In reflecting-telescopes with glass mirrors, effective views of the Sun are obtainable by employing unsilvered mirrors; for sufficient light is reflected by the glass surfaces to form good images of solar detail.
What, perhaps, interferes more than any other circumstance with successful observation of the Sun, is the fact that the rays, falling upon the telescope and objects near, induce a good deal of radiation, the direct tendency of which is to impair the definition and give a rippling effect to the disk. This is sometimes present in such force that the spots are subject to an incessant commotion, which serves to obliterate their more delicate features. A shady place is best, therefore, for such work; and if the observer leaves his telescope for a short time, intending to resume observations, it should never be placed broadside to the Sun, or the tube wall get hot, and heated currents must be generated in the interior, to the ruin of subsequent views.
A large sun-spot consists of an apparently black nucleus, a brown umbra, divided possibly by veins of bright matter or by encroachments of the penumbra which surrounds it. The latter is of much lighter tone than the umbra, though often similar in its general form. The outer edges of the umbra are serrated or scalloped by rice-grain protuberances. The inner region of the penumbra is much brighter than the outer, and the latter often exhibits quite a dusky fringe, induced by lines of dark material intervening with the brighter particles. The filaments forming the penumbra—often grouped in a radial manner with reference to the centre of a spot—would appear to be more widely separated near the outer border of the penumbra, and sufficiently so to allow sections of the umbral layer of the Sun to be observed through the interstices. The lighter tint of the interior part of the penumbra is stated to be due to contrast; but this is a mistake. The difference is too definite and distinct to permit such an explanation. Mr. Maunder says “that usually (not invariably) the penumbra darkens towards the umbra, and that the phenomenon as ordinarily described is merely an effect of contrast.” My own observations, however, appear to show that there is an actual difference of detail in the outer and inner portions of the penumbra, which gives a darker tone to the former.
In drawing the forms of sun-spots the observer must be expeditious, because of the variations which are quickly and constantly affecting them. In concluding a sketch I find it essential to make several alterations in it, owing to the changes which have occurred in the spots during the interval of a quarter of an hour or so since it was commenced. The details must be filled in consecutively, each one being the result of a careful scrutiny. When finished, the whole sketch should be compared with the object itself and amended if found necessary. The observer should also mark upon the sheet the measured or estimated latitude and longitude of the spot, and make a finished drawing from the basis of his sketch as soon as possible afterwards. At Stonyhurst Observatory excellent delineations of solar phenomena are made; and the late Father Perry, who lost his life in the cause of science, thus described the method:—“On every fine day the image of the Sun is projected on a thin board attached to the telescope, and a drawing of the Sun is made, 10½ inches in diameter, showing the position and outline of the spots visible. It is the first duty of the assistant who makes the drawings to note the position of the spots, and sketch their outlines. He then proceeds to shade in the penumbra and to draw the finer details, comparing the drawing from time to time by placing it alongside the projected image of the spots. The position of the faculæ is then filled in with a red pencil, so that the eye can at once recognize their grouping with respect to sun-spots, and the other details drawn with a black pencil.” The same astronomer also stated that, “as a general rule, careful drawings of the projected image of the Sun give much more satisfactory pictures of the solar surface than the photographs taken even at our best observatories. It is quite true that occasionally an exquisite photograph on an enlarged scale may be obtained, which exhibits features such as no pencil could portray as accurately, but rarely indeed will the photograph furnish all the details that a practised eye and hand, kept patiently at the sketch-board, will detect and faithfully describe. And the reason is not far to seek; for any experienced observer knows that, even on the finest day, the definition is continually changing with the sky, and that it is only at comparatively rare moments we can expect those perfect conditions that enable the finest details to stand out sharply, as Schiaparelli expresses it, like the faintest lines of a steel engraving. A photograph may be accidentally taken during one of these exceptionally favoured moments; but a patient draughtsman is almost sure to secure several of these best opportunities at each prolonged visit to his sketch-board. What would, therefore, be a great acquisition at present is a series of careful solar drawings, taken at short intervals of time, on days when characteristic spots are visible upon the Sun; and this would be the surest way of adding much valuable information to that already possessed concerning the changes that take place in the solar photosphere.”
With regard to ascertaining the dimensions of sun-spots, very precise results require accurate means of measurement and some mathematical knowledge. For the general purposes of the amateur, who will only want round numbers, simple methods may be adopted with success. I have used, on a 4-inch refractor, a graduated piece of plane glass, mounted suitably for insertion in the focus of the eyepiece, and marked with divisions 1/200 of an inch apart. With power 65 I find the Sun’s disk at max. distance covers 83 divisions of the graduated lens; so that one division = 22″·8, the Sun’s min. diameter being 1892″. Each division, therefore, is equal to 10,434 miles, the Sun’s real diameter being 866,000 miles.
Sun-spot of June 19, 1889, 2h P.M.
I viewed a large spot on June 19, 1889, and found its major axis covered 2·6 divisions, = 59″·3[9]; so that its apparent length was about 27,000 miles. For
1892″:866,000 miles :: 59″·3:27,143 miles.