242 



• KNOWLEDGE ♦ 



[Jax. 20, 1882, 



of a schcuip of circling worlds. It is estimated that the 

 intensity of sunlight at the sun's surface exceeds 19,000 

 tuues tliat of aeandin flame ; S.SOO times that of metal in 

 a Bessemer idiivrrti'r : 111) times tlmtof the calcium light ; 

 3'4 times that of the electric arc at its greatest attainaliLu 

 brilliancy. Tiio lieat euiittt^il l>y tlu^ .sun in every second of 

 time is as much as would be obtained by the consumption 

 of 1G,43G billions (millions of millions) of tons of coal per 

 second. 



Passing tO the consideration of the aspect of the mighty 

 globe, which thus rules, light-s, and nourishes the earth 

 and her follow planets, we find in this treatise one of 

 the most complete, though conci.se, accounts of telescopic 

 study of the sun which has yet been published. The 

 nature and appearance of the spots, the facula' and the 

 granules, the way in wliich the spots are formed, vary in 

 structure and disappear, their motions in different solar 

 latitudes, ami other phenoiiiena of interest, are described 

 succinctly and reasoned upon with skill and caution. We 

 luust leave, however, our remarks on this important part 

 of Professor Young's subject to another occasion. 



PRECESSION OF THE EQUINOXES. 



]}y THE Editoh. 



SINCE the whole globe of the earth reels in the way 

 described in Ko. 11, while the sphere of the li.ved 

 stars remaiiiB all the while unchanged in position, the poles 

 of the heavens (the points towards which the axis of the 

 earth is directed) must move in circles around the pules of 

 the ecliptic (the points towards which a perpendicular to 

 the plane of the earth's motion is directed). The inclination 

 of the earth's axis being about 2.3^°, the circles thus de- 

 scribed by the poles have an arc diameter of about -Ll^. 

 In our illustrative map we show a part of the circle de- 

 scribed by the north jiolo of the heavens around the pole of 

 the ecliptic. This part shows the course pursued by the 

 pole from between live and six. thousand years B.C. to 

 about 8500 A.d. 



We must, however, note two cii'cumstances in the part 

 of the Xorth Pole's path thus given. There is a reeling 

 motion due to what is called nutation, a sort of small reel 

 executed in about eighteen and a half years, the effect of 

 which is to make the course of the pole wavy, instead of 

 the simple circular path we have shown ; this does not 

 much affect the accuracy of the picture. There is another 

 and somewluit more important correction, which would not, 

 indeed, make much difference in our map, but in a map of 

 the pole's course, on a larger scale, would have to be taken 

 into account The pole of the ecliptic is itself moving. The 

 plane of the earth's ])ath is slightly inclined to the median 

 plane — the so-called invariable plane — of the solar system. 

 This invariable plane is nearly coincident with the plane 

 of Jupiter's orbit, but not (juite. It crosses the eclip- 

 tic ascendingly, in about longitude 103°, and is inclined 

 to that plants at an aiigh^ of about 1° 'S'^\' (the 

 plane of Jupiter's orbit crosses the ecliptic ascendingly 

 in longitude 9'J", and is inclined to it at an angle of aljout 

 1° 18 J'). The pole of the ecliptic describes then a small 

 circle about the pole of tlu- invariable plane, tliis small 

 circle having a diameter of about 3° 11'. The time of 

 circuit is not as yet known, because of uncertainties which 

 exist as to the exact ma,sses of tlie members of the .solar 

 system. Sutfice it hero to notice that, owing to this change, 

 it is calculated that, during the last 3,000 year.s, the pole 

 of the ecliptic luis diminished its distance from the pole of 

 the equator by about 25'. That the physical tlieory is not far 



from correctness is shown by the circumstance that, from 

 observations made Viy Tclieou Kin^ 1,100 years licfore the 

 Christian em, the obliquity v. to have Ijeen then 



about '26' greater than at pr' 



Apart from this slight cji...... . j:i.th shown for the 



pole in our illustrative map is sufficiently near to correct- 

 ne.s.s. Wo note that the only eonsjjicuous star which has 

 been very near the ])ole during the last live or six thousand 

 years is the star Alpha Draconis, or Thubaii, yet this star 

 mu.st have heen far more conspicuous in Uayer's time (still 

 more, therefore, in all probability, in tlio time when it was 

 tlie Pole; Star) than it is in our own ; for Bayer lettered 

 th<" stai-s in each constellation in the order of their briglit- 

 nes.s, as nearly as he could estimate that wder with the 

 conqjaratively rough light-measuring methods available in 

 his time ; and the most cursory study of the stars of the 

 Dragon shows that both Beta and Gamma are now much 

 brighter than AljJia. 



Thubau was nearest the pole about 2,700 years ago; 

 but, of course, it was the Pole Star for a long time befor* 

 and after tlie period when it wa.s nearest the pole (just as 

 Alpha of the Little Bear is now the Pole Star, though 

 some 300 years will elapse liefore it is at its nearest 

 to the pole). In dealing with the Great Pyramid, 

 and trying to determine when it was built, it is not 

 the time when Tliuban was nearest the pole that we have 

 to consider, but the time when it was at a certain definite 

 distance (about 3'^ 40') from the pole. Now, a study of 

 the pole curve in our map shows that the star Thuban was 

 at this distance from the pole (about I Jths of the distance 

 between the successive circular lines in the map) at two 

 epochs. We open out a pair of compasses to the distance 

 just named, and placing one point on « Di-aconis, describe 

 with the other a circle ; this will cut the path of the pole 

 in two points, one corresponding to aViout 3350 b.c., the 

 other to about 2170 b.c. Either of these would correspond 

 with the position of the descending passage in the Great 

 Pyramid ; but Egyptologists tell us there can absolutely 

 be no doubt that the later epoch is far too late. If, then, 

 we regard the slant passage as intended to bear on the 

 Pole Star at its subpolar passage, we get the date of the 

 Pyramid assigned as about 3350 years B.C., with a pro- 

 bable limit of eiTor of not more than 200 years either way. 



Be this as it may, we know that in tlie past the constella- 

 tion of the Dragon was at the pole or boss of the celestial 

 sphere. In stellar temples, like those of which Rawlinson 

 gives examples, the Dragon would l>e the uppermost or 

 ruling constellation. And here, in passing, it may interest 

 the reader to note that some find evidence in this relation, 

 that when writei-s of old spoke of the Old Dragon as having 

 been cast from heaven, carrying two-thirds of the celestial 



• Fellows of the Astronomical Society will rciiipinber, with sotuo 

 amusement, how, a few vears ago, Lieru.-Col. DrnysOn (.*> mathnma* 

 tician of some skill) pravely told astronomir.'* at tho obsrrvcil 

 diminution of the ecliptic obliqnity, as if it wore siinic iinoxplaincd 

 chanfje, instead of being one of the most beautiful oontirmation.s of 

 the tlicorA- of pr.ivity. On the one hand are the calculated effects 

 of the pcrlmbing action of the st-vcral planets; on tho other, on 

 observed change, precisely corresponding; with calculation; yet, 

 instead of "confirmation strong" of accept ihI vimvs. Col. Drayson 

 could only find a source of perptoxity. (In this itnnginary dilficuJtT 

 he based an entii-oly imjiossible theory of tho glacial period. Yet, 

 when he described his views about tho ohaujring oliliqnity of tho 

 ecliptic, one of the greatest mathematiciuns living explained to him 

 that the change ho found BO peri)loxinjy was a necessary conw- 

 (pience of the action of gravity. He had been too many years 

 fostering his delusions to give them up. Such is the birth and 

 growth of pai-adoxes : an cm>r which could have been readily cor- 

 rected at the outset, seta a man wn><iin.' v.ars ,.l l:ib(.nr in devclo|v 

 iug a false theory j and when, a.t '■ ■ nocte<l, ho is 



too much in love with his o«Ti W' : ;>. 



