This variation is also accompanied by a variation in the latitude of the spots. Generally they follow certain definite zones, mostly lying between 10° and 35° on either side of the solar equator. As a minimum approaches, they tend to appear nearer to the equator than usual; and when the minimum has passed the reappearance of the spots takes the opposite course, beginning in high latitudes.
It has further been ascertained that a close connection exists between the activity which results in the formation of sun-spots, and the electrical phenomena of our earth. Instances of this connection have been so repeatedly observed as to leave no doubt of its reality, though the explanation of it has still to be found. It has been suggested by Young that there may be immediate and direct action in this respect between the sun and the earth, an action perhaps kindred with that solar repulsive force which seems to drive off the material of a comet's tail. As yet not satisfactorily accounted for is the fact that it does not always follow that the appearance of a great sun-spot is answered by a magnetic storm on the earth. On the average the connection is established; but there are many individual instances of sun-spots occurring without any answering magnetic thrill from the earth. To meet this difficulty, Mr. E. W. Maunder has proposed a view of the sun's electrical influence upon our earth, which, whether it be proved or disproved in the future, seems at present the most living attempt to account for the observed facts. Briefly, he considers it indubitably proved—
1. That our magnetic disturbances are connected with the sun.
2. That the sun's action, of whatever nature, is not from the sun as a whole, but from restricted areas.
3. That the sun's action is not radiated, but restricted in direction.
On his view, the great coronal rays or streamers seen in total eclipses (Plate [VIII.]) are lines of force, and similarly the magnetic influence which the sun exerts upon the earth acts along definite and restricted lines. Thus a disturbance of great magnitude upon the sun would only be followed by a corresponding disturbance on the earth if the latter happened to be at or near the point where it would fall within the sweep of the line of magnetic force emanating from the sun. In proportion as the line of magnetic force approached to falling perpendicularly on the earth, the magnetic disturbance would be large: in proportion as it departed from the perpendicular it would diminish until it vanished finally altogether. The suggestion seems an inviting one, and has at least revived very considerably the interest in these phenomena.
Such, then, are the solar features which offer themselves to direct observation by means of a small telescope. The spots, apart from their own intrinsic interest, are seen to furnish a fairly accurate method by which the observer can determine for himself the sun's rotation period. Their size may be approximately measured, thus conveying to the mind some idea of the enormous magnitude of the convulsions which take place upon this vast globe. The spot zones may be noted, together with the gradual shift in latitude as the period approaches or recedes from minimum; while observations of individual spots may be conducted with a view to gathering evidence which shall help either to confirm or to confute the Wilsonian theory. In this latter department of observation the main requisite is that the work should be done systematically. Irregular observation is of little or no value; but steady work may yield results of high importance. While, however, systematic observation is desirable, it is not everyone who has the time or the opportunity to give this; and to many of us daily solar observation may represent an unattainable ideal. Even if this be the case, there still remains an inexhaustible fund of beauty and interest in the sun-spots. It does not take regular observation to enable one to be interested in the most wonderful intricacy and beauty of the solar detail, in its constant changes, and in the ideas which even casual work cannot fail to suggest as to the nature and mystery of that great orb which is of such infinite importance to ourselves.
A small instrument, used in the infrequent intervals which may be all that can be snatched from the claims of other work, will give the user a far more intelligent interest in the sun, and a far better appreciation of its features, than can be gained by the most careful study of books. In this, and in all other departments of astronomy, there is nothing like a little practical work to give life to the subject.
In the conduct of observation, however, regard must be paid to the caution given at the beginning of this chapter. Various methods have been adopted for minimizing the intense glare and heat. For small telescopes—up to 2½ inches or so—the common device of the interposition of a coloured glass between the eye-piece and the eye will generally be found sufficient on the score of safety, though other arrangements may be found preferable. Such glasses are usually supplied with small instruments, mounted in brass caps which screw or slide on to the ends of the various eye-pieces. Neutral tint is the best, though a combination of green and red also does well. Red transmits too much heat for comfort. Should dark glasses not be supplied, it is easy to make them by smoking a piece of glass to the required depth, protecting it from rubbing by fastening over it a covering glass which rests at each end on a narrow strip of cardboard.
With anything larger than 2½ inches, dark glass is never quite safe. A 3-inch refractor will be found quite capable of cracking and destroying even a fairly thick glass if observation be long continued. The contrivance known as a polarizing eye-piece was formerly pretty much beyond the reach of the average amateur by reason of its costliness. Such eye-pieces are now becoming much cheaper, and certainly afford a most safe and pleasant way of viewing the sun. They are so arranged that the amount of light and heat transmitted can be reduced at will, so as to render the use of a dark glass unnecessary, thus enabling the observer to see all details in their natural colouring. The ordinary solar diagonal, in which the bulk of the rays is rejected, leaving only a small portion to reach the eye, is cheaper and satisfactory, though a light screen-glass is still required with it. But unquestionably the best general method of observing, and also the least costly, is that of projecting the sun's image through the telescope upon a prepared white surface, which may be of paper, or anything else that may be found suitable.