The Sun, February 3, 1905. Royal Observatory, Greenwich.
The first telescopic view of the sun is apt, it must be confessed, to be a disappointment. The moon is certainly a much more attractive subject for a casual glance. Its craters and mountain ranges catch the eye at once, while the solar disc presents an appearance of almost unbroken uniformity. Soon, however, it will become evident that the uniformity is only apparent. Generally speaking, the surface will quickly be seen to be broken up by one or more dark spots (Plate [V.]), which present an apparently black centre and a sort of grey shading round about this centre. The margin of the disc will be seen to be notably less bright than its central portions; and near the margin, and oftenest, though not invariably, in connection with one of the dark spots, there will be markings of a brilliant white, and often of a fantastically branched shape, which seem elevated above the general surface; while as the eye becomes more used to its work it will be found that even a small telescope brings out a kind of mottled or curdled appearance over the whole disc. This last feature may often be more readily seen by moving the telescope so as to cause the solar image to sweep across the field of view, or by gently tapping the tube so as to cause a slight vibration. Specks of dirt which may have gathered upon the field lens of the eye-piece will also be seen; but these may be distinguished from the spots by moving the telescope a little, when they will shift their place relatively to the other features; and their exhibition may serve to suggest the propriety of keeping eye-pieces as clean as possible.
Photograph of Bridged Sunspot (Janssen). Knowledge, February, 1890.
The spots when more closely examined will be found to present endless irregularities in outline and size, as will be seen from the accompanying plates and figures. On the whole, there is comparative fidelity to two main features—a dark central nucleus, known as the umbra, and a lighter border, the penumbra; but sometimes there are umbræ which have no penumbra, and sometimes there are spots which can scarcely be called more than penumbral shadings. The shape of the spot is sometimes fairly symmetrical; at other times the most fantastic forms appear. The umbra appears dark upon the bright disc, but is in reality of dazzling lustre, sending to us, according to Langley, 54 per cent. of the amount of heat received from a corresponding area of the brilliant unspotted surface. Within the umbra a yet darker deep, if it be a deep, has been detected by various observers, but is scarcely likely to be seen with the small optical means which we are contemplating. The penumbra is very much lighter in colour than the umbra, and invariably presents a streaked appearance, the lines all running in towards the umbra, and resembling very much the edge of a thatched roof. It will be seen to be very much lighter in colour on the edge next the umbra, while it shades to a much darker tone on that side which is next to the bright undisturbed part of the surface (Figs. 14 and 15). Frequently a spot will be seen interrupted by a bright projection from the luminous surface surrounding it which may even extend from side to side of the spot, forming a bridge across it (Plate [VI.], and Figs. 16, 17, and 18). These are the outstanding features of the solar spots, and almost any telescope is competent to reveal them. But these appearances have to be interpreted, so far as that is possible, and to have some scale applied to them before their significance can in the least be recognised. The observer will do well to make some attempt at realizing the enormous actual size of the seemingly trifling details which his instrument shows. For example, the spot in Figs. 14 and 15 is identical with that measured by Mr. Denning on the day between the dates of my rough sketches; and its greatest diameter was then 27,143 miles. Spots such as those of 1858, of February, 1892, and February, 1904, have approached or exceeded 140,000 miles in diameter, while others have been frequently recorded, which, though not to be compared to these leviathans, have yet measured from 40,000 to 50,000 miles in diameter, with umbræ of 25,000 to 30,000 miles. Of course, the accurate measurement of the spots demands appliances which are not likely to be in a beginner's hands; but there are various ways of arriving at an approximation which is quite sufficient for the purpose in view—namely, a realization of the scale of any spot as compared with that of the sun or of our own earth.
FIG. 14.—SUN-SPOT, JUNE 18, 1889.
