Spots on the Sun

SUN-SPOTS appear to have been noticed occasionally for more than a thousand years, being mistaken at first for planets crossing the disk of the sun. One of the first fruits of the invention of the telescope was their discovery by Galileo and Fabricius to be appendages to the sun ; and its rotation on its own axis, in a little less than a month, determined from them. From that time they have been objects of frequent observation and increasing interest, as the principal means of studying the physical constitution of the sun ; and never so much so as at the present time, when the spectroscope and photography are added to the means of observation.

Many theories have been advanced with regard to them. Galileo thought they were pitchy scum, surmounted by clouds, thrown up from the heated body of the sun, and wafted round on an illuminated atmosphere. But his means of observation were imperfect. It did not occur to him to use a colored glass, and his observations were confined to periods when the sun was near the horizon or veiled by clouds or fog. When they came to be better observed, this theory was abandoned, as well as many others that succeeded it About 1773 Professor Alexander Wilson, of Glasgow, advanced another theory, founded upon a careful examination of the aspect of the spots ; which, with some modifications from late discoveries, obtains at the present time. His idea was that they were conical cavities in the illuminated surface of the sun, analogous to cyclones in the atmosphere of the earth, showing a darker stratum of the sun at the bottom. The normal condition of a well-developed sun-spot may be described as consisting of a very dark centre, varying in shape, called the umbra, which is the lower and darker stratum at the bottom of the vortex, surrounded by a gray marginal disk, much larger, called the penumbra, which is the conical or funnel-shaped cavity in the luminous surface. Sometimes a still darker spot is seen on the umbra, called the nucleus, which is presumed to be an opening into a still lower stratum. The sun rotates in about 25 1/2, days ; but as the earth advances in its orbit in the same direction, its apparent rotation to us is 27.3 days ; and the average passage of a spot across the disk is about 13 1/2 days. As the spot first makes its appearance on the eastern edge of the hemisphere, it is seen only as a long, narrow line of the penumbra, — as if one were approaching the crater of a volcano from a level, and got sight of the opposite side of the crater. The next day, as it advances, a glimpse of the umbra is seen, as it over the edge of the nearest side of the cone, and so on, till it approaches the centre of the disk, and gives a view into the whole cavity, as if one were suspended in a balloon directly over the crater. As the spot rotates towards the western edge these aspects are reversed. It was from these appearances that Wilson founded his theory. We know now, that what we see of the sun, except through these spots, and at the time of a total eclipse, is an intensely luminous envelope, called the photosphere, consisting of incandescent metallic gases, in which iron, copper, sodium and many other metals are held in a gaseous condition by the intense heat. The idea now is that these gases, becoming cooled by contact with external space, or a cooler external atmosphere, are precipi tated, and rush in descending currents towards the centre, breaking away the photosphere, and exposing lower and less luminous strata. This is confirmed by the spectroscope, which shows the presence of descending currents in the penumbra ; and by stereoscopic views, which present the spots as funnel-shaped cavities. It was once thought that the umbra was part of a solid crust of the sun, it there is anything solid there, but the spectroscope shows it to be composed of incandescent gases, in constant agitation, and it would doubtless appear luminous, were it not in such juxtaposition with the photosphere.

But this is not the only aspect of sun-spots. They are often seen in small patches of dark umbra, without any penumbra, sometimes singly, but oftener in groups ; appearing, through a telescope of moderate size, like black beads dropped upon the luminous surface. They must be several hundred miles in diameter, however, to be seen at all. Many of these close up without any further development ; and late observations this season suggest the possibility that they may sometimes be vents, through which ascending currents of flame may be rising. Others increase in size, develop the penumbra, and assume the normal type. In this condition they seem to be endowed with a degree of permanence ; not only passing across the disk of the sun, but returning in subsequent rotations. A few have been traced through six rotations. Sometimes they appear and disappear quite suddenly. Welldeveloped spots, two or three times the diameter of the earth, were seen one day last summer, where there was nothing but the unbroken photosphere the day before ; others have disappeared in one or two days. Sometimes they increase to vast proportions, separate into fragments, throw off large patches of penumbra, with little or no umbra, scatter, and disappear. In this last condition the cavities seem to become much more shallow than before, and the penumbra widely diffused, so that the umbra may be traced quite near the edge of the sun.

Accompanying the spots are to be seen lines and wreaths of luminous matter, more brilliant than the photosphere, reminding one of snow-drifts seen in sunshine, which are called flaculæ. Stereoscopic views show them to be elevated ridges of the luminous matter of the photosphere, crowded up into corrugations by the disturbing forces which form the spots.

Sun-spots have received more or less attention from almost every astronomer since their discovery. But two men have distinguished themselves by their devotion to them, — Holfath Schwabe of Dessau, and Richard C. Carrington of Red Hill in England. Schwabe has spent forty years in observing them, and collecting and analyzing all the information to be obtained from other sources. Carrington erected an observatory for their examination, and made very accurate drawings of them, from a graduated screen, every fine day for eight years, from 1853 to 1861 ; the result of which was published in a beautiful quarto volume, which received the gold medal from the French Academy of Science. From all these sources some progress has been made in the study of their habits. It has been ascertained that they are governed by periodic laws. They increase gradually, in numbers and size, till they reach a maximum ; and then decrease to a minimum, going through the cycle in about 10 1/2 years. During the maximum not a day passes without their appearance, and they may be often counted by scores and hundreds, while the region they frequent is corrugated with flaculæ. In the minimum they become very rare, many weeks passing without one, and the surface of the photosphere is quite smooth. This periodicity is found to coincide with the distance of the planet Jupiter from the sun, the maximum with the greatest distance ; though the nature of the relation has not yet been ascertained. A careful analysis of Carrington’s observations has lately shown other minor fluctuations, which coincide with the movements of Venus and the Earth, thus confirming the probability of planetary influence ; and it will be fair to infer that the other planets exert similar influences, in proportion to their size and distance.

This becomes very suggestive with regard to the phenomena of those variable stars which periodically vary in brilliancy-. It has been calculated that the difference in the luminous surface of our sun, at the maximum and minimum of spots, is sufficient to make it a variable star, with a cycle of 10 1/2 years. We have only to suppose a variable star to be accompanied by planets of adequate size, or orbits sufficiently eccentric, to account for almost any amount of variability.

It has also been ascertained that the spots are chiefly confined to two zones, between the latitude of 10° and 30°, corresponding to the cyclonic regions on the earth. At the minimum periods a few lap over into the equatorial regions ; then, as they increase, they break out about the latitude of 30° and gradually' spread over the whole zone. A few extend beyond 30°, but never into the polar regions. In five thousand three hundred observed by Carrington, but one reached the latitude of 47 1/2°.

While they rotate with the photosphere they also have a proper motion of their own, the laws of which are not yet established. They recede from, and approach each other. The late W. C. Bond noted one that passed over the sun’s disk in twelve days, which would indicate a movement of its own of more than ten thousand miles a day. Carrington found that they moved faster in low latitudes than in high, and that they had a drift towards the equator. Whether these movements are owing to proper motions in the spots, or currents in the photosphere, is yet to be ascertained. There seems to be a disposition in them to preponderate in the northern hemisphere. It has been decidedly so this season ; and one side of the sun has been more spotted than the oppoposite. This suggests the question, whether they may not be connected with tides in the photosphere. We know it is relatively thin, and an ocean of gas must be much more sensitive to external influences than one of water.

It has also been ascertained that the maxima and minima coincide with the variations of the magnet on the earth, thus establishing a correlation with our planet. This has been sometimes manifested in a more spasmodic manner by magnetic storms and brilliant auroras. The most remarkable of these occurred on the 1st September, 1859. On that day Messrs. Carrington and Hodgson were simultaneously observing the same spot, at places remote from each other, when they were surprised by the sudden appearance and passage of a brilliant stream of light over the umbra. It was found that at the same moment the self-registering magnets at Kew indicated a violent and unusual oscillation, which was followed by a great magnetic and electric storm. Telegraphic communication was interrupted. In this country the operators disconnected the batteries and worked by the fluid with which the atmosphere was surcharged. Some stations were set on fire, and some operators injured by excessive discharges of electricity ; and very brilliant auroras were seen in both hemispheres for two or three days, that at the north extending to Cuba. Thus the whole earth seemed to thrill in response to this movement on the sun.

There is a strong impression that hot and dry weather on land is coincident with the maximum of sun-spots. But we have not yet accumulated accurate meteorological observations sufficiently extended in time and space to decide this question. During the past year we have been passing through one of the maximum periods of demonstration, and not a day has passed without their appearance, while they have often been present in scores and even hundreds. Sometimes large spots, groups, and bands have been perfectly visible to the eye, only screened by a colored or smoked glass. In March there was a magnificent pearshaped cluster, probably one hundred and fifty thousand miles long, nearly twenty times the diameter of the earth. In November an immense spot was visible, apparently breaking up, throwing off large patches of penumbra, and exhibiting an uncommonly large nucleus. In the latter half of August a grand procession of groups occupied the northern zone, visible to the screened eye as a dusky girdle, like one of the bands of Jupiter. When the opposite side of the sun was turned, only three small spots were seen, the minimum of the season. But on the next rotation the whole procession reappeared, somewhat altered, but quite recognizable. One of those spots was very remarkable, passing through extraordinary changes and most picturesque forms. On the 20th of September it was measured at an observatory in England, the penumbra found to be seventy-two thousand miles long and fifty thousand miles wide, and the umbra thirty-one thousand five hundred miles long. There was room enough on the penumbra to place fifty spheres the size of the earth. On the 26th it had separated into two penumbræ, each containing two pear-shaped umbra, with all their apices pointing towards a common centre of motion, while a vast river of penumbra, marked with small dots of umbra, flowed around the southern spot and through the space between them, more than one hundred thousand miles long, towards the sun’s centre. But another interest attaches to this spot.

It is well known that when the sun is totally eclipsed, there are tongues or columns of rosy light, seen here and there along the edge of the sun, projecting beyond the disk of the moon, called prominences or protuberances, which are entirely invisible at other times to the most powerful telescope, owing to the superior brilliancy of the photosphere, and which the spectroscope has shown to be luminous gas, in which the lines of hydrogen and magnesium have been found. During the last two years these have not only been analyzed, but also revealed to sight by the spectroscope.

During the past summer Professor C. A. Young of Dartmouth College has been diligently observing them, with a new compound spectroscope, designed by himself, and constructed by Alvan Clark and Sons of Cambridgeport. He has not only seen them around the sun, but also on its disk, and has even photographed one of them. One of them he describes as rising in a column of light ninety thousand miles high, and then spread out in the shape of a mushroom. Others rose in parallel columns, and were then deflected at right angles towards each other. One was composed of separate luminous clouds ten or fifteen thousand miles long, one above the other, as if thrown out in separate jets. Once he caught sight of a single cloud soaring up diagonally at the rate of one hundred and twenty miles in a second ; all indicating the existence of an extensive atmosphere outside the photosphere.

On the 28th of September, at 4 P. M. he was observing the southern umbra of this spot, when he discovered one of these protuberances issuing, or as he expresses it “originating from,” this spot. He held it in sight for an hour, during which it gradually faded. He analyzed it, and found it identical with the protuberances around the edge of the sun. He drew its form. He measured it and found it to be one hundred and thirty thousand miles long; thus establishing for the first time a connection between the spots and the protuberances. This spot, considering the vast area over which it was spread, the velocity of motion indicated by its rapid changes of form, and its connection with the protuberance, is the most interesting of the season, and illustrates in a striking manner the prodigious forces active on the sun’s surface.

It will be seen that sun-spots are of much interest as objects of astronomical observation, and important as elucidating that great problem which is now exercising so many acute minds, What is a sun ? Whence does it derive its light and heat, the sources, under Providence, of all life and energy to the surrounding planets? We have just been passing through one of the interesting periods of their maximum demonstration, and yet no regular and systematic observations of these spots have been made at any New England observatory; not for want of interest, but for lack of means. At Cambridge, with a telescope and meridional circle unsurpassed in size and excellence, and a superintendent able, skilful, and earnest, the funds are insufficient to employ an adequate corps of observers to work the instruments to their full capacity of usefulness. At Dartmouth one of our ablest observers is deficient in both instruments and assistants. Will not some of our wealthy men, who are looking for wise methods of bestowing their abundance, place our observatories in a condition of efficiency ? They are means of education, not local or temporary, but for the whole world and all time.

NOTE. I learn from Professor Young that the magnetic record at Greenwich for 28th September indicated an unusual disturbance, simultaneous with the appearance of the protuberance which he observed.

J. J. Dixwell.