130 



KNOWLEDGE. 



[Jink, 1902. 



• ,. bri 

 " ifi wh 



With 

 iKithins; 



;lit isliind in tlio Mure Nubium Lii:-tl_v, Oic Mnn> Scroiiitiitis 

 iier in (lie ci'iitiv liv reason of llic (;nii( wliilr strciik loiniiig 

 Tiilio wliicli rros9er< it." 



regiircl to Ww sun, it iiiiiy l)e th(iiij,'ht tbat absolutely 

 fiiu Ix' (liiiic witlioiit i)|itic"il iissistiiiice. But any 



U 



A, Mare Crisium. \', l\Iare Fi.fcuiiditalis. ('. M;iri 

 (]Uillitatis. 1>, Mare Iiul-rium. E, OceiinusProceUarum, 

 li, Marc Niibium. H, Apeuniiies and Copernicns. 



Urawin'r of M. Maurice Petit. 



F, Mare Humoru 



observer who will j,'ive himself day by day to its patient 

 scrutiny will be astonished ere long at the result. Of 

 rourse, in this case, the term " naked eye " is no longer 

 ajiplicable, since the eye must be shielded from the over- 

 ) lowering light of the sun either by a dark glass or the 

 sun's light may be admitted through a small hole into a 

 darkened chamber and received upon a white screen. But 

 thus observed the mm at maximum activity will show a 

 sjiot large enough to be easily seen on fully one dav in 



])r 



of M. Autuiiiadi. 



lour ; not infrequently two or thi'oc spots may lie seen at 

 the same time. Had it occurred to the classical and 

 mediaeval astronomers to watch the sun systematically in 

 this wav, thev would not onlv have detected the existence of 



ajiots on its disc, but have demonstrated as certainly as 

 we know it to-day the |>eriod of the sunspot cycle, and the 

 value to us of that information would have been incal- 

 culable. More than that, it would have been possible for 

 them, from a long series of observations, to have fixed the 

 solar rotation period fairly exactly and to have made a 

 first approximation to the determination of the position of 

 the axis. 



The accompanying little table and diagram give a coni- 

 ])arison between the number of days in each year during 

 the last twt) decades in which there were spot groups the 



area of which as seen covered more thau one-thousandth 

 of the sun's apparent disk, and the mean daily spotted 

 area expressed iu milliouths of the visible hemisphere as 

 deducetl from the j>hotographic record at Greenwich. It 

 will be seen that the first record comes as sharjjly to a 

 maximum as the second, and falls off again as unmistak- 

 ably to a minimum. Even the minor details of the second 

 curve are faithfully indicatcil in the first. The limit 

 chosen — one-thousiuidili of the visible disc, or almost 



