FIG. 15.—SUN-SPOT, JUNE 20, 1889.
Of these methods, the simplest on the whole seems to be that given by Mr. W. F. Denning in his admirable volume, 'Telescopic Work for Starlight Evenings.' Fasten on the diaphragm of an eye-piece (the blackened brass disc with a central hole which lies between the field and eye lenses of the eye-piece) a pair of fine wires at right angles to one another. Bring the edge of the sun up to the vertical wire, the eye-piece being so adjusted that the sun's motion is along the line of the horizontal wire. This can easily be accomplished by turning the eye-piece round until the solar motion follows the line of the wire. Then note the number of seconds which the whole disc of the sun takes to cross the vertical wire. Note, in the second place, the time which the spot to be measured takes to cross the vertical wire; and, having these two numbers, a simple rule of three sum enables the diameter of the spot to be roughly ascertained. For the sun's diameter, 866,000 miles, is known, and the proportion which it bears to the number of seconds which it takes to cross the wire will be the same as that borne by the spot to its time of transit. Thus, to take Mr. Denning's example, if the sun takes 133 seconds to cross the wire, and the spot takes 6·5, then 133 : 866,000 : : 6·5 : 42,323, which latter number will be, roughly speaking, the diameter of the spot in miles. This, method is only a very rough approximation; still, it at least enables the observer to form some conception of the scale of what is being seen. It will answer best when the sun is almost south, and is, of course, less and less accurate as the spot in question is removed from the centre of the disc; for the sun being a sphere, and not a flat surface, foreshortening comes largely and increasingly into play as spots near the edge (or limb) of the disc.
Continued observation will speedily lead to the detection of the exceedingly rapid changes which often affect the spots and their neighbourhood. There are instances in which a spot passes across the disc without any other apparent changes save those which are due to perspective; and the same spot may even accomplish a complete rotation and appear again with but little change. But, generally speaking, it will be noticed that the average spot changes very considerably during the course of a single rotation. Often, indeed, the changes are so rapid as to be apparent within the course of a few hours. Figs. 14 and 15 represent a spot which was seen on June 18 and 20, 1889, and sketched by means of a 2½-inch refractor with a power of 80. A certain proportion of the change noticeable is due to perspective, but there are also changes of considerable importance in the structure of the spot which are actual, and due to motion of its parts. Mr. Denning's drawing ('Telescopic Work,' p. 95) shows the spot on the day between these two representations, and exhibits an intermediate stage of the change. The late Professor Langley has stated that when he was making the exquisite drawing of a typical sun-spot which has become so familiar to all readers of astronomical text-books and periodicals, a portion of the spot equal in area to the continent of South America changed visibly during the time occupied in the execution of the drawing; and this is only one out of many records of similar tenor. Indeed, no one who has paid any attention to solar observation can fail to have had frequent instances of change on a very large scale brought under his notice; and when the reality of such change has been actually witnessed, it brings home to the mind, as no amount of mere statement can, the extraordinary mobility of the solar surface, and the fact that we are here dealing with a body where the conditions are radically different from those with which we are familiar on our own globe. Changes which involve the complete alteration in appearance of areas of many thousand square miles have to be taken into consideration as things of common occurrence upon the sun, and must vitally affect our ideas of his constitution and structure (Figs. 16, 17, 18).
FIG. 16.—SUN-SPOT SEEN IN 1870.
Little more can be done by ordinary observation with regard to the spots and the general surface. Common instruments are not likely to have much chance with the curious structure into which the coarse mottling of the disc breaks up when viewed under favourable circumstances. This structure, compared by Nasmyth to willow-leaves, and by others to rice-grains, is beautifully seen in a number of the photographs taken by Janssen and others; but it is seldom that it can be seen to full advantage.
FIG. 17.—ANOTHER PHASE OF SPOT (FIG. 16).
