Earth-Pulse

The day is now one-thousandth of a second longer than it was a century ago. Although this change has had no effect on clocks, it is highly significant for astronomers. Just before he died last July, venerable, 71-year-old Professor Ernest William Brown of Yale, who spent half a century studying the passage of time, sent the Smithsonian Institution an original hypothesis on the lengthening of the day.

Last week it appeared in the Institution's annual report.

According to mathematicians, the length of the day is determined by the speed of the earth's rotation on its axis. Slower speed means a longer day. According to astronomers, speed of the earth's rotation on its axis is affected by the moon. Reason: the moon pulls the tides, and a strong pull creates friction on the earth's surface, slows down its motion. This well-known fact, said Professor Brown, accounts for slight variations in the day's length, but the moon's influence is not powerful enough to cause such large changes in time as that which occurred in 1897, when the apparent length of the year was changed by one second. Practical reason for studying the phenomenon, he added, is that the length of the day is more variable now than ever and a new change may soon occur.

According to astronomical theory no force external to the earth can produce large changes in its rotation on its axis. As for surface changes on the earth. Dr. Brown rules them out. Older theories held that accumulations of ice and snow at the poles might slow up the earth's speed. But the least amount of frozen water necessary to slow up the earth would have changed the average sea level all over the world by about a foot. This has not happened. The weight of mountains and force of volcanoes are also inconsiderable. Even if the whole group of Himalayas could be razed and piled up at the pole, the earth's speed would not be altered.

Thus ''driven to changes below the surface," Dr. Brown offered the following hypothesis : astronomers believe that certain stars pulsate bodily, and it is not unreasonable to suspect that the earth, a star, pulsates too. If there is a uniform contraction and expansion of the entire globe a raising of five inches in the crust is sufficient to slow down the earth and account for the maximum lengthening of the day which has so far been observed. The process of expansion, said Dr. Brown, might conceivably take place if there were a layer of material near the earth's surface which was at a critical temperature (one in which a small change of temperature produces a relatively large change of volume). Thus a slight change in the temperature of the earth's interior would produce a considerable alteration of the crust. How the earth contracts and expands Dr. Brown could only speculate. Evidence for his theory lies in the fact that the fissures all over the earth (such as occur in mountain building) constantly repeat their openings and closings. Last advice of Astronomer Brown was the suggestion that sensitive instruments be set up near large fissures to measure the rate of their changes. Exact measurements might then lead to some predictions.

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