WHEN A MONTH WAS A DAY

Evening Post, Volume CIII, Issue 83, 8 April 1922, Page 16

WHEN A MONTH WAS A DAY

REMARKABLE FACTS ABOUT THE MOON.

An interesting article on the Origin ,of the Moon appears in Part VI.. of the "Outline of Science." It is explained that in the hands of Sir George H. Darwin, the distinguished so'- of Charles Darwin, the tides have been made to throw light on the evolution of our solar 6.ystem. j

"It is quite certain that, long ages ago, the earth was rotating immensely faster than it is now, and' that the moon was so near as to be actually in contact with the earth. In that remote age the moor was just on the point of separating from the earth, of being thrown off by the earth. Earth and moon were once one body, but the high rate of rotation caused this body to split up into two pieces; one piece became the earth we now know, and the othe. "came the moon. Such is the conclut,... i which we are led by an examination of tha tides." ' - " . Where does the energy come from that causes the tides to eat into rocks, to crumble away cliffs and form huge estuaries? "The source of tidal energy if Hie rotation ot the earth. The massive '.mlk of the earth, turning every tweni'-four hours on its axis, is like a gigantic flywheel. In virtue of its rotation it possesses an enormous store of energy." But the greatest flywheel must slow down. "It is the rotation of the earth which supplies the energy of the tides, and, as a consequence, the tides must be slowing down the earth. The tidea act as a kind of- brake on the earth's 1 rotation. These masses of water, held back by the moon, ,exert a kind of dragging effect on the rotating earth. Doubtless this effect, measured by our ordinary, standards, is very small; it is, however, continuous, and in the course pf the millions of years dealt with in astronomy, this small but constant effect may produce very considerable results."

The tides also have an effect on the moon. "It is the moon's action on the earth which produces the tides, but they also react on the moon. The tides are slowing down the earth, and they are also driving the moon farther and farther away. This result, strange as it may seem, does not permit of doubt, for it is the result of an indubitable dynamical principle, which cannot be made clear without a mathematical discussion."

At the beginning, when the moon was very near the earth, it went round the earth in the same time as the earth took to rotate, 3nd so the day and month were of the same length. But when the moon got farther away, and took longer to rotate round the earth, the month became longer. " The day also became longer, because the earth was slowing down, taking longer to rotate on its axis, but the month increased at a greater rate than the day. Presently; the month became equal to two days,' then to three, and so on. It has been calculated that this process went on until there were twenty-nine days in the month. After that the number of days in the month began to decrease until it reached its present value or magnitude, and will continue to decrease until once more the month and the day are equal. In that age the earth will be rotating very slowly." .

That is the future; it is interesting to look backward: "At present {he moon is getting farther and farther away. It follows, therefore, that when the day was shorter the moon was nearer. As we go farther back in time we find the moon nearer and nearer to an earth rotating faster and faster. When we reach the period when the earth completed a revolution in three or four hours, we find that the moon was so near as to be almost grazing the earth.,

The conclusion is irresistible. In an age more remote the earth did fly in pieces, and one of those.pieces is the moon. Such, in brief outline, is the tidal theory of the origin of the earthmoon system.',' Part VI. of the " Outline.. finishes with the first part of an interesting account of the " Body Machine," The author says: "The machine of the animal body is not only the most.perfect in nature; it is immeasureably the oldest. For at least 50,000,000 million years—how much longer no man knows —the world forces have been making the animal body, developing and improving the various organs and co-ordinating their functions. During all these tens of millions of years the machine has been subject to the fiercest stresses and trials, and the human body, as we know it, is the final and finished outcome." The human, body tells the story of evolution. The human hair is a dwindling vestige of the warm fur coat, which mammals developed to meet the conditions of an Ice Age. It is vestigial, not rudimentary, as is sometimes said: "The pieces of gristle or cartilage on the sides of the head which we call our ' ears ' are similar organs. They do not catch" waves of sound, as many suppose, and guide them into the real ear inside the skull. They are too flat to do so. But if we compare them with the useful, pointed, movable ear of a horse, we see what they mean. They were once similar organs, but they have fallen out of use, and are dwindling away. Underneath the skin we still have seven, muscles attached to the shell of cartilage, from which it is obvious that the ear could once be moved in every direction to catch the waves of sound. Now only an individual here and there can use one or two of these muscles. The pinna, or 'ear trumpet,' is a surviving structure that tells us a little about the body's remote past."

There are very many similar muscles in the body, to-day which merely tell u« about a strange past.