SOLAR SPOTS.] 



ASTRONOMY. 



929 



spots have at any time been remarked to produce any 

 degree of cold on the earth ; for numerous and large as 

 they seem to be, they bear but a small proportion to the 

 total extent of the surface of the sun. The contrary 

 opinion is much more generally regarded as true, and 

 seems more accordant with the observed facts viz., that 

 the warmest seasons are those in which the sun is most 

 plentifully covered with spots. 



The bright spots which appear on the solar surface, 

 either occur under the forms of long irregular veins, or 

 minute specks, and are both perceptibly brighter than the 

 general surface of the sun ; the former being known as 

 faculce, and the latter as the luculi. The luculi, like the 

 fores, are situated at all parts of the sun's disc, and 

 assist in giving it that mottled appearance which it has. 

 The faculse are mostly confined to a zone of 60 in 

 breadth, in which the spots make their appearance, and 

 generally accompany the latter as invariably as the sur- 

 rounding penumbra. The faculse are only visible at 

 the edges of the sun, as, when they are carried by the 

 rotation of the sun to the centre, they are seldom, if 

 ever visible. When the faculae appear in great numbers 

 and brightness on the limb of the sun, they are certain 

 forerunners of large spots, which are almost sure to make 

 their appearance in a few days afterwards. Cassini states 

 that faculoe ordinarily show themselves in those places 

 where spots have previously appeared, and that they have 

 sub-' . ucntly again become spots. Durham noticed a 

 similar appearance, and, on one occasion, relates that he 

 perceived changes in the spots with his eye at the tele- 

 scope a black spot appearing and disappearing succes- 

 sively in the centre of a brilliant faculw. Herschel has 

 likewise perce'ved extraordinary rapid changes with 

 elongated facultc ; although Scheiner did not give credit 

 to the existence of the faculof or luculi, or anything 

 on the sun brighter than its surface, yet they were 

 detected with much less powerful instruments than he 

 possessed, by earlier observers. Galileo first remarked 

 the existence of the faculce, and Scheiner that of the 

 luculi. (See, also, page 934). 



PHYSICAL CONSTITUTION OF THE SUN. Until compara- 

 tively recent times, those spots were held to be the dark 

 smoke or vapour which floated over the solar surface, 

 although this did not explain the penumbra, or the shape 

 which it takes when seen at different parts of the disc, 

 when the penumbra surrounds a spot pretty equally at 

 all sides near the centre of the sun, as at Fig. 24 ; for as 

 it passes to the margin, and is received obliquely, the 

 portion most distant would appear the narrowest. This, 

 however, is directly contrary to observation, as it has 

 been noticed that the nucleus and penumbra, when seen 

 at the edge of the sun, appear as at Fig. 25, the part of 

 the penumbra most distant from the sun's limb dis- 

 appearing entirely, whilst the opposite side is only 

 slightly diminished in breadth. Taking these facts into 

 consideration, Dr. Wilson became convinced that the 

 Fig. 24. nucleus was in reality 



a deep hollow in the 

 surface of the sun, 

 and that the penum- 

 bra was the shelving 

 sides surrounding it. 

 If such a hollow as 

 this be received ob- 

 liquely, it is evident that it will appear as in Fig. 24. 

 Dr. Herschel confirmed this theory in many respects ; 

 but, instead of holding the opinion that they were inden- 

 tations on the surface, he considered, rather, that they 

 were openings in the luminous atmosphere of the sun. 

 The sun itself he considered to be a dark body sur- 

 rounded by two envelopes, the interior one being formed 

 of very luminous clouds and very bright, whilst the one 

 lying between the photosphere and the body of the sun, 

 is formed of clouds, very little, if at all luminous. 



If we suppose an opening be formed in these envelopes,- 

 by a gas ascending from the body of the sun, and driving 

 the atmosphere away, the opening at the centre of the 

 sun (Fig. 25), to an observer at A, and the nucleus, will 

 appear of the breadth a a', surrounded equally on all 



VOL. I. 



sides by a penumbra, whose breadth is 6 6'. If, however, 



the observer is at B, he views the spot obliquely ; the 



Fig. 25. 



sides of the two openings, a and 6, will coincide, and will 

 lie in the same direction. On the opposite sides, how- 

 ever, at a' and 6', they will still be fully apparent. If 

 the gas drive away the clouds of the two atmospheres, 

 they will, of course, accumulate about the opening ; and 

 this may account for the facuke which are to be per- 

 ceived about the nucleus and penumbra. In regard to 

 the luculi or points of light which cover the surface of 

 the sun, they may be due to the roughness which would 

 result from such a cloudy and irregular mass as the 

 photosphere is imagined to be. An observation which 

 tends to prove the unstable and cloudy nature of the 

 outer envelope is, that they shift their positions from day 

 to day ; and it is very difficult to determine the exact 

 period of rotation of the sun from these observations, 

 some showing much longer periods than others. 



From observations on a great number of spots, M. 

 Langier, of the Paris Observatory, has recently de- 

 termined the exact period of rotation to be 25 '34 days. 

 Since, however, the earth is pig. 26. 



moving around the sun during 

 this interval, we must wait 

 nearly two days longer before 

 we perceive the spot again at 

 the centre of the sun. To 

 explain this (Fig. 26) : If T be 

 the earth, S the sun, and a the 

 spot as seen in its centre, whilst 

 the spot appears to make a 

 complete revolution, and to 

 arrive again at the centre, the 

 sun passes from S to S' ; and 

 when it arrives at S', the spot appears at a'. If the sun 

 had made exactly one revolution on its axis in the 

 direction of the arrow, the radius S a would have taken 

 the position S 6, parallel to its first position. When it 

 arrives in a', it must, therefore, have made more than 

 one revolution by the angle 6 S' a'. To pass through 

 360 plus the angle 6 S' a', it requires 27 '3 days ; whilst 

 to make a rotation on its own axis it only requires 25 -34 

 days. 



As the solar equator is inclined 7 9' 12" to the ecliptic 

 (according to M. Langier's investigations), the path which 



the spots appear to de- 

 scribe on the sun will vary 

 at the different seasons of 

 the year, the concavity 

 being sometimes turned 

 towards the north, and at 

 other times to the south. 

 In the beginning of De- 

 South. Sonth. cember, the spots will ap- 

 pear to describe straight lines with reference to the 

 ecliptic ee, the lines being inclined to it by an angle of 7 

 (Fig. 27). From the 1st of December to the 1st of June, 

 they described curved lines, the convexity being turned 

 to the north (Fig. 28). At the commencement of 

 June they again describe straight lines, but in a con- 



60 



Fij. 27. 



North. 



North. 



