Solar Observations.—Early notices of Spots.—Difficulties of the old observers.—Small instruments useful.—Tinted glass.—Solar Diagonal.—Structure of a Spot.—Methods of Drawing.—Ascertaining Dimensions.—Observer’s aims.—Eclipses of the Sun.—Periodicity of Spots.—Crateriform structure.—“Willow-Leaves.”—Rotation of the Sun.—Planetary bodies in transit.—Proper motion of Sun-spots.—Rise and decay of Spots.—Black Nuclei in the umbrÆ.—Bright objects near the Sun.—Cyclonic action.—Sudden outbursts of FaculÆ.—Shadows cast by FaculÆ.—Veiled Spots.—Recurrent disturbances.—Recurrent forms.—Exceptional position of Spots.—The Solar prominences.

The Sun is not an object comprehended in the title of this volume. But to have omitted reference to a body of such vast importance, and one displaying so many interesting features to the telescopic observer, would have been inexcusable. We may regard the Sun as the dominant power, the controlling orb, and the great central luminary of our system. The phenomena visibly displayed on his surface assume a particular significance, as affecting a body occupying so high a place in the celestial mechanism.

The mean apparent diameter of the Sun is 32' 3·6, and his real diameter 866,000 miles. The apparent diameter varies from a minimum of 31' 32 at the end of June to a maximum of 32'36; at the end of December; and the mean value is reached both at the end of March and September. The Sun’s mean distance from the Earth is about 92,900,000 miles, computed from a solar parallax of 8·8, which appears to agree with the best of recent determinations. At this distance the linear value of 1 of arc is 447 miles.

The Sun’s apparent diameter is as follows on the first day of each month:—

Solar observations may be pursued with a facility greater than that attending work in some other departments of practical astronomy. The Moon, planets, and stars have to be observed at night, when cold air, darkness, and other circumstances are the cause of inconvenience; but the student of the Sun labours only in the light and warmth of genial days, when all the incidentals to observation may be agreeably performed. There are, however, some drawbacks even in this pleasant sphere of work. The light of the Sun is so great that much persistent observation is apt to have an injurious effect on the eye, and will certainly deaden its sensitiveness on faint objects. In the summer months the observer experiences discomfort during a lengthy observation from remaining so long in the powerful rays of the Sun, some of which must fall upon his face unless measures are adopted to shield it. During the progress of solar work the student should always provide for himself as much shelter as possible from the glare, which must otherwise disturb that equanimity of feeling in the absence of which no delicate research is likely to be successfully conducted.

“Spots on the Sun” were remarked long before the telescope came into service. In the early Chinese annals many references are made to these objects; thus, in A.D. 188, February 14, it is recorded—“The colour of the Sun reddish-yellow; a fleckle in the Sun (bird-shaped).” Other ancient notices compare the spots to a flying bird, an apple, or an egg. Many spots were seen in later years, especially in 321, 807, 840, 1096, &c. In 807 a large black spot upon the Sun was watched during a period of eight days. It reflects much credit upon observers of a past age that they performed so many useful feats of observation, though relying simply upon the powers with which Nature alone had endowed them. They anticipated the telescope in some important discoveries. Large sun-spots are not, it is true, difficult features to perceive with the naked eye under certain circumstances; for whenever there is a fog or haze sufficiently dense to veil the lustre of the Sun in suitable degree, they can be readily seen, presuming, of course, that such spots are in existence at the time. They are sometimes observed, in a purely casual way, by people who may happen to glance at the Sun when he is involved in fog and looks like a dull, red ball suspended in the firmament. On one occasion, near sunset, in the autumn of 1870, I saw four large spots on different parts of the Sun, and these phenomena were very numerous at about this time. When spots attain a diameter of 50 or more they may be detected by persons of good sight; but if the Sun is high and clear, coloured glass must be used to defend the eye.

Doubt hangs over the question as to the first telescopic observer of the spots. It is certain that Fabricius, Galilei, Harriot, and Scheiner all remarked them in about the year 1611; and of these Fabricius perhaps deserves the chief praise, as the first who published a memoir on the subject. Galilei appears undoubtedly to have had priority in recognizing the bright spots, or faculÆ. Scheiner discovered that the black spots, or maculÆ, are composed of a dark umbra and a fainter outlying shade, called the penumbra. Arago quotes him as having also described the Sun as “covered over its whole surface with very small, bright, and obscure points, or with lively and sombre streaks of very slender dimensions, crossing each other in all directions.” He announced, too, that the spots were confined to a narrow zone on the north and south sides of the equator, and this he termed the “Royal Zone.”

Some grave difficulties appear to have marked the attempts of the earlier observers; for they did not all use coloured glasses, and the dazzling light of the Sun, intensified by their lenses, often overpowered the sight, and so we find them awaiting opportunities when fog partly obscured the Sun near his rising or setting. Thus Harriot, who seems to have noticed and figured three sun-spots as early as 1610, Dec. 8, says:—“The altitude of the Sonne being 7 or 8 degrees, and it being a frost and a mist, I saw the Sonne in this manner.” His drawing followed. On another occasion he says:—“A notable mist: I observed the Sonne at sundry times, when it was fit.” Fabricius advised other observers to commence their observations by admitting only a small portion of the Sun into the field, so that the eye might be prepared to receive the light of the entire disk. Galilei was equally unaware of the advantage of tinted glass, and adopted the expedient of scanning the Sun when placed in the vicinity of the horizon. He remarks that “the spot of 1612, April 5 appeared at sunset;” and his writings contain other references of similar import. Scheiner, however, appears to have been more alive to the requirements of the work, and employed a plain green glass placed in front of the object-lens of his telescope.

Under the various circumstances we have been alluding to, the views obtained of the solar surface must necessarily have been of a very defective character, and the old observers at least deserve our sympathy in their exertions. No such obstacles confront the observer now. He has everything provided for him. Instrumental devices rob the Sun of his noonday brilliancy, and the eye serenely scans the details of his expansive image without the slightest pain or effort.

Small telescopes are peculiarly well adapted for solar observations. A good 3-inch refractor or 4-inch reflector will reveal an astonishing diversity of structure in the spots, and show something of the complicated minutiÆ of the general surface. If the aperture of either instrument is 2 inches more than that stated, so much the better; but further than this it is rarely advisable to go. When the objective or mirror exceeds a diameter of 5 or 6 inches a stop often improves the images, and even smaller instruments will perform better when a little contracted. Definition is here the point to be desired; of light we have a superabundance. But if the observer meditates a critical analysis of the detail, either of a single spot, of a group of spots, or of a small area of the luminous surface, then a fair amount of aperture should be used, because greater aperture means greater separating power, and the latter will be useful in resolving the network of fibrous materials of which apparently the whole surface is composed. But for the common requirements of the observer an instrument of 3 or 4 inches will be found very effective, and it can either be used on a short tripod stand, placed on a steady table near a window having a south aspect, or it may be mounted on a tall garden stand and, according to the owner’s pleasure, either fixed at his window or in his garden. Two powers will be really necessary—one of about 60 and a field of quite 33 to contain the entire disk and give a good general view, and another of 150 to which the observer will have recourse when examining details. Additional eyepieces will be sometimes useful, especially one of about 100; but the power of 60 previously recommended will, if a Huygenian, answer the same purpose, for if the field-lens is removed it will be increased to about 90. And should the observer think that anything is to be gained by a higher magnifier than 150, let him use the eye-lens only of that power. I have obtained many exquisite views of sun-spots with a single lens, and, instead of purchasing new eyepieces, a real advantage will be derived in adopting the plan suggested. There will be a smaller field and more colour about the image, but the improvement in definition is considerable, and more than balances these disadvantages.

Tinted glass must always be employed, unless a dense fog prevails, in which case the example of the old observers may be emulated. Several coloured glasses, of various depths, are needed for use according as the occasion requires. With a high Sun on a bright June day a darker tint will be necessary than in the winter, when the Sun’s rays are but feebly transmitted through the horizontal vapours. Red glass is unsatisfactory, as there is much heat and glare with it; but when used in combination with green the effect is excellent. Green alone is often used, and answers well; but it is not always thick and dense enough for the purpose. The plan of Sir W. Herschel, to interpose a glass trough of diluted ink, has never become popular, though he found it to succeed admirably. Smoked glass is also adapted for solar work, and recommends itself as being always obtainable at a minute’s notice. Some observers use a Barlow lens, with a thin film of silver deposited on the surfaces. It is then sufficiently transparent to give a neutral tint when held before a light, and sharp definition is said to be obtained without additional protection. Mr. Thornthwaite has also employed a coloured Barlow lens with effect.

A solar diagonal is a very necessary appliance if the observer would ensure perfect safety; for any refractor exceeding 2-inches aperture may, when turned on the Sun, focus enough heat to fracture the tinted sun-glass. The diagonal, by preserving a part only of the solar rays which are transmitted by the object-glass, enables observations to be made in security. This little instrument is comparatively cheap, and no telescope is complete without one. Dawes’s solar eyepiece serves the same purpose in a different manner, but it is an expensive luxury. In the latter construction there is a perforated diaphragm fixed near the eyepiece and so arranged that the quantity of admitted light may be modified consistently with the observer’s wishes.

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.

I viewed a large spot on June 19, 1889, and found its major axis covered 2·6 divisions, = 59·39; so that its apparent length was about 27,000 miles. For

1892:866,000 miles :: 59·3:27,143 miles.

The same method may be adopted if the image is thrown upon a screen.

Approximate values are to be obtained by means of fine cross wires fixed in the eyepiece. Note the exact interval occupied by the Sun in crossing the vertical wire, and also the interval occupied by the large spot or group. If the Sun is 133 seconds in passing the wire, and the group 6·5 seconds, then

133 seconds:866,000 miles :: 6·5 seconds:42,323 miles.

This plan is likely to be most successful when the Sun is near its meridian passage; but it may be applied at any hour, if care is taken to adjust the eyepiece so that the Sun’s motion is precisely at right angles to the vertical wire. One other plan may be mentioned. Draw on cardboard, with compasses, a circle about 10 or 12 inches diameter, and divide this with 31 parallel lines. Subdivide each of the spaces into 5, less prominently marked. Then, during observation, keep both eyes open, and hold or fix the circular disk at a distance enabling it to coincide with the telescopic image of the Sun. By carefully noting how many divisions the group covers on the cardboard, its dimensions may be readily found, because one division will be equal to about 5410 miles. Of course these methods10 are simply approximate, and only strictly applicable to objects not far removed from the central regions of the Sun, because the spots are portions of a sphere, and not angles subtended by a flat surface. When close to the E. or W. limbs, foreshortening is considerable, though the polar diameter of a spot is not affected by it then.

Presuming an observer to have his 3-or 4-inch telescope duly fitted with a solar diagonal and tinted glass, he may naturally ask, after his curiosity has been satisfied by the contemplation of his first sun-spot, what he can do further: What special features is he to look for? What changes ought to be recorded? What are the doubtful points that require to be cleared up as regards the Sun’s physical appearance? In what way are new and novel facts likely to be glimpsed? In a word, he desires to know in what manner he may employ his eyes and instrument usefully for science, while also gaining pleasure for himself. Information like this is often needed by the young student, and sometimes indeed by men who have already gained a little experience, and who possess much larger instruments than we have intimated above. In endeavouring to offer suggestions in response to such inquiries, I would remark that the nature and direction of a research essentially depend upon several conditions, viz. the observer’s inclination, his instrumental equipment, his place of observation, and the amount of time he can devote to the pursuit of his object. There are very few men who, like Schwabe of Dessau, will confront the Sun on nearly every day for more than forty years in order to learn something of its secrets. Such extraordinary pertinacity is fortunately not required, except in special cases. Amateurs may effect much valuable work in the short intervals which many of them steal either from business or domestic ties and offer at the shrine of astronomy.

There are quite a considerable number of attractive phenomena and features on which the solar observer will find ample employment, and to the principal of these it may be as well to make individual references.

Eclipses of the Sun.—These phenomena deservedly rank amongst the most important and impressive events displayed by the heavenly bodies, and they are specially interesting to the possessors of small telescopes. Solar eclipses have been so often made the subject of observation and discussion, that our knowledge of the appearances presented may be considered to be nearly complete. The various aspects of Nature on such occasions have been so attentively studied in their manifold bearings, that virtually nothing remains for the ordinary observer but to reexamine and corroborate facts already well ascertained. He can expect to glean few materials in a field where a plentiful harvest has just been reaped. But the eclipsed Sun, if it has revealed most of its secrets to previous investigators, has certainly not declined in attractiveness; and the amateur will find the spectacle still capable of exhibiting features which, though not full of the charms of novelty, will be sufficiently striking and diversified to be remembered long after the event has passed.

Eclipses recur in cycles of 18 years and 10 days (= 6585 days). This period was determined by the ancients, and called the saros. By its means the times and magnitudes of eclipses were roughly computed long before astronomy became an exact science.

A solar eclipse is really an occultation of the Sun by the Moon; for the word eclipse, in its usual reference, denotes the obscuration of one body by its immersion in the shadow of another. During any single year there are never less than two eclipses, nor more than seven. Whenever there are two only, both are solar.

Since the fine solar eclipse of December 22, 1870, no large eclipse of the Sun has been visible in England. It is remarkable that during the thirty years from 1870 to 1900 these phenomena are all of an unimportant, minor character. Within the thirty years following 1891 there will be twelve solar eclipses, for which the Rev. S. J. Johnson has given projections (as shown on p. 98) for the period of greatest obscuration.

Total eclipses are extremely rare as regards their visibility at a given station. Thus between 878 and 1715 not one was observed at London, and during the next 500 years there will be a similar absence of such a phenomenon. The observer of total eclipses must perforce journey to those particular tracts of the earth’s surface over which the band of totality passes. On such occasions photography plays an important part; and the corona, the red flames, the shadow-bands, and numerous other features become the subjects of necessarily hurried observation and record, for totality endures for very few minutes11.

As regards ordinary partial eclipses, amateurs usually find ample entertainment in noting the serrated aspect of the Moon’s contour projected on the bright Sun. It is also interesting to watch the disappearance and reappearance of the solar spots visible at the time. Rather a low magnifying power, with sufficiently expansive field to include the entire disk, is commonly best for the purpose of these observations.

Periodicity of Spots.—This detail may be said to have been fully investigated. Schwabe and Wolf have accomplished much in this direction. A work of this kind must, by the nature of it, extend over many years and entail many thousands of observations. It is therefore more suited to the professional astronomer than to the amateur, whose attention is more or less irregular owing to other calls. The sun-spot cycle is one of about 11 years, during which there are alternately few and many spots on the Sun. There appear to be some curious fluctuations, disturbing the regular increase and decrease in the number of spots; and these variations are worthy of more attention. The following are the years of observed maxima and minima of sun-spot frequency:—

These phenomena have been rare during the past few years. The next maximum may be expected in about 1894, when solar observers will probably have an abundance of new materials to study.

Crateriform Structure.—In 1769 Prof. Wilson, of Glasgow, while watching a sun-spot with a Gregorian reflecting-telescope, remarked that, as it approached near the limb, the penumbra became much foreshortened on the interior side. He inferred from this that the spots were cavities, and the idea has been generally accepted; so that these objects are sometimes termed solar craters, and commonly regarded as openings in the luminous atmosphere of the Sun. But the conclusion appears to be based on data not uniformly supporting it. In 1886 the Rev. F. Howlett published some observations which “entirely militate against the commonly received opinion that the spots are to any extent sunk in the solar surface as to produce always those effects of perspective foreshortening of the inner side of the penumbra (when near the limb) which have been described in various works on astronomy.” In a number of instances the penumbra is wider on the side nearest the Sun’s centre, whereas the converse ought to be the case on the cavity theory. The fine sun-spot of July 1889 offered an example of this; for when it was near the W. limb the W. side of the penumbra was obviously much narrower than the E. side, so that the appearance would indicate the object as an elevation rather than a depression. The observer should keep a register of the aspect of all pretty large spots near the limb, and note the relative widths of the E. and W. sides of the penumbra. An extensive table of such results would be interesting, and certain to throw some light on the theory of spot-structure. It is of course possible that occasionally the inner side of the penumbra is broader than the outer, and thus appears wider even on the limb, though really forming the side of a shallow depression.

“Willow-Leaves.”—In 1861 the late Mr. Nasmyth announced that the entire solar surface was composed of minute luminous filaments in the shape of “willow-leaves,” which interlaced one another in every possible variety of direction. This alleged discovery only met with doubtful corroboration. The objects were stated by some authorities to be simply identical with the “corrugations” and “bright nodules” of Sir W. Herschel. Mr. Stone called them “rice-grains.” The eagle-eyed Dawes thought “granulations” a more appropriate term, as it implied no consistency of form and size. Secchi referred to them as oblong filaments, and “rather like bits of cotton-wool of elongated form.” The Rev. F. Howlett described the Sun as presenting a granulated, mottled appearance in a 3-inch Dollond refractor, and mentioned that on the morning of June 9, 1865, the aspect of its surface was like that of new-fallen snow, the objects “being not rounded but sharply angular.” The opinions of observers were thus singularly diverse, and the result of several animated discussions at the Royal Astronomical Society was that little unanimity was arrived at, except as to the fact that the Sun’s surface was crowded with small luminous filaments of elongated form, and either rounded or angular at the ends. There was no accord as to their precise forms or distinctive manner of grouping. Some of the observers averred that the “willow-leaves” or “rice-grains” had no title whatever to be regarded as a new discovery, the same appearances having been recognized long before. Gradually the contention ceased, and though more than a quarter of a century has passed since the discussion arose there has been little new light thrown on the subject.

Amateurs will therefore do well to probe deeper into this promising branch of solar observation. As Mr. Nasmyth himself stated, considerable telescopic power is required, combined with a good atmosphere. But comparatively small instruments will also be useful, because of their excellent definition and efficacy in displaying details on a brilliant orb like the Sun. A power of 150 should be employed in examining small regions of the general surface, and also the edges of the umbra and penumbra of sun-spots. When definition is unusually sharp, and the details very distinct, the magnifying power should be increased if it can be done with advantage; and the observer should utilize an occasion like this to the utmost extent. On a really excellent day more may be sometimes detected than during several weeks when the atmosphere is only moderately favourable. The observations, being of a critical nature, should not be attempted in winter, when the Sun is low. I have frequently secured fine views of the delicate structure of the solar surface between about 8 and 9 A.M. in the summer months; and this is often a convenient time for amateurs to snatch a glimpse, before going to business.

With reference to the general question as to the existence of the “willow-leaves,” my conception of the matter is that the features described by Mr. Nasmyth are not new. His drawing of a spot in Sir J. Herschel’s ‘Outlines’ and Chambers’s ‘Descriptive Astronomy’ exhibits objects extremely uniform in shape and size, and this uniformity I have never observed in the penumbra of spots. As to the engraving in the ‘Outlines,’ showing the aspect of the interlaced “willow-leaves” on the general surface, this is also not realized in observation. The “corrugations” and “bright nodules” of Sir W. Herschel aptly represent what is seen, and they are possibly identical with the “very small bright and obscure points” and “lively and sombre streaks” of Scheiner, though seen much better and in more profusion of detail through the improved modern telescopes. The so-called “willow-leaves” are rounded at the ends, and are consistent neither in size nor shape. They encroach upon the umbra of the spots, and give a thatched appearance to the edges. The penumbra also shows this in its outer limits, where it is also fringed with lenticular particles. Drawings by Capocci and Pastorff seventy-five years ago, and published in Arago’s ‘Popular Astronomy,’ show the thatching at the edges of the umbra quite as palpably as it is represented in recent drawings.

Rotation of the Sun.—By noting when the same individual spots return to the same relative places on the disk, the approximate time of rotation is easily deduced. This varies according to the latitude of the spots12; whence it is evident the solar atmosphere is affected by currents of different velocities, causing the spots to vary in their longitudes with reference to each other. The Earth’s motion round the Sun causes the spots to travel apparently more slowly than they really do; for observations prove that a spot completes a rotation in 27 days 5 hours, whereas the actual time, after making allowance for the earth’s orbital motion, is about 25 days 7-3/4 hours. The period of rotation may be roughly found as follows, supposing a spot to return to precisely the same part of the disk in 27 days 5 hours:—

365^d 5^h 49^m + 27^d 5^h = 392 10^h 49^m.

Then

392^d 10^h 49^m (= 565,129^m) : 365^d 5^h 49^m (= 525,949^m)

:: 27^d 5^h (= 39,180^m) : 25^d 7^h 44^m (= 36,464^m).

For exact results several circumstances have to be considered, such as the direction of the spot-motions across the disk, as the chords vary according to the season; thus in June and December the spots traverse straight lines, while in March and September their paths are curved, like a belt on Saturn when the planet is inclined. Some of the spots display considerable proper motion; so that it is best to observe a number of these objects, and reduce the times to a mean result. They are not very durable, rarely lasting longer than a few weeks; but some of the more extensive disturbances are sustained for several months, during which many singular changes are effected. The period of rotation, as determined by several observers, is as follows:—

The motion of rotation is similar in direction to that in which the planets move found the Sun, namely from west to east. Hence the spots come into view on the east limb of the Sun, and disappear at the west.

Planetary Bodies in transit.—During observation the observer should particularly watch any very dark, small spots that may be visible, such as are isolated and pretty circular and definite in outline. If an object of this character is seen it should be examined with a high power, and its aspect critically noted. Should the observer entertain any suspicion of its being of a planetary nature, he should carefully determine its position on the disk, and, after a short interval, re-observe it for traces of motion. If it remains stationary, its true solar origin will be proved. If motion is shown, then the successive positions of the object during its transit, and its place of egress, with the time of each observation, should be recorded. In such a case it would be a good plan to project the Sun’s image, and mark the place of the suspicious object and chief sun-spots at short intervals. This would be more accurate than mere eye-estimation. The observer who scans the solar surface for intra-Mercurial planets must remember that, if any such bodies exist, they will probably be very diminutive. Venus, when on the Sun in December 1882, was a spot 63 in diameter, and easily perceptible to the naked eye. Mercury, at the transits of 1861, 1868, and 1881, was a little less than 10, but in 1878 was 12. If “Vulcan,” the suspected interior planet, has any existence it may possibly be much smaller than Mercury, and will thus escape observation, unless the observer exercises great care in the search. The mobile, planetary spots asserted to have been seen on the Sun in past years prove nothing definite, and appear to have been illusory.

Proper Motion of Sun-spots.—This feature is one deserving more investigation. The distances separating individual spots should either be measured with a micrometer or determined by transits across a wire, and the displacement recorded from hour to hour or from day to day. Spots in different latitudes will almost certainly exhibit some change of relative place; and objects in the same latitude must be watched, for similar variations probably affect them. The physical peculiarities of such spots should be remarked, and also the alterations of appearance they undergo during the time they approach or recede from each other.

Rise and Decay of Spots.—Occasionally large spots are formed in an incredibly short time, and the disappearance of others has been equally sudden. Schwabe found, from many observations, that the western spots of a group are obliterated first; but authorities differ. I have usually observed that the smaller, outlying members of a group vanish before the larger spot, which then contracts and is invaded by tongues of faculÆ; so that its effacement soon follows, and nothing remains to indicate the disturbance but bright ridges of faculÆ, which are very conspicuous near the limb.

Black Nuclei in the UmbrÆ.—Dawes was the first to announce that the umbra sometimes included a much darker area or nucleus. This is present in nearly all large spots. A part of the umbra seems covered or veiled by a slightly luminous medium, and the portion unaffected looks black by contrast. On October 1, 1881, with a 2½-inch refractor, I saw a large sun-spot, the umbra of which was broken up into 7 fragments, and the S. preceding part appeared very black while the others showed a much lighter tint. In the fine spot of June 1889 a nucleus was also distinctly apparent; and this feature is sometimes so obvious in large spots that it may be observed with an instrument of only 2-inches aperture. I have usually remarked the nucleus on one side of the umbra, and abutting the penumbra. It may be formed by light patches of transparent material floating over the umbra, and leaving a part free where the Sun’s dark body is fully exposed. This light material is possibly suspended far above the umbra and inconstant in its position; so that the place and form of the nucleus should always be noted for traces of change. It is necessary that such details should be closely watched during an entire day, or several days; for the variations could then be followed, and perhaps reduced to some law. This persistence is very necessary, in order to solve many of the peculiarities of sun-spots, which, though pretty well known in appearance, have not been thoroughly studied in their various developments.

Bright Objects near the Sun.—Small, rapidly moving bodies have been occasionally reported as seen passing over the Sun. In several cases these have been prematurely assigned a meteoric origin. They have been described as luminous bodies of irregular shape, as moving in a common direction, and as being very distinct when projected on the dark sky just outlying the bright limb of the Sun. There is little doubt they are either the pappus of different kinds of seed, or convolutions of gossamer, which have been lifted to great heights in the air, and are rendered bright by reflection from the bordering Sun. In this connection I may mention some observations of my own with a 4-inch refractor:—

“1889, May 20, 0^h 30^m P.M.—Bright points and little misty forms kept passing from the Sun’s limb, at the average rate of 13 in a minute. They moved in the same direction as the clouds and wind. Some of them were followed by tails, which were far from straight. I saw them best when I focused the telescope for an object much nearer than the Sun. One of these forms would occasionally halt and pursue an irregular flight. It was evident they were terrestrial objects, with motions controlled by the wind.

“3^h P.M.—Many bright objects still passing from the Sun’s limb.”

“1889, May 22, 9^h A.M.—Observed vast numbers of luminous particles floating about contiguous to the Sun’s margin. They were clearly carried along by the wind; but this being very slight, their motions were extremely slow, and now and then many of them became nearly stationary. Their directions were far from uniform, though the general tendency was obviously in a common line of flight. I watched them for some time passing in a plentiful shower.”

These objects are always noticed in summer-time, and I believe they would much more frequently attract remark but for the fact that they require a longer focus than the Sun and cannot be recognized when on the disk, to which the observer is usually giving the whole of his attention. Those who are often employed in solar work will find it an interesting diversion to look for these bodies. The instrument should be focused as for a distant terrestrial object, and only a part of the Sun’s limb should be retained in the field of view of an eyepiece of moderately low power. Then, looking intently at the dark sky near the limb, the bright objects will be sometimes seen sailing past in considerable numbers.

Cyclonic Action.—The appearance in detail of certain spots, coupled with evidences of rotatory motion round their own centres, has induced the belief that they are liable to action in some degree similar to the cyclonic storms13 which disturb and rend the terrestrial atmosphere. Such indications should be looked for in fairly conspicuous spots, and any peculiarities of the nature alluded to made the subject of close investigation. A spot showing features having a spiral tendency may not, however, have a gyratory movement about its centre. This can only be determined by critically noting the details, and frequently reobserving them for traces of motion. The penumbra always shows radiations converging on the umbra as a centre; but this is merely a form of structure, and proves nothing in evidence of a revolving storm.

Sudden Outbursts of FaculÆ.—In September 1859 Carrington and Hodgson independently observed a striking outburst of faculÆ in front of a large group of spots which they were examining. It remained visible about five minutes, during which interval several patches of light travelled over a region nearly 34,000 miles in extent. An extraordinary magnetic disturbance was simultaneously recorded at the Kew Observatory, and sixteen hours afterwards there followed a magnetic storm of unusual severity. On another occasion Dr. Peters observed flashes of light cross and recross the umbra of a prominent spot with electric velocity. Some other startling observations of solar phenomena have been effected, and there is no question as to their having been matters of fact. In the presence of effects so sudden, so obvious, and so unexpected, no wonder the observers at first doubted the evidence of their eyes and suspected the cause to lie in a fractured glass or a fault of adjustment. But the corroboration afforded the clearest proof as to the actuality of the events described. They will doubtless occur again; but these phenomena cannot be definitely predicted as to time, so that students of the solar surface should be prepared for a repetition of them whenever they may occur.

Shadows cast by FaculÆ.—M. Trouvelot, while examining a large sun-spot on May 26, 1878, noticed that it was “completely surrounded by very brilliant and massive faculÆ.” “On one part of the penumbra an extraordinary appearance was perceived, which resembled so closely a shadow, as it would have been cast by the overhanging faculous mass, that it seemed useless to seek, and it was impossible to admit, any other explanation. This shadow, the outline of which was a little diffused, had the same shape as, and reproduced with great exactness, the outline of the faculous mass situated above it. It was not so black as the opening in spots called the umbra, but of a very dark tint.” A similar feature was seen by Kirk and Maclean on May 2, 1884, and the ‘Observatory,’ vol. vii. pp. 146, 170, and 197, contains some interesting particulars on this subject. Fig. 24 is a drawing by Kirk, in which the shadow is represented by A, B; at C “it accurately followed the outline of the intensely white margin of the spot.”

Veiled Spots.—The late Father Perry described these objects at the R.A.S. meeting on May 9, 1884, and said they are to be seen all over the face of the Sun. They only exist for two or three minutes, and then disappear. In one instance he observed a train of these veiled spots stretching over “a tenth part of the Sun’s diameter, which was nearly as obvious to the eye as the penumbra of an ordinary spot; it split into two throughout its whole length, and disappeared in a minute. The veiled spots seem to be of two classes: the one appear like small greyish clouds, which disappear after a few minutes, as if they were formed and rapidly evaporated by the Sun’s heat, and the others seem to be connected with the umbra of ordinary spots; they appear about them, and are more permanent than the ordinary veiled spots, lasting sometimes two or three days, but never longer.” These markings appear to have been first detected by Trouvelot in 1875, and he gives some information as to this class of phenomena in the ‘Observatory,’ vol. viii. pp. 228 et seq.

Recurrent Disturbances14.—It is supposed, and with good evidence affirming the idea, that certain regions of the Sun’s surface are subject to frequent outbursts of spots, which are possibly due to forces acting from below the Sun’s bright atmosphere. After the disappearance of large groups or isolated spots it is therefore advisable to watch the same region for some time afterwards, to find whether it remains perfectly quiescent, or whether it soon again becomes a seat of activity and change.

Recurrent Forms.—Certain spots observed at different times have exhibited appearances so nearly resembling each other that it has been considered the likeness may be due to something more than mere accident. Whenever such suggestive coincidences are recognized the observer should note them particularly, and secure drawings. It should be his aim to determine the exact intervals elapsing between the presentation of spots or groups of this character, and also whether they occupy the same latitude and longitude on the Sun’s disk.

Exceptional Position of Spots.—The ordinary spots are rarely seen more than 35° distant from the solar equator or within 8° of it. They usually appear in the zones from 8° to 20° N. and S. of the equator. A few exceptions may be mentioned15. Mechain saw a spot in July 1780 having a latitude of 40-1/3°; in April 1826 Capocci recorded one having 49° of S. latitude; Schwabe and Peters observed spots 50° from the equator. Lahire, in the last century, described a spot as visible in a latitude of 70°; but the accuracy of this observation has been questioned. Whenever a spot is seen near the equator, or very far removed from it, measures should be taken of its exact place; for it is desirable to learn something more of those disturbances which occasionally affect the more barren regions of the solar envelope.

The Solar Prominences.—Those amateurs who have included a spectroscope in their instrumental outfit will find the study of the chromosphere and prominences a most productive one. Huggins and ZÖllner were the first to apply the “open-slit” method; and the study of the shape of the hydrogen prominences commenced in 1869. Tupman details (‘Monthly Notices R.A.S.,’ vol. xxxiii. p. 106) a series of observations which he secured in 1872 with a refractor of 3-inches aperture and a direct-vision spectroscope of five prisms. He mentions the cost of the entire apparatus as only £18, and says he entertains “no doubt that an equally effective instrument could be made for much less.” The prominences appear to be of different kinds, and are known as “cloud”- and “flame”-prominences. Both are liable to rapid changes. Trouvelot, in June 1874, noticed “a gigantic comma-shaped prominence, 82,000 miles high, which vanished from before his eyes by a withdrawal of light as sudden as the passage of a flash of lightning.” Since the study of these remarkable forms was rendered feasible by using a greater dispersion to open the slit of the spectroscope wide enough to see them, they have been made the subject of daily study and record. The results, so far as they have been investigated, show that the region of the Sun’s limb in which the prominences are most frequent reaches to some 40° on either side of the equator, which is somewhat greater than the area of sun-spot frequency. The chromosphere itself is probably of much the same character as the erupted prominences, and formed of little flames arranged thickly together like “blades of grass.”

In observing the Sun with a telescope the amateur will soon notice that the surface is far more brilliant in the central parts than round the margin of the disk. VÖgel has estimated that immediately inside the edges the brightness does not amount to one seventh that of the centre. The difference is entirely due to the solar atmosphere, which is probably very shallow relatively to the great diameter of the Sun.