PHENOMENA OF SOLAK ECLIPSES.] 



ASTRONOMY. 



983 



moon, it must happen that the moon penetrates the cone 

 of shadow BOB' at C (Fig. 142) ; and it is easily seen, 



Fig. 142. 



that in order that there be an eclipse of the sun at some 

 portion of the earth, it must penetrate into the cone 

 A O A' at the point D. As the diameter of the cone is 

 larger at "D than at C, it consequently happens that the 

 moon passes more frequently through the former than the 

 latter, and that eclipses of the sun will be more frequent 

 than those of the moon. Theory and observation alike 

 prove, that in the period of 18 years 11 days, in which, 

 as before mentioned, the moon passes to the same posi- 

 tion in respect to the sun and its nodes, there are in 

 general seventy eclipses, of which forty-one are of the 

 sun, and twenty-nine of the moon. There cannot be 

 more than seven eclipses in one year, nor less than two ; 

 and if the latter, they must both be eclipses of the sun. 

 I n any <j iren place, however, the eclipses of the moon 

 will appear to be more frequent than those of the BUII, 

 as will be apparent when we come to consider that the 

 fi.ni'i' are due to the total or partial extinction of light 

 on the surface of our satellite, and are seen on every oc- 

 casion on that hemisphere of the earth, where the moon 

 is above the horizon. Eclipses of the sun, on the con- 

 trary, can only be seen at one zone of the earth, and that 

 a very narrow one ; for, in addition, it will be seen that 

 the dimensions of the shadow of the moon are small as 

 compared with that of the earth. Out of the forty-one 

 solar eclipses, which may occur in eighteen years, there 

 are in gi-ncral twenty-eight of those which may become 

 central according to circumstances that is, either total 

 or annular. It results from Du Sejour's calculations, 

 that the greatest possible duration of an eclipse of the 

 mm cannot be more than 4h. 29m. 44s. for a place situated 

 at the equator, or 3h. 26m. 32s. for the latitude of Paris. 

 ] n total eclipses, the greatest possible duration of total 

 obscuration may only be 7m. 68s. at the equator, and 

 Cm. 10s. at the latitude of Paris. In the case of annular 

 eclipses, the greatest possible duration of the phase is 

 12m. 24s. at the equator, and 9m. 56s. at the latitude of 

 1'aris. Such cninliinatioiiR are of course very rare. The 

 total cclij.se of 1706 lasted for 4m. 10s. ; that of 1715, at 

 London, Hm. 57s. ; the total eclipse of 1806, at Kinder- 

 hook, in America, continued for 4m. 37s. ; and that of 

 1724, at Paris, for 2m. 16s. In the eclipse of 1778, the 

 darkness continued for four minutes. 



Phenomena observed during a Total Eclipse. The great 

 and sudden darkness which takes place at the moment of 

 a central eclipse is, of course, the great phenomenon to be 

 witnessed in solar eclipses, but many other curious cir- 

 cumstances have been unexpectedly noticed by the tele- 

 scopic observers during the eclipses of 1842 and 1851, 

 which serve to throw some light on the constitution of 

 the sun ; so true is it, as Arago observes, that, in the in- 

 tellectual world as in the terrestrial, we cannot advance 

 a step without discovering a new horizon. It is not, 

 however, to be strictly understood that those appear- 

 ances were first perceived ou these occasions, but rather 

 that they were observed with greater care ; and from the 

 number of observers engaged, and the different instru- 

 ments they made use of, the results are placed beyond 

 doubt, and confirm the desultory notes of former 

 astronomers. 



The darkness which follows the total obscuration of 

 the sun's disc, though momentary, is generally very 

 great, though not complete. The total eclipse which 

 occurred in England, on June 17th, 1433, was long 

 ]<i n < mitred under the name of the "Black Hour." 

 That of 1598 was equally imprinted on the memories of 

 the peasantry, and called to mind by the name of the 

 "lilack Saturday;" whilst the total eclipse of 1652, is 

 ided in Scotland by the name of "Mirk Monday." 



Nor, if we are to allow the testimony of Dr. Halley, are 

 those titles misapplied in such cases. "1 forbear," he 

 says, in his communication to the Royal Society, 

 on the eclipses of 1715, "to mention the chill and 

 damp which attended the darkness of this eclipse, 

 of which most spectators were sensible and 

 equally judges. Nor shall I trouble you with the 

 concern that appeared in all sorts of animals, birds, 

 beasts, and fishes, upon the extinction of the sun, 

 since ourselves could not behold it without some 

 sense of horror." In the eclipse of 1706, during the 

 period of total darkness at Geneva, bats flew about as at 

 dusk, swallows were seen flying about, and cage-birds 

 put their heads under their wings. The stars appeared 

 as thickly strewed as at the time of full moon. In the 

 eclipse of 1842, several stars of the first magnitude, as 

 well as the planet Mars, were distinctly visible. The 

 colour of the sky at those times does not appear, how- 

 ever, either to belong to the darkness of night nor the 

 hue of twilight ; in some places it was even noticed to be 

 of a violet tint ; and red stars, such as those of a Orionis, 

 Aldebaran, and others, were noticed to be white. At 

 MontpelHer, in the eclipse of 1842, the light had acquired 

 a livid tint, imparting to the hun an countenance an 

 aspect which it was painful to contemplate ; and the 

 spectacle was generally allowed to be of an extraordinary 

 and appalling character. An owl was seen to leave the 

 tower of St. Peter, the bats left their retreats, the swal- 

 lows disappeared, the fowls went to roost, and the catlle 

 stood still in the field. A heavy dew fell at Perpignan, 

 Turin, and Vienna, during the obscuration. Similar 

 appearances were noticed during the total eclipse of 1851, 

 by the numerous observers stationed in the path of 

 central eclipse in Sweden. 



Even during the greatest obscuration, and when the 

 disc of the sun is completely hidden by that of the moon, 

 its place is still made apparent by the brightness in the 

 part of the heavens in which it is situated. This light 

 appears in the remarkable form of a corona or lustrous 

 ring, and it has sometimes been so bright as to be 

 observed and mistaken for an annular eclipse. This 

 luminous ring was seen during the eclipse of 15G7, 

 and was mistaken for the margin of the solar disc ; it was 

 equally visible in that of 1598. Its radiating appearance 

 seems to have been first noticed in the eclipse of 1652, 

 and appeared to be endowed with a sort of rotatory mo- 

 tion. In every subsequent eclipse, observed under any 

 favourable circumstances, the corona has been observed 

 with equal plainness, generally of about one-tenth or one- 

 twelfth of the diameter of the moon, and of a pearl-white 

 colour, or, in the words of an observer, "of that bluish 

 tint which distinguishes the colour of quicksilver from 

 that of a dead white." In the eclipses of 1842 and 1801, 

 it was the principal circumstance to which the attention 

 of the observer was directed ; and M. Arago was able to 

 perceive what might be termed two rings the inner one, 

 or that which bordered on the moon's limb, being of a 

 uniform brightness, whence it faded imperceptibly out- 

 wards, and terminated irregularly. The inner ring was 

 3' or 4' in breadth ; but the whole breadth of the corona 

 was differently estimated by various observers, as 8', 1C', 

 and 25' in breadth. Several luminous jets of light were 

 noticed in the corona by M. Struve, at Lipesk, as in the 

 following diagram (Fig. 143) ; and it had a decidedly 

 radiating appearance ; but the observers in France only 

 perceived one or two radiations, and in some cases only 

 the brighter or inner ring of light was observed. In the 

 eclipse of 1851, the same discordance in the estimation of 

 the breadth, and in the radiating aspect of the corona, 

 existed between the several observers. The direction of 

 the diverging rays did not seem to be perpendicular to 

 the circumference of the moon, but were greatly inclined ; 

 and M. Arago noticed, that at one part of the corona, the 

 rays were entwined within each other, like a skein of 

 thread which was entangled. (See, also, page 165, ante). 

 The appearance of this ring of light is now generally 

 attributed to the effect of the atmosphere of the sun, 

 rather than to anything of the same nature in the case of 

 the moon. The latter was the supposition of Kepler; 



