294

ASTRONOMY.

on account of the greater distance of the Sun . The actions of the |primary planets on one another will produce similar irregularities,though very minute on account of their small size ; however, theactions of Jupiter and Saturn are very discernible.

Sect.. V. Of the Phenomena of the Sum.

(116.) The Sun to an observer on the Earth appears the mostglorious of all the heavenly bodies ; but. when viewed through atelescope dark spots are observed on his surface, of different mag-nitudes at different times ; some have been so large, as by compu-tation, to equal live times the surface of the Earth , others are ob-served of comparatively small extent. The diameter of a spot,when near the middle of the disk, is measured by comparing thetime it takes in passing over a cross-hair in a telescope, with thetime wherein the whole disk of the Sun passes over the same hair;it may also be measured hv the micrometer ; and, bv either ofthese methods, we may judge how many times the diameter ofthe spot is contained in the diameter of the Sun . Spots are subjectto increase and diminution of magnitude, and seldom continuelong in the same state. They are of various shapes ; most of themhaving a deep black nucleus surrounded by a dusky cloud, where-of the inner parts near the black are a little brighter than the out-skirts. They change their shapes, something in the manner thatour clouds do ; though not often so suddenly : thus, what is of acertain figure to-day, shall, to-morrow, or perhaps in a few hours,be of a different one; what is now but one spot, shall, in a littletime be broken into two or three, and sometimes two or threespots shall coalesce, and be united into one. The number of spotson the Sun is very uncertain ; sometimes there are a great many,sometimes very few; and sometimes none at all. Ilevelius ob-served one that arose and vanished in 16 or 17 hours; and no onehas been observed to continue longer than 70 days: those spotsthat are formed gradually, are gradually dissolved; while thosethat arise suddenly, are for the most part suddenly dissolved.When a spot disappears, that part where it was generally becomesbrighter than the rest of the Sun, and continues so for several days;on the other hand, those brig,lit parts called faculte (as the othersare called maculae) sometimes turn to spots.

(117.) The solar spots appear to have a motion across the Sun ’sdisk. Every spot, if it continues long enough without being dis-solved, appears to enter the Sun ’s disk on the east side, to go fromthence with the velocity continually increasing till it has gone halfits way ; and then to move slower and slower, till it goes off at thewest side ; after which it disappears for about the same space oftime that it spent in crossing the disk, and then enters upon theeast side again, nearly in the same place, and crosses it in the sametrack, and with the same unequal motion as before ; from this welearn that the Sun is a globe, and revolves on his axis. The mo-tion of the spots is in the order of the signs ; and therefore, as IheEarth revolves round the Sun the same way with the solar spots,one of these will appear to remain longer on the disk than it wouldif the Earth remained at rest. By comparing many observations ofthe intervals of time in which the spots made their revolution, byGalileo, Cassini, Scheiner, Ilevelius, Dr. Ilallev, Dr- Derham,and others, it is found that 27 days, 12 hours, 20 minutes, is themeasure of one of them at a mean ; but in this time the Earth de-scribes the angular motion of 27° 7' 8" about the Sun ’s centre:therefore, as the angular motion 360° -j- 27 9 7' 8" is to 360°; sois 27 days, 12 hours, 20 minutes, to 23 days, 14 hours, 8 minutes;the time of the Sun ’s revolution about his axis. Had the spotsmoved over the Sun in right-lined directions, it would have shewnthe Sun ’s axis to have been perpendicular to the plane of the eclip-tic, blit since they move in a curvilinear path, it proves his axisinclined to the axis of the ecliptic, and it is found by observation,that the angle is equal lo 7° 20'. The plane of the Sun ’s equatorcuts the ecliptic in two opposite points called the Sun ’s nodes,being in 10° of n, and 10° of f. The face of the Sun , whenclear of spots, seen by the naked eye through a smoked or colour-ed glass, or through a thin cloud, or the vapours near the hori-zon, appears all over equally luminous: but when viewed throughthe telescope, the glasses being smoked or coloured, the middle ofthe disk appears brighter than the outskirts ; because the light isdarted more directly towards us from the middle than from anyother part, and the faculas appear more distinctly near the sides, asbeing on a darker ground than in the middle.

(118.) All the phxnoinena of the solar spots, as delivered by

Scheiner and Ilevelius, may be summed up in the following par-ticulars: 1. Every spot which hath a nucleus, or considerablydark part, hath also an umbra, or fainter shade, surrounding it.2. The boundary betwixt the nucleus and umbra is always distinctand well-defined. 3. The increase of a spot is gradual, thebreadth of the nucleus and umbra dilating at the same time. 4. Inlike manner, the decrease of a spot is gradual ; the breadth of thenucleus and umbra contracting at the same time. 5. The exteriorboundary of the umbra never consists of sharp angles ; but is al-wavs curvilinear, how irregular soever the outline of the nuclei!?mav be. 0. The nucleus of a spot, whilst op the decrease, oftenchanges Us figure by the umbra encroaching irregularly upon it.insomuch that in a small space of time new encroachments are dis-cernible, whereby the boundary betwixt the nucleus and umbrais perpetually varying. 7. It often happens, bv these encroach-ments, that the nucleus of a spot is divided into two or more nu-clei. 8. The nuclei of the spots vanish sooner than the umbra:.9. Small umbra are often seen without nuclei. 10. An umbra ofany considerable size is seldom seen without a nucleus in the mid-dle of it. 11. When a spot which consists of a nucleus and umbrais about to disappear, if it be not succeeded by a facula, or spotbrighter than the rest of the disk, the place where it was is soonafter not distinguishable from the rest.

(119.) There have been a variety of opinions respecting the na-ture and formation of the solar spots. Some have supposed theSun an opaque mountainous body covered over with a fiery andluminous iluid, subject to ebb and flow after the manner of ourtides, so as sometimes to leave uncovered the tops of rocks orhills,which appear like black spots. Others suppose the Sun to con-tain a nucleus subject to frequent volcanoes which cause the spots.

(120.) According to Dr. lierschcl’s theory, the Sun is an opaqueglobe, surrounded by a double set of clouds ; the upper stratum,which is the apparent globe, is more luminous and less closelyconnected than the lower one, and hence the spots are consider-ed as accidental openings between the luminous clouds, throughwhich we behold the opaque body of the Sun , or the inferior lessluminous clouds ; lienee the spots appear of different shades. Inconsequence of this theory Dr. Herschel rejects the old names ofspots, nuclei, penumbra:, lacuhv, and luculi, which other astrono-mers had given to the various appearances on the surface of theSun ; and adopts the following terms : Openings, or the placeswhere the body of the Sun may he seen by the accidental removalof the luminous clouds. Shallows, or the extensive and level de-pressions of the luminous solar clouds. Ridges, or the bright ele-vations of luminous matter, extended in rows of an irregular ar-rangement. Nodules, are also bright elevations of luminous mat-ter, but confined to a small space. Nodules and ridges have been1 called faculx and luculi. Corrugations, or that very particularand remarkable unevenness, ruggedness, or asperity, which ispeculiar to the luminous solar clouds, and extends all over thesurface of the Sun . indentations are the depressed parts of thecorrugations. Pores are very small holes or openings, about themiddle of the indentations.

(121.) The appearance termed semita luminosa, or the zodiacallight, indicates that the Sun is surrounded by a dense atmo-sphere. This was discovered by Cassini in 1683. In brightnessit equals the milky way, the tail of a comet, or a faint twilight,and seems to surround the Sun , in the form of a lens, the planewhereof is nearly coincident with that of the Sun ’s equator. Itis seen along the zodiac, and accompanies the Sun in his annualmotion through the twelve signs. Each end terminates in an an-gle of about 20°: the extent of it in length from either of the an-gular points varies from SO to 100° ; it reaches beyond the orbitof Venus, but not so far as that of the Earth . The breadth of itnear the horizon is also various; from 12° almost to 30°: nearthe Sun , where it may reasonably be supposed to be broadest, itcannot be seen.

(122.) This light is weakest in the morning, and strongest atnight: disappearing in full moonlight, or in strong twilight, andtherefore is not at all visible about midsummer in places sonear either of the poles as to have their twilight all the night, butmay be seen in those places, in the middle of winter, both mom-in" and evening, as it may in places under and near the equator,alfthe year round. In north latitude, it is most conspicuous afterthe evening twilight, about the latter end of February, and be-fore the morning twilight, in the beginning of October; for at

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