THE APPROACHING TOTAL SOLAR ECLIPSE. 



291 



eiice is quite sufficient to produce an appreciable 

 effect, though not so much as the other, because 

 the moon's apparent size changes slowly both 

 when she is near her largest and when she is near 

 her smallest. The sun's change of size is of 

 course small in nine days, the difference between 

 the period we are considering (111 lunations, or 

 3277.9 days) and an exact number of years. 



Since, then, it seems that 111 lunations do not 

 bring about an exact, or even a nearly exact, re- 

 turn to the state of things which had existed at 

 their commencement, it appears at a first view 

 that the singularly close resemblance between the 

 total eclipse of 1S69 and that of the present 

 month was an abnormal, and one may almost 

 say a portentous, phenomenon. But it is readi" 

 !y explained, though it remains an altogether ex- 

 ceptional event in the history of astrouomy. It 

 so chanced that on the occasion of the former 

 eclipse the moon, on her course from the place 

 where she had just crossed the sun's track as. 

 cendingly, had got almost exactly as far above 

 (or to the north of) that track, as she will be at 

 the hour of totality on July 29th, when on her 

 way to the place where she will cross the sun's 

 track descendingly. In 1S69, central eclipse oc- 

 curred at a few minutes before 10 p. m., Green- 

 wich time; but the moon had crossed the sun's 

 track ascendingly at 9.5 a. m. that day. On the 

 other hand, the moon will not cross the sun's 

 track descendingly till near 9 a. m. on the morn- 

 ing of July 30th next ; whereas, central eclipse 

 will take place about 9.23 p.m., Greenwich time, 

 on the preceding day. Thus, ihe interval of time 

 between the two passages of the sun's track, on 

 or about August 7, 1869, and July 30, 1S78, is 

 3,079 days, or one day longer than the interval 

 between the two eclipses ; but because it so 

 chances that this interval of "one day is divided 

 almost equally between the two eclipses, these 

 resemble each other very closely so far as they 

 depend on the moon's distance from the sun's 

 track. As to the moon's apparent size, again, it 

 so chanced that the resemblance is much closer 

 than usual at the beginning and end of a period 

 of exactly 111 lunations. Moreover, while the 

 moon will be smaller (in appearance) on the 29th 

 inst. than she was on August 7th, the sun will 

 also be somewhat smaller ; so that the moon's ex- 

 tension beyond the sun at the time of mid-total- 

 ity, though less than in 1869, will not be so much 

 less as it would be if the change in her own 

 diameter only were considered. 



The reader who examines the above state- 

 ments closely, however, will still recognize some 



difficulties ; for the actual epochs mentioned in 

 the last paragraph do not correspond exactly 

 with the durations of the various kinds of lunar 

 months mentioned in the preceding paragraph. 

 The discrepancy is due simply to the fact that the 

 durations given are only the average durations. 

 The moon's path is constantly undergoing changes 

 of shape and position, in consequence of the dis- 

 turbing action of the sun and (in less degree) of 

 the planets. These perturbations are of the most 

 varied kind, now acting in one way, now in an- 

 other ; now quickly, now slowly. It is only in 

 the long-run that they produce their full effects, 

 or rather their average effects. So that in any 

 special case the moon may cross the sun's track, 

 or reach the points marking her least and great- 

 est distance from the earth, some considerable 

 time before or after the epochs which we should 

 calculate for such passages if we considered only 

 the average periods. 



It will thus be readily understood not only 

 that the close resemblance between the eclipse 

 of this month and that of August 7, 1869, accords 

 perfectly with the theory of the motions of the 

 sun and moon, but that the coincidence really is 

 altogether remarkable and exceptional. The an- 

 nals of solar eclipses might be searched through 

 from beginning to end, without any recorded in- 

 stance of such close resemblance being found ; 

 and the solar eclipses which preceded the histori- 

 cal period, as well as those which are to come 

 during many hundreds, even thousands of years, 

 would not, if carefully calculated, be found to 

 afford more than two or three similar cases. 



It is singular, and altogether encouraging, to 

 note how very rapid has been the progress of 

 discovery effected during solar eclipses in the 

 course of the last ten years. Only a quarter of a 

 century ago, even when astronomers talked about 

 eclipses, they had very little to say about the 

 probability that something might be learned re- 

 specting the sun. They spoke a gcod deal about 

 such utterly insignificant phenomena as Baily's 

 Beads ; they had also a good deal to say about 

 meteorological phenomena to be observed during 

 solar eclipses. But the idea does not seem to 

 have occurred to them that it might be possible 

 to learn something about the sun himself on these 

 occasions. 1 Nay, when phenomena had been ob- 



1 Admiral Smytli, a fine representative astronomer 

 of the last generation, gives what he describes as a 

 sample of a solar eclipse, in the account of which, 

 from the beginning of totality, not one word is said 

 about the sun himself. The description is in such 

 curious contrast with that which we expect to receive 

 from American observers of the approaching eclipse, 



