THE SUN. The number of prominences represented around the disk of the Sun in the above view is probably much greater than the number of prominences ever actually visible at one time; but the form and height of each prominence are derived from actual observation. The granulation shown in the figure is coarser than that which is usually seen. A few faculæ appear near some of the spots close to the limb; and the two zones of spots are shown. 63. The average number of spots visible at once varies from time to time. A maximum of spots is said to have occurred at any time when the spots were more abundant than they were for some seasons before or after; and a minimum of spots occurs when spots are comparatively very scarce, so that there are often days when no spots can be seen. A maximum of spots occurs about once in every eleven years; but two maxima are not always just eleven years apart. For example, there was one maximum in 1848; the next did not come till 1860; and the next came in 1870. A minimum of spots usually comes not half-way between two maxima, but rather nearer the maximum that is to come next than to the previous one. For example, the minimum between the maxima of 1860 and 1870 came early in 1867. About the time of a minimum the spots on the whole lie nearer the Sun's equator than usual. 64. Besides this period of eleven years, there is another longer period of variation in the numbers of the solar spots. But the spots have not been carefully observed for a sufficient time to determine the exact length of this long period. It apparently occupies five of the short periods. 65. The reason why spots are more abundant at one time than at another is still unknown. There are some grounds, however, for thinking that any large object, like a planet, not too far from the Sun, tends to diminish the spotted area of that part of the photosphere nearest it, and perhaps to increase the spotted area on the opposite side of the Sun. 66. The photosphere is too bright to allow us to see much of what goes on in the Sun's atmosphere between us and the disk. Even round the limb, where there is of course much of this atmosphere which has not the disk behind it, an ordinary telescope is prevented by the sunlight which our own atmosphere reflects and scatters in all directions from showing us any thing remarkable in the atmosphere of the Sun. But this difficulty has been overcome by the invention of the spectroscope, an instrument which can be so applied to a telescope as to enable us to learn much about the Sun's atmos The unber of prominences represented around the disk of the Sun inte a ove view is probabiv much greater than the number of prominet ever actually visible at of time; but the form and height of promenence are der ed fro.: actual observation. The granulation shown in the figure is coarser th: that which is usually seen. few face appear near so.ne of the pots close to the lunb, and the two zone of spots are shown A |