290 



THE POPULAR SCIENCE MONTHLY.— SUPPLEMENT. 



tory and present position of that part of solar re- 

 search which is connected with the observation 

 of total eclipses, I may make a few preliminary 

 remarks on the laws regulating the recurrence of 

 total eclipses. The circumstances I have men- 

 tioned above are such as to render a few words 

 of explanation necessary. For it might seem to 

 the reader that, since what happened on the occa- 

 sion of the eclipse of August 7, 1869, will be al- 

 most exactly repeated on July 29, 1878 — that is, 

 nine years later, less nine days — the circumstances 

 of every solar eclipse ought either to be exactly, 

 or almost exactly, repeated nine years less nine 

 days later. If this idea were entertained without 

 inquiry, the reader would fall into an error. If, 

 on the other hand, the reader inquired whether 

 other solar eclipses were thus repeated, and found 

 — as he would — that they were not, he would be 

 perplexed, and might probably fall into an error 

 more serious than the one he had avoided, infer- 

 ring that the motions of the sun and moon were 

 not so regular as in reality they arc. 



Suppose, for instance, he took the celebrated 

 eclipse of August 18, 1868. He might reason 

 thus : On August 7, 1869, the sun, moon, and 

 earth, were in a line thus : 



Si 



-moon- 



-earth. 



Nine years less nine days later they will be in the 

 same relative position ; moreover, the earth will 

 be in the same rotation-position, turning Asia and 

 North America sunward ; the moon must be near- 

 ly at the same distance from the earth, while the 

 earth will be also nearly at the same part of her 

 orbit round the sun. Such relations being thus 

 almost exactly restored in this case, it seems to 

 follow that the remarkable eclipse of August 18, 

 1868, when the sun, moon, and earth, were as ob- 

 served above, ought to have been nearly repro- 

 duced on August 9, 1877. Any solar eclipse 

 ought, it should seem, to be repeated nine years 

 less nine days later ; but certainly an eclipse oc- 

 curring, like that of 1869, in August, might be 

 expected to be repeated with as close, or very 

 nearly as close, a degree of resemblance. But, on 

 turning to the " Nautical Almanac " for 1877, the 

 student would find that, though a solar eclipse 

 did take place on August 9th (civil date: the as- 

 tronomical date is August 8th, half-past sixteen 

 o'clock), yet the eclipse was utterly unlike that 

 of August 18, 1868. The latter was one of the 

 greatest total eclipses ever known, totality lasting 

 more than six minutes; whereas the solar eclipse 

 of August 9, 1877, was so insignificant that it was 

 not thought necessary to give a map of it in the 



" Nautical Almanac : " it was partial for the whole 

 earth, only two-fifths of the sun's diameter being 

 covered where the eclipse was greatest, a place 

 near the antarctic circle, whereas the eclipse of 

 1868 was greatest near the equator. 



The real fact is that it was merely by a chance, 

 so to speak, and a very unusual chance, that the 

 eclipse of 1S69 so closely resembled that which is 

 to occur on the 29th of the present month of 

 July. Nine years less nine days correspond close- 

 ly enough with an exact number of lunations (or 

 intervals from new moon to new moon). The 

 interval, allowing two leap-years (there may be 

 three, in which case, of course, we must take nine 

 years less ten days), contains 3,278 days ; and 

 111 lunations, each of 29.5304 days, contain 

 3,277.90 days, 1 or only about 2J hours less. But 

 we do not get eclipses of the sun at every new- 

 moon, only at new moons occurring when the 

 moon is close to the sun's track. She crosses the 

 sun's track at intervals averaging 13.606 days, 

 this being half what is called the nodical month. 

 Now, it will be found that 241 sjch half-months 

 contain 3,279.07 days, or 1 day 1J hour more 

 than 3,278 days, and about 1 day 4 hours more 

 than 111 lunations. This is a considerable dif- 

 ference, insomuch that, if there was a central solar 

 eclipse at the beginning of a period of 111 luna- 

 tions, the eclipse which would occur at the end 

 of the period would be only partial, the moon be- 

 ing l£ day's journey from the point where she 

 crosses the sun's track. 



But there is yet another kind of month to be 

 considered. The moon may be exactly on the 

 sun's track when she overtakes him, and so may 

 pass centrally athwart bis face ; and yet the 

 eclipse may not be total. For the moon to hide 

 the sun, she must be not too far from perigee, 

 that part of her oval course, at the time, which 

 is nearest to the earth, and where, therefore, she 

 looks largest. And for a solar eclipse to resem- 

 ble a former one, the moon must at the latter be 

 just as much larger, to look at, than the sun, as 

 she had been at the former. Now, the average 

 length of the moon's passage, from perigee to 

 perigee again, is 27.555 days. This is what as- 

 tronomers call, for the sake of convenience and 

 simplicity, an anomalistic month. And 115 of 

 these months contain 3,279.00 days, or 1-fr day 

 more than 111 lunations. Here again the differ- 



1 The zero here is not a mistake. In statements 

 of this kind .90 is by no means identical, as many ima- 

 gine, with .9. The former means something: between 

 905 and .895; the latter means something between .95 

 and .85. 



