I If any of our readers had provided themselves with a piece of smoked glass during any of the bright days which were so plentiful during the middle of last April, they might have seen through it a group of remarkable spots on the sun's disk. These spots have been observable more or less during the entire summer. On the fourth of September five of these spots reappeared, after a short period during which the sun was almost wholly free from spots. Two of these spots were of very great size, the entire surface covered by these and other smaller spots being more than one fifth the sun's diameter. These spots are of frequent occurrence, and although they cannot always be detected by the naked eye, there are few intervals when they cannot be detected with a telescope. We should not have felt called upon to say much about these spots at this time were it not for the fact that we are approaching a period when they are to be expected in greater numbers than at ordinary times. Mr. W. T. Lynn, of the Royal Observatory at Greenwich, England, says all things indicate that we are rapidly approaching a period of maximum of abundance and frequency of the solar spots. He estimates the most probable length of the interval between two consecutive maxima, as one ninth of a century, or eleven years and one month ; this would bring us to another maximum in the course of the year after next, 1871, probably about the middle or towards the end of it. As the period of the sun's rotation on its axis is 2534 days, and its apparent revolution is 27'3 days in consequence of the change in the position of the earth during a rotation of the sun, the time for the reappearance of spots after having passed behind the western limit of the sun, unless they should be dispersed before his semi-rotation is completed, may be readi ly computed. The April spots, or rather the spot, as although there were five distinct nuclei observed they were included in one penumbra, were estimated on the 13th of that month as being 55, 000 miles in length, and 30,000 in breadth, covering an area of about 150,000 square miles. Recent observations seem to put beyond all question that there is an intimate connection between disturbances in the sun's photosphere (light-sphere) and meteorological condi tions of the earth's atmosphere. Some of these observations have found a record in the late volumes of this journal, and our readers will recollect them perfectly, particularly an article entitled " Storms in the Sun," published on page 139, current volume. It is no wonder then that all solar phenomena should at the present time be of the most absorbing interest. We are probably on the eve of remarkable discoveries. The spectroscope is, in the hands of able investigators, throwing light on much that has been hitherto mysterious, and opening new avenues of research, the future of which it is impossible to predict. Two hypotheses have hitherto been entertained in regard to the nature of the solar spots, The first is, that the vaporous envelope is deeper and of greater density where the spots are seen, thus partially intercepting the light from the photosphere ; and the second is that the photosphere is broken up where the spots appear. The latter has been supposed by some to be caused by an upward rush of vapor from beneath ; but we need not say that all this speculation has been of no real value to science in the absence of any facts tending to support them. Mr. J. Norman Lockyer, P. R A. S a young English astronomer, who has achieved an enviable reputation as a sagacious and careful investigator, undertook, in 1866, to demonstrate if possible which of the two hypotheses, if either, was correct. Wo cannot, in the limits of this article, follow Mr. Lockyer through the extended labors ha brought to bear upon this subject. Suffice it to say he has by his perseverance developed a mass of facts in regar to the constitution of the photosphere which alone would render his name famous in tfie scientific world. The instrument upon which he chiefly relied was the spectroscope, and the conclusion at which he arrives is that neither of the hypotheses above stated is correct, but that the spots are produced by the sudden and downward rush of portions of the sun's atmosphere. It must be confessed, however, that Mr. Lockyer has yet to demonstrate the hypothesis—for hypothesis it must yet be called—which he Seeks to substitute for the ones he has discarded. Whatever these spots may be they must indeed be obstinate spots if they refuse to surrender to the " artillery of science " now leveled against them.
This article was originally published with the title "Solar Spots" in Scientific American 21, 21, 329-330 (November 1869)