The sun outweighs fully seven hundred and forty times the combined mass of all the planets which circle around him, so that, when we regard the energy of his attraction, we still find him a worthy ruler of the planetary scheme. Viewed with the naked eye, the sun appears only as a luminous mass of intense and uniform brightness; but when examined with the telescope, his surface is frequently observed to be mottled over with a number of dark spots, of irregular and ill-defined forms, constantly varying in appearance, situation, and magnitude. These spots are occasionally of immense size, so as to be visible even without the aid of the telescope; and their number is frequently so great that they occupy a considerable portion of the sun’s surface. Sir W. Herschel observed one in 1779 the diameter of which exceeded 50,000 miles, more than six times the diameter of the earth; and Scheiner affirms that he has seen no less than fifty on the sun’s disk at once. Most of them have a deep black nucleus, surrounded by a fainter shade, or _umbra_, of which the inner part, nearest to the nucleus, is brighter than the exterior portion. The boundary between the nucleus and umbra is in general tolerably well defined; and beyond the umbra a stripe of light appears more vivid than the rest of the sun. [Illustration: Fig. 30.—Phase of Spot] The discovery of the sun’s spots has been attributed to Fabricius, Galileo, and Scheiner, and has been claimed for the English astronomer Harriot. Amid these conflicting pretensions it is perhaps impossible to arrive at the truth; but the matter is of little importance; the discovery is one which followed inevitably that of the telescope, and an accidental priority of observation can hardly be considered as establishing any claim to merit. The study of solar physics may be said to have commenced with the discovery of the sun spots, about two hundred and sixty years ago. These spots were presently found to traverse the solar disk in such a way as to indicate that the sun turns upon an axis once in about twenty-six days. Nor will this rotation appear slow, when we remember that it implies a motion of the equatorial parts of the sun’s surface at a rate exceeding some seventy times the motion of our swiftest express train. Next came the discovery that the solar spots are not surface stains, but deep cavities in the solar substance. The changes of appearance presented by the spots as they traverse the solar disk led Dr. Wilson to form this theory so far back as 1779; but, strangely enough, it is only in comparatively recent times that the hypothesis has been finally established, since even within the last ten years a theory was put forward which accounted satisfactorily for most of the changes of appearance observed in the spots, by supposing them to be due to solar clouds hanging suspended at a considerable elevation above the true photosphere.