Astronomy for Everybody/Part 3/Chapter 3

The globe on which we live, being one of the planets, would be entitled to a place among the heavenly bodies even if it had no other claims on our attention. Insignificant though it is in size when compared with the great bodies of the universe, or even with the four giant planets of our system, it is the largest of the group to which it belongs. Of the rank which it might claim as the abode of man we need not speak.

What is the earth? We may describe it in the most comprehensive way as a globe of matter nearly eight thousand miles in diameter, bound together by the mutual gravitation of its parts. We all know that it is not exactly spherical, but bulges out very slightly at the equator. The problem of determining its exact shape and size is an extremely difficult one, and we cannot say that an entirely satisfactory result is yet reached. The difficulty is obvious enough. There is no way of measuring distances across the great oceans. The measurements are necessarily limited to such islands as are visible from the coasts of the continents or from each other. Of course, the measures cannot be extended to either pole. The size and shape must therefore be inferred from the measures across or along the continents. Owing to the importance of such work, the leading nations have from ​time to time entered into it. Quite recently our Coast and Geodetic Survey has completed the measurement of a line of triangles extending from the Atlantic to the Pacific Oceans. North and south measurements both on the Atlantic and Pacific coasts have been executed or are in progress. The English have from time to time made measures of the same sort in Africa, and the Russians and Germans on their respective territories. Nearly all these measures are now being combined in a work carried on by the International Geodetic Association, of which the geodetic authorities of the principal countries are members.

The latest conclusions on the subject may be summed up thus. We remark in the first place that by the figure of the earth geodetists do not mean the figure of the continents, but of the ocean level as it would be if canals admitting the water of the oceans were dug through the continents. The earth thus defined is approximately an ellipsoid, of which the smaller diameter is that through the poles, and which has about the following dimensions:

Polar diameter, 7,899.6 miles, or 12,713.0 kilometres.

Polar diameter,Equatorial" 7,926.6 miles, or 12,756.5 kilometres.

It will be seen that the equatorial diameter is twenty-seven miles or forty-three kilometres greater than the polar.

What we know of the earth by direct observation is confined almost entirely to its surface. The greatest depth to which man has ever been able to penetrate com- ​pares with the size of the globe only as the skin of an apple does to the body of the fruit itself.

I shall first invite the reader's attention to some facts about weight, pressure, and gravity in the earth. Let us consider a cubic foot of soil forming part of the outer surface of the earth. This upper cubic foot presses upon its bottom with its own weight, perhaps one hundred and fifty pounds. The cubic foot below it weighs an equal amount, and therefore presses on its bottom with a force equal to its own weight with the weight of the other foot added to it. This continual increase of pressure goes on as we descend. Every square foot in the earth's interior sustains a pressure equal to the weight of a column of the earth a foot square extending to the surface. Not many yards below the surface this pressure will be measured in tons; at the depth of a mile it may be thirty or forty tons; at the depth of one hundred miles, thousands of tons; continually increasing to the centre. Under this enormous pressure the matter composing the inner portion of the earth is compressed to the density of a metal. By a process which we will hereafter describe, the mean density of the earth is known to be five and one half times that of water, while the superficial density is only two or three times that of water.

One of the most remarkable facts about the earth is that the temperature continually increases as we penetrate below the surface in deep mines. The rate of increase is different in different latitudes and regions. The general average is one degree Fahrenheit in fifty or sixty feet.

​The first question to suggest itself is, how far toward the earth's centre does this increase of temperature extend? The most that we can say is that it cannot be merely superficial, because, in that case, the exterior portions would have cooled off long ago, so that we should have no considerable increase of heat as we went down. The fact that the heat has been kept up during the whole of the earth's existence shows that it must still be very intense toward the centre, and that the rate of increase near the surface must go on for many miles into the interior.

At this rate the material of the earth would be red hot at a depth of ten or fifteen miles, while at one or two hundred miles the heat would be sufficient to melt all the substances which form the earth's crust. This fact suggested to geologists the idea that our globe is really a molten mass, like a mass of melted iron, covered by a cool crust a few miles thick, on which we dwell. The existence of volcanoes and the occurrence of earthquakes gave additional weight to this view, as did also other geological evidence, showing changes in the earth's surface which appeared to be the result of a liquid interior.

But in recent years the astronomer and physicist have collected evidence, which is as conclusive as such evidence can be, that the earth is solid from centre to surface, and even more rigid than a similar mass of steel. The subject was first developed most fully by Lord Kelvin, who showed that, if the earth were a fluid, surrounded by a crust, the action of the moon would not cause tides in the ​ocean, but would merely tend to stretch out the entire earth in the direction of the moon, leaving the relative positions of the crust and the water unchanged.

Equally conclusive is the curious phenomenon which we shall describe presently of the variation of latitudes on the earth's surface. Not only a globe of which the interior is soft, but even a globe no more rigid than steel could not rotate as the earth does.

How, then, are we to reconcile the enormous temperature and the solidity? There seems to be only one solution possible. The matter of the interior of the earth is kept solid by the enormous pressure. It is found experimentally that when masses of matter like the rocks of the earth are raised to the melting point, and then subjected to heavy pressure, the effect of the pressure is to make them solid again. Thus, as we increase the temperature we have only to increase the pressure also to keep the material of the earth solid. And thus it is that, as we descend into the earth, the increase of pressure more than keeps pace with the rise of temperature, and thus keeps the whole mass solid.

Another interesting question connected with the earth is that of its density, or specific gravity. We all know that a lump of lead is heavier than an equal lump of iron, and the latter heavier than an equal lump of wood. Is there any way of determining what a cubic foot of earth would weigh if taken out from a great depth of its vast interior? If there is, then we can determine what the ​actual weight of the whole earth is. The solution depends on the gravitation of matter.

Every child is familiar with gravitation from the time it begins to walk, but the profoundest philosopher knows nothing of its cause, and science has not discovered anything respecting it except a few general facts. The widest and most general of these facts, which may be said to include the whole subject, is Sir Isaac Newton's theory of gravitation. According to this theory, the mysterious force by which all bodies on the surface of the earth tend to fall toward its centre does not reside merely in the centre of the earth, but is due to an attraction exerted by every particle of matter composing our globe. Whether this was the case was at first an open question. Even so great a philosopher and physicist as Huyghens believed that the power resided in the earth's centre, and not in every particle, as Newton supposed. But the latter extended his theory yet farther by showing that every particle of matter in the universe, so far as we have yet ascertained, attracts every other particle with a force that diminishes as the square of the distance increases. This means that at twice the distance the attraction will be divided by four; at three times by nine; at four times by sixteen, and so on.

Granting this, it follows that all objects around us have their own gravitating power, and the question arises: Can we show this power by experiment, and measure its amount? The mathematical theory shows that globes should attract small bodies at their surfaces with a force proportioned to their diameter. A globe two ​feet in diameter, of the same specific gravity as the earth, should attract with a force one twenty-millionth of the earth's gravity.

In recent times several physicists have succeeded in measuring the attraction of globes of lead having a diameter of a foot, more or less. This measurement is the most delicate and difficult that has ever been made, and the accuracy which seems to have been reached would have been incredible a few years ago. The apparatus used is, in its principle, of the simplest kind. A very light horizontal rod is suspended at its centre by a thread of the finest and most flexible material that can be obtained. This rod is balanced by having a small ball attached to each end. What is measured is the attraction of the globes of lead upon these two balls. The former are placed in such a position as to unite their attraction in giving the rod a slight twisting motion in the horizontal plane. To appreciate the difficulties of the case, we must call to mind that the attraction may not amount to the ten-millionth part of the weight of the little balls. It would be difficult to find any object so light that its weight would not exceed this force. To compare the weight of a fly with it would be like comparing the weight of an ox with that of a dose of medicine. Not only the weight of a mosquito but even of its finest limb might exceed the quantity to be measured. If a mosquito were placed under a microscope an expert operator could cut off from one antenna a piece small enough to express the force measured.

Yet the determination of this force has been made with ​such precision that the results of the two latest investigators do not differ by a thousandth part. These were Professor Boys, F.R.S., of Oxford, England, and Dr. Karl Braun, S.J., of Marienschein, in Bohemia. They worked independently at the problem, meeting and overcoming innumerable difficulties one after another, getting greater and greater delicacy and precision in their apparatus, and finally published their results almost at the same time, the one in England, the other in Austria. The outcome of their experiments is that the mean density of the earth is slightly more than five and a half times that of water. This is a little less than the density of iron, but much more than that of any ordinary stone. As the mean density of the materials which compose the earth's crust is scarcely more than one half of this amount, it follows that near the centre the matter composing the earth must be compressed to a density not only far exceeding that of iron, but probably that of lead.

The attraction of mountains has been known for more than a hundred years. It was first demonstrated by Maskelyne about 1775 in the case of Blount Schehallion, in Scotland. In all mountain regions where very accurate surveys are made the attraction of mountains upon the plumb line is very evident.

We know that the earth rotates on an axis passing through the centre and intersecting the earth's surface at either pole. If we imagine ourselves standing exactly ​on a pole of the earth, with a flagstaff fastened in the ground, we should be carried round the flagstaff by the earth's rotation once in twenty-four hours. We should become aware of the motion by seeing the sun and stars apparently moving in the opposite direction in horizontal circles by virtue of the diurnal motion. Now, the great discovery of the variation of latitude is this: The point in which the axis of rotation intersects the surface is not fixed, but moves around in a somewhat variable and irregular curve, contained within a circle nearly sixty feet in diameter. That is to say, if standing at the north pole we should observe its position day by day, we should find it moving one, two, or three inches every day, describing in the course of time a curve around one central point, from which it would sometimes be farther away and sometimes nearer. It would make a complete revolution in this irregular way in about fourteen months.

Since we have never been at the pole, the question may arise: How is this known? The answer is that by astronomical observations we can, on any night, determine the exact angle between the plumb line at the place where we stand and the axis on which the earth is rotating on that particular day. Four or five stations for making these observations were established around the earth in 1900 by the International Geodetic Association. One of these stations is near Gaithersburg, Md., another is on the Pacific coast, a third is in Japan, and a fourth in Italy. Before these were established observations having the same object were made in various parts of Europe and America. The two most important stations ​in the latter region were those of Professor Rees of Columbia University, New York, and of Professor Doolittle, first at Lehigh, and later at the Flower Observatory, near Philadelphia.

The variation which we have described was originally demonstrated by S. C. Chandler, of Cambridge, in 1890 by means of a great mass of astronomical observations not made for this special purpose. Since then investigation has been going on with the view of determining the exact curve described. What has been shown thus far is that the variation is much wider some years than others, being quite considerable in 1891, and very small in 1894. It appears that in the course of seven years there will be one in which the pole describes the greater part of a comparatively wide circle, while three or four years later it will for several months scarcely move from its central position.

If the earth were composed of a fluid, or even of a substance which would bend no more than the hardest steel, such a motion of the axis as this would be impossible. Our globe must therefore, in the general average, be more rigid than steel.

The atmosphere is astronomically, as well as physically, a most important appendage of the earth. Necessary though it is to our life it constitutes one of the greatest obstructions with which the astronomer has to deal. It absorbs more or less of all the light that passes through it, and thus slightly changes the colour of the ​heavenly objects as we see them, and renders them somewhat dimmer, even in the clearest sky. It also refracts the light passing through it, causing it to describe a slightly curved line, concave toward the earth, instead of passing straight to the astronomer's eye. The result of this is that the stars appear slightly higher above the horizon than they actually are. The light coming directly down from a star in the zenith suffers no refraction. The latter increases as the star is farther from the zenith, but even forty-five degrees away it is only one minute of arc, about the smallest amount that the unaided eye can plainly perceive; yet this is a very important quantity to the astronomer. The nearer the object is to the horizon the greater the rate at which the refraction increases; twenty-eight degrees above the horizon it is about twice as great as at forty-five degrees; at the horizon it is more than one half a degree, that is more than the whole diameter of the sun or moon. The result is that when we see the sun just about to touch the horizon at sunset or sunrise its whole body is in reality below the horizon. We see it only in consequence of the refraction of its light. Another result of the rapid increase near the horizon is that, in this position, the sun looks decidedly flattened to the eye, its vertical diameter being shorter than the horizontal one. Anyone may notice this who has an opportunity to look at the sun as it is setting in the ocean. It arises from the fact that the lower edge of the sun is refracted more than the upper edge.

When the sun sets in the ocean in the clear air of the ​tropics a beautiful effect may be noticed, which can rarely or never be seen in the thicker air of our latitudes. It arises from the unequal refraction of the rays of light by the atmosphere. Like a prism of glass the atmosphere refracts the red rays the least and the successive spectral colours, yellow, green, blue, and violet, more and more. The result is that, as the edge of the sun is disappearing in the ocean, these successive rays are lost sight of in the same order. Two or three seconds before the sun has disappeared, the little spark of its limb which still remains visible is seen to change colour and rapidly grow paler. This tint changes to green and blue, and finally the last glimpse which we see is that of a disappearing flash of blue or violet light.