Astronomy for Young Folks

by Isabel Martin Lewis

Now, a freely suspended compass needle carried to all parts of the earth will behave very much in the same manner as the needle moved over the magnetized steel sphere. There are two points on the earth's surface, known as the North and South Magnetic Poles, where the needle points vertically downward and approximately midway between is the Magnetic Equator where the compass needle places itself in a perfectly horizontal position and the "dip" of the needle is zero. In other words, the earth acts as a huge magnet and possesses a magnetic field with lines of force converging towards its poles similar to the lines of force of the steel sphere.

There are, however, some very important differences between the sphere of steel and our earth. The matter of which the earth is composed is not homogeneous. It is believed to possess an iron core of considerable size, it is true, but its outer shell is composed of heterogeneous masses that in certain regions cause very appreciable local deflections of the needle. It is surrounded, moreover, by an atmosphere permeated by electrified particles of matter shot forth from the sun, which we now know is a still greater magnet surrounded by a magnetic field that is of the order of 50 gausses at the poles and about eighty times more powerful than that of the earth.

It is now a well-established fact that the sun's magnetic field exerts a powerful influence over the condition of the earth's magnetic field, and that vast solar disturbances affect very materially the direction and intensity of the lines of force.

It is thus little wonder that this non-homogeneous and rapidly rotating terrestrial globe, surrounded by an electrified atmosphere and subject to the action of a still more powerful magnet, the sun, should not behave in a manner exactly analogous to a uniformly magnetized steel sphere.

The earth's magnetic poles are neither symmetrically placed nor absolutely fixed in position. There is every reason to suspect that they shift about from year to year, and possibly fluctuate irregularly in position in the course of a few days or hours under the influence of disturbing forces. The position of the earth's North Magnetic Pole, last visited by Amundsen in 1903, now lies approximately in Latitude 70° N. Longitude 97° W. The position of the South Magnetic Pole, according to the latest determinations, is, in round numbers, in Latitude 73° S. and Longitude 156° E. of Greenwich. It is evident, therefore, that the magnetic poles of the earth are not symmetrically placed and that they lie fully 30° from the geographical poles. The chord connecting the magnetic poles passes 750 miles from the earth's center, and it is about 1,200 miles from the geographic pole to the nearest magnetic pole. There exist, moreover, in high latitudes local magnetic poles, due possibly to heavy local deposits of ore. One such pole was discovered at Cape Treadwell, near Juneau, Alaska, during Dr. L. A. Bauer's observations there in 1900 and 1907. In the center of the observing tent at this point the needle pointed vertically downward and the compass reversed its direction when carried from one side of the tent to the other.

It is a well-known fact that there are very few points on the earth's surface where the compass needle points either to the true geographical pole or to the magnetic pole, and if it does chance to do so, it is a transient happening. The "variation of the compass" or the declination of the needle, as it is called, is the angle that the compass needle makes with the true north and south line or the meridian. It is an angle of greatest importance to navigators and explorers, for it gives them their bearings, yet it is unfortunately subject to ceaseless variations of a most complicated nature, since it depends on the constantly pulsating and never ceasing magnetic changes that sweep over the surface of the earth and through its crust. It is affected by long period or secular changes, as they are called, progressing more or less regularly in obscure cycles of unknown period. It is subject to a diurnal change that depends on the position of the sun relative to the meridian, and that varies with the seasons and with the hour of the day. It is affected by the sun spot cycle of 11.3 years which has a direct effect upon the intensity of the earth's magnetic field. The intensity of the magnetic field in sun spots is, according to Abbot, sometimes as high as 4,500 gausses or 9,000 times the intensity of the earth's field. At times of maximum spottedness of the sun the intensity of the earth's magnetic field is reduced.

Moreover, when great and rapidly changing spots appear upon the sun, electrified particles are shot forth from the sun with great velocity and in great numbers, and are drawn in towards the magnetic poles of the earth. Meeting the rarefied gases of the earth's upper atmosphere, they illuminate them as electric discharges illuminate a vacuum tube. Some of these electrons are absorbed by gases at high elevations, other descend to lower levels. The most penetrating rays have been known to descend to an altitude of twenty-five miles which is about the lowest limit yet found for auroral displays. It is the passage of these rays through the atmosphere that cause the magnetic disturbances known as magnetic storms, that are associated with the appearance of great sun spots and auroral displays. At such times sudden changes take place in the intensity of the earth's magnetic field that cause the compass needle to shiver and tremble and temporarily lose its directive value. These magnetic storms have been known to produce great temporal changes in the intensity of the earth's field. According to Dr. L. A. Bauer, Director of the Department of Terrestrial Magnetism of the Carnegie Institute of Washington, the earth's intensity of magnetization was altered by about one-twentieth or one-thirtieth part by the magnetic storm of September 25, 1909, which was one of the most remarkable on record, and the earth's magnetic condition was below par for fully three months afterwards as a result.

In addition to these various regular and irregular changes in the variation of the compass, or declination of the needle, due to changes in the earth's magnetic field as a whole, there are local effects due to restricted regional disturbances of the earth's magnetic field or to local deposits of ore, or to volcanoes or other local causes. The effect of all these disturbances upon the declination of the needle must be determined by continual magnetic surveys of all portions of the earth's surface.

As a whole the earth's magnetic field is more uniform over the oceans than over the land, with all its disturbing topographical features. Yet this advantage is offset largely in navigation by the fact that every steel ship that sails the seas is a magnet, with its two magnetic poles and its neutral line where the two opposite magnetic forces are neutralized, as is the case with every magnet. The direction in which a steel ship lies with reference to the earth's magnetic field while it is being built determines the position of the magnetic poles in its hull and the position of its neutral line and this distribution of magnetism over a ship's hull must be taken account of in the installation of its standard compass. Every piece of horizontal and vertical iron aboard ship has an effect upon the variation of the compass and compensation must be made for such disturbing forces. The direction of sailing, the position in which a ship lies at dock, storms encountered at sea, the firing of batteries (on warships) are some of the factors that are operative in producing changes in the variation of the magnetic compass aboard a ship.

Every ship must undergo at frequent intervals magnetic surveys for the purpose of determining its magnetic constants and its "Table of Deviations of the Compass."