ASTRONOMY. 



[p.ci.irsr 



The consequence would be, that u the moon passed into 

 the part C B B' C' before meeting with the real shad..*, 

 it would iiiMiuibly loae a portion of its light, and iu 

 luntre would become gradually leu intense as it ap- 

 proached the real shadow. As, however, the breadth of 

 the penumbra is equal to the angle O B - A B A' or 

 the diameter of the sun as seen from the earth ; and as 

 the apparent diameters of the moon and sun are nearly 

 equal, it follows that the whole disc of the moon may be 

 within the penumbra C B O. When any part of the 

 disc of the moon is obscured by the real shadow, it fol- 

 lows that the whole of its surface is more or leas hidden 

 by the penumbra. The insensible molting away of the 

 real shadow into the penumbra is noticed at every lunar 

 eclipse, but it is impossible to tell precisely where the 

 penumbra ends, and the shadow begins. 



EFFECT OF THE ATMOSPHERE OF THE EARTH. Hitherto 

 we have supposed the rays proceeding from the sun to 

 pas8 in straight lines until they meet the lunar disc, but 

 this is not the case ; for those which pass through the 

 earth's atmosphere are subject to refraction, and change 

 tlu-ir direction in the manner already explained in the 

 ease of a star. This will be more apparent when wo 

 consider the direction of the ray S A (Fig, 124), which 



Flit. t54. 



traverses the atmosphere of the earth, and passes beyond 

 it The direction which it takes previous to entering 

 the atmosphere of the earth makes an angle of 33' with 

 its direction when it arrives at that point ; and after 

 passing through it, it is still further deflected from its 

 original course, and by the same amount as before. The 

 consequence of this will be, that it is altogether deflected 

 more than a degree, and that the solar rays will meet before 

 arriving at the point O (Fig. 125); and, instead of the rays 

 A I! and A' B' taking a straight direction, they will be 

 deflected to the i>oiiit D (Fig. 125), which is considerably 

 nearer the earth than the point O. Thus it will be only the 



Fig. 125. 



inner cone B D B', which is the real cone of the shadow ; 

 the remaining parts of the mathematical cone BOB' will 

 be traversed by the solar rays, which are bunt from tlu-ir 

 primitive direction by the action of the terrestrial atmo- 

 sphere. The distance of the point D from the centre of 

 the earth can be calculated as in the former case, and 

 it has been found that its average is forty-two terrestrial 

 radii ; and we conclude from this, that a* the moon's 

 mean distance is equal to sixty-four radii of the earth, 

 it can never be inclosed within the real shadow of the 

 earth B D B'; but at the time of total eclipse it falls 

 within that part of the shadow where the rays, refiacted 

 by the atmosphere, penetrate. This is the cause that 

 when the moon is wholly eclipsed, it still shines with a 

 reddish light, being illumined by the faint rays refracted 

 by the earth's atmosphere. This, however, is not the 

 I- effect of the atmosphere, for tin; apparent dUineter 

 of the earth's shadow has been found by observation to 

 be ynattr than might be expected from the preceding 

 investigation ; and it has been accounted for, by sup- 

 paring that the solar rays do not really touch the surface 

 of the earth, but that the lower strata of the air absorbs 

 which approach iU margin ; and if such be the 



case, the diameter of the globe of the earth and it* 

 shadow would accordingly be greater. 



The condition of the atmosphere through which the 

 rays of the sun pass, produces considerable changes in 

 tho appearance of the lunar disc at the times of total 

 eclipse. In ordinary oases, as before mentioned, tho 

 colour of the moon is of a red or coppery tint, similar to 

 that frequently assumed by the setting sun, and pro- 

 duced by the same causes viz., an absorption of the blue 

 rays of light when passing through great depths of the 

 atmosphere. If that portion of the atmosphere be charged 

 iu addition with cloud and dense vapours, the whole of 

 the elementary rays will be stopped in their passage 

 to the lunar surface ; and several instances are on record 

 in which the disc of the moon was completely invisible at 

 the time of total eclipse. In the eclipses of 1601 and of 

 June 1620, this was the case, although the air was suffi- 

 ciently clear to allow the light of stars of the fifth magni- 

 tude to be distinguishable ; and, in a similar manner, 

 the moon could not be perceived (even with the aid of a 

 telescope) during the eclipse of April, 1642, although the 

 sky was equally as clear as in the former case. When, how- 

 ever, the sky is clear at those portions of the earth's sur- 

 face through which the solar rays pass, the red rays are 

 transmitted in great number, and the lustre of the lunar 

 surface is but slightly dimmed in consequence. Su.-U 

 was the case with the eclipse of the moon which happened 

 in March, 1848, when the lunar disc was almost as ap- 

 parent as on ordinary dull nights, and when many per- 

 sons who were observing it could scarcely be persuaded 

 that it was really eclipsed. The dark spots and bright 

 places were as well seen on its surface as if viewed 

 through thin cloud or vapour ; and one observer, 

 who had witnessed lunar eclipses for more than sixty 

 years, never remembered one m which the illumination 

 was so strong appearing like the glowing heat of tiro 

 from the furnace, and tinged with a deep rod. Some- 

 times the moon becomes invisible during the progress of 

 its immersion in the shadow, as was the case with that 

 observed by Wargeutin, in 1761, when the moon was very 

 bright for ten minutes after its total immersion; but for 

 an hour afterwards became so completely invisible that 

 not the slightest trace of it could be detected, either with 

 the naked eye or telescope, although the sky was clear, 

 and stars, in the immediate vicinity of the moon, ap- 

 peared bright and distinct. In other cases, tho region of 

 the atmosphere, through which the rays have passed, 

 has been clear at some parts, and obscured at others : 

 and the consequence has been, that whilst the lunar disc 

 appeared clear and bright in some portions, it was very 

 dark at others. Kepler states that, during the total 

 eclipse of August, 1598, one half of the moon appeared 

 so bright that it seemed doubtful if it was eclipsed at all, 

 whilst tho other was seen with the utmost difficulty. A 

 similar variegated appearance was presented by the lunar 

 disc in October, 1837. Sir W. Herschel perceived many 

 bright spots on tho occasion of a total eclipse, and was 

 induced to believe that they were volcanoes in action. 

 Such an appearance, however, it will plainly be seen, may 

 be supposed due to the various reflective qualities of tho 

 surface of the moon, and that the bright spots noticed Ity 

 Herschel were only the bright mountainous districts illu- 

 minated bv tho red light of the sun. (See Note, p. '. 



PREDICTION, DURATION, AND MAGNITUDE OF ALU.NA.I 

 ECLIPSE. If the moon did not depart from the plane of 

 the ecliptic, there would be a lunar eclipse at each full 

 ni' .on, and a solar one at new moon ; but as it is sometimes 

 above and sometimes below this plane, an eclipse can only 

 take place when the moon approaches the nodes of its 

 orbit. It was known to the ancients that at certain in- 

 tervals the new and full moon returned again on the 

 same day of the month; and they had observed that at 

 the end of eighteen years, eleven days, or a period of 223 

 lunations, there wns a return of the same eclipses, and 

 were thus rn.-iWod to foretel them with considerable, hut 

 not with perfect accuracy ; for the exact recurrence, if it 

 took place, would depend upon the return of the sun's 

 place, tho moon's place, the position of the moon's 

 apogee, and that of tho ascending node of the moon to 



