Toward a Longer Day

Russian Scientist Vladimir Kotelnikov checked and rechecked the calclations, but the answer remained essentially the same: between March 1963 and October 1965, the rotation of the earth slowed down so much that the average day lengthened by 1.6 milliseconds--or about one six-hundredth of a second. The result was "extremely unexpected," a surprised Kotelnikov told the Soviet Academy of Sciences. The length of a day had increased only one millisecond (one-thousandth of a second) during the previous 120 years.

Was something unusual happening? Not likely. While most scientists found no reason to doubt Kotelnikov's figures, they did not share his surprise. Records of solar and lunar eclipses from as far back as 500 B.C. prove that days have been lengthening by an average of 1.8 milliseconds every century as tidal drag on the earth caused by both the moon and sun gradually slows terrestrial rotation. The same records confirm that sudden changes in the rate of slowdown have occurred before, probably because of varying interaction between the earth's mantle and its molten core, or shifts in atmosphere circulation and ocean currents.

Even a climatic change can affect the rate of rotation. When the earth's weather becomes warmer, for example, some of the ice concentrated at the North and South poles melts, releasing water into the world's oceans. The mass of ice near the earth's axis of rotation is reduced, and the amount of water in the oceans (which are farther from the axis) is increased. As a result, the earth's moment of inertia becomes greater and--like a twirling ice skater who moves his arms out from his body --its speed of rotation decreases.

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