Iu6 



* KNOWLEDGE 



lSept. 7, 1883. 



THE HARVEST MOOK 

 By Richard A. Pkoctor. 



FROM numbers of letters which have reached ma within 

 the last few weeks — to say nothing of past years — I 

 infer that the explanations of the Harvest Moon in our 

 books of astronomy have not prove i altogetlier clear and 

 convincing, or else perhaps they have not been very care- 

 fully read. In fact I have reason to believe that the 

 moon's motions are not generally understood, or even 

 known. I receive the most marvellous letters of inquiry 

 respecting the moon's motions and phases, — some of them 

 perfectly astounding in the calmness with which they 

 demand explanations of peculiarities which have no 

 existence in nature. One correspondent asks me why 

 the moon is always full in the Arctic regions and never 

 sets there, why she does not behave in the same way 

 within the Antarctic as within the Arctic circle, and so 



point, S the south, the east and west points (Orient and 

 Occident) being both at O (the former nearer the latter 

 farther from the eye, which is supposed to view the celestial 

 sphere from outside). Let Z be the zenith, Z' the 

 nadir, POP' the polar axis, E O E' the celestial equator 

 ( ^ P O N = 51i- and E S=3Si°). 



Suppose the sun's path or the ecliptic represented by 

 EOE' inclined 23/;^ to EOE'. This path is of course 

 carried round by the diurnal rotation, so that in our tigure 

 it might either be shown as a straight line or as an ellipse, 

 according to the time of day and year. But once m each day 

 the sun's path would have the position shown in the figure, 

 the further half of the path passing through the west 

 point at O and the nearer part through the east point 

 at O. 



Draw M M' m O m\ inclined 5^ 8' to e O e'. Then 

 MOM' lines would represent the moon's path as pro- 

 jected into a straight line at the time when the inclination 

 to the equator E E' is a maximum — or about 28''; while 



forth. Another asks me whether there is any other ex- 

 planation of the Harvest Moon than the effects of cold 

 condensing the air ; and so forth, till I feel disposed to 

 lay down in despair the explanatory pen in order that I 

 may take up that particular soft-nibbed quill with which 

 I endeavour to impart information to those who chance not 

 to possess it in any abnormal degree. 



In my treatise on the Moon, the usual explanation of the 

 harvest moon is given, with some details which are wanting 

 from most accounts. Some errors also are corrected which 

 have resulted from the careless way in which the compara- 

 tively rough statements of Ferguson have been copied and 

 re-copied in our books on astronomy. I propose now to 

 give an explanation which I believe to be new (at least in 

 form). It also includes some information about the lunar 

 movements usually overlooked. 



First let it be noted that the path of the moon among 

 the stars is somewhat like that of the sun, but inclined 

 5° 8' (on the average) to his. 



Thus suppose Z N Z' S represents the celestial sphere, 

 S O N the horizon plane for London, N being the north 



m O m! would represent the moon's path projected into a 

 straight line at the time when the inclination to the 

 equator is a minimum, or about 1S;|°. The ellipse 

 e n' e' n (ii 7i' = M m) would represent the moon's path 

 (sun's path being brought to position e O «') at the time 

 when the inclination has its mean value, about 23i^, k 

 and I being the points (/ on the nearer, k on the farther 

 hemisphere) where the path at this time cuts the 

 equator. 



About 18.\ years pass while the moon's path is changing 

 from the position ?« m', to the position e a' e' n, thence to 

 the position M 'M', and so through the position e n' e' n 

 again, to the position in O //;'. 



We need not further amuse ourselves with these changes 

 of inclination to the equator, but taking the mean inclina- 

 tion, as when the path is e ii e' n, consider the moon's 

 monthly and diurnal motions, — and first without reference 

 to changes of phase. 



But it will be well to bring the celestial sphere to such a 

 position that the path en'e'nis projected into a straight 

 line, manifestly in the position e O e' which before repre- 



