have slowed .. isn't wholly understood," and further reported that scientists at the US Naval Observatory and at the Jet Propulsion Lab found that the "earth, like an unbalanced washing machine," has developed "wobbles as it spins." Still later in July, 1990, Omni Magazine reported that between January 24 to February 3, 1990, the Earth's rotation suddenly and unexpectedly slowed down again. US Naval Observatory scientists reported that the slowdown was more abrupt than usual. On August 9, 1991, the New York Times speculated as to the causes of the slowdown in the following article.

New York Times
August 9, 1991

As the world turns, its rotational speed is slowing down by an average of 1.4 milliseconds each century. But now scientists have determined that the deceleration is occurring somewhat fitfully. The slowing causes the Moon's orbit to expand slightly and a day on Earth to grow ever so much longer. Scientists analyzing new, more precise measurements have determined that winds and ocean currents, ice melting and turbulence in the Earth's liquid metallic core are causing detectable variations in the rotation on time scales of days, months or decades. Besides the long-recognized tidal forces from the gravitational action of the Moon, Sun and other astronomical bodies, the forces generated by atmospheric motion have the most significant effect on the slowing down or speeding up of the spinning Earth.

Recent studies, for example, have established a correlation between these variations and the large-scale redistribution of atmospheric mass associated with El Nino, the periodic phenomenon of strong winds and ocean currents in the southern Pacific basin. The growing appreciation of the important role of atmospheric forces on Earth's rotation is described in the issue of the journal Science being published Friday. The report was written by Dr. Raymond Hide, a geophysicist at Oxford University in England, and Dr. Jean O. Dickey, a geodesist at the Jet Propulsion Laboratory here. "Ninety percent of these variations are caused by changes in wind," Dr. Dickey said in an interview. "If the atmosphere is speeding up, Earth is slowing down." Some of the short term fluctuations can change the length of a day by as much as five milliseconds. Not that this should disrupt anyone's biological clock, but the fluctuations can affect navigational computations for space craft traveling far out in the solar system.

On a more fundamental level, geophysicists are keenly interested in learning how to use Earth's erratic motions as a key to interpreting the dynamics of its deep interior. They suspect that perturbations in the circulation of the hot fluid metallic core contributes to the rotational changes. Some of these disturbances may be caused by the rough topography, like inverse mountains, where the liquid core meets the solid structure, or mantle, of Earth's depths. "The roughness at the core-mantle boundary," Dr. Dickey said, "causes most of the variations that occur over periods of decades." In their report, Dr. Hide and Dr. Dickey concluded that the irregular rotational variations are the result of "fluctuating fluid flow in the underlying hydrosphere and atmosphere, and non-periodic mass distribution associated with earthquakes, the melting of ice" and also convection forces within Earth and the movement of crustal plates near the surface.

The analysis was based on years of precise measurements of Earth's motions. These were achieved by observing shifts of distant celestial objects in relation to Earth and by bouncing laser signals off artificial satellites and off quartz reflectors left on the Moon by Apollo astronauts. The scientists said that such measurements are 10 to 20 times more accurate now than they were 10 years ago.