re MOON. 



B, C, R. E, CornioulaU, Faloata, eurvaU in cornua. 

 D Primus sextilis aspeotus, et E secuudus. 



F IMraa Quadrature, O secunda; luna dividua, I 

 bisccta, diiuidiaU, semiplena. 



t'l! I 



H, K, L, I. Luna Olbba, gibbom ; H primus. If 



secundus aspectus trinus; Luna in triquetro. \ 

 M I'lcniiunium, Oppositio, Luna Totilunis, in dia- 1 



metro sinuate in orbem, medius mentis. J 



Luna eresoen* ab A per F in M, Luna descrescens / 



sea senescens ab M per U in A. \ 



If the moon moved in the plane of the ecliptic, or of the sun's 

 motion, as in the figure, there would be an eclipse of the sun at every 

 new moon (A), and of the moon at every full moon (u) ; since in the 

 former case the moon would hide the sun, and in the latter the earth, 

 would intercept the sun's light. The moon however is generally a 

 little on one side or the other of the ecliptic, not enough to introduce 

 any sensible error into the preceding explanation of the phases, but 

 enough to hinder the eclipses from taking place, except now and then : 

 we shall see more of this presently. Again, if the sun remained in the 

 line i s, the lunation, or complete cycle of phases, would be of the 

 same duration as the actual revolution of the moon round the heavens. 

 Since however the sun moves slowly forward in the same direction as 

 the moon, the latter does not alter iU phases so rapidly as in the figure, 

 nor U the cycle of phases complete until the moon has overtaken 

 the sun. 



It U usual to divide the whole lunation into four quarters, the first 

 from new moon to increasing half moon, the second from half moon to 

 full moon, the third from full moon to waning half moon, the fourth from 

 half moon to new moon. Each of these is called the change of the moon, 

 and it is a very common belief that a change of weather and wind is 

 to be expected, if not at every change of the moon, at least much 

 at the changes than in the intervals. This opinion, when not absolutely 

 received as true, is usually treated as the extreme of absurdity. It is in 

 truth neither one thing nor the other, as the following considerations 

 will show. 



The atmosphere is continually undergoing a slight alteration from 

 the effect* of the tide. At new and full moon (or rather a little after 

 these phenomena) there are those great tides called the spring-tides, 

 arising from the action of both luminaries ; at the two quarters the 

 same luminaries oppose each other, and the quarters are followed by 

 the smaller floods, called neap-tides. What effect may be produced 

 by this succession of smaller and greater oscillations of the sea, which 

 must produce oscillations of the atmosphere, it is impossible to say 

 beforehand. Again, we know nothing of the electric action of either 

 luminary upon the earth, or whether any and what electric state may 

 depend upon their relative position. We have therefore abundant 

 grounds d priori to abstain from forming any opinion upon the effect 

 of the heavenly bodies upon the weather ; and we shall now K" 

 results of such facts as observation has furnished. M. Arago collected 

 the evidence on this subject in an article published in the ' Annuaire' 

 for 1833 ; the general conclusion derived from them is, that upon the 

 whole- there is a Unit more rain during the second quarter than during 

 either of the others ; but that there is no evidence to confirm the 

 common notion that a change of the moon is accompanied by change of 

 weather. It has also been observed that the height of the barometer is, 

 one time with another, less in the middle of the second quarter than in 

 that of either of the others ; and that it is somewhat greater at new 

 and full moon than at the quarters. With regard to a great many 

 other asserted effects of the moon upon animal and vegetable life, it 

 can only be said that there is no conclusive evidence for or against 

 them ; nothing but a long series of observations can settle such 

 points, and this is not likely to be made (or if made, to be made 

 fairly) by those who have predetermined tho questions in one way or 

 the other. 



The moon's age is usually reckoned from the new moon, and tin' rule* 

 by which Easter in found depend, or should depend, upon a correct 

 knowledge of this age' at the beginning of the year, called the i 

 But all readers should remember that the sun and moon by wlii.-h 

 Easter is found are not the real bodies, l.ut fictitious ones, m..ving 

 not with the real but the average motions, and then <foie w.i 

 before and sometimes behind the real bodies. It should then be no 

 matter of surprise if, as will happen. Easter Sunday should soi. 

 be seven days sooner or later titan it would be if tho real bodies were 

 employed. [KASTKR, METHOD or Km.iv.. | 



We now come to the actual motion of the moon round the earth, 

 which is the most complicated question in astronomy. Roughly speak- 

 ing, it may be said thai the moon's motion is circular, which u sufficient 

 for the explanation of thf phases ; it U somewhat, l.ut very little, more 

 correct to say it is elliptical. If the moon's url.it were actually 

 exhibited in space, an elli|e might be found win. h would nearly fit 

 one of its convolutions ; l.ut the succeeding convolutions would .1. pait 

 further and further from such an ellipse, and it would 1m i 

 years before a convolution would again occur which is situated in spac* 

 near to the ellipse with which we started. And though astronomers 

 have found a way of simplifying the question, by supposing the moon 



MOON. Tel 



to move in an ellipse which itself moves in space, jet we may better 

 explain the subject by arriving at that ellipse from the real motion 

 than by beginning with it 



When the motion of the moon is watched in the heavens with 

 instruments fitted to measure her apparent diameter, it is soon found 

 that she changes her distance from the earth, becoming alternately 

 larger and smaller. Her path is not very much inclined to the . 

 so that she is never 5J from some one of the positions which tho sun 



has had or will have in the course of tho year. We may explain the 

 i i path in the heavens by the annexed figure, which represents 

 a portion of the apparent heavens : T is the earth in the centre ; 

 is the eircle of the ecliptic ; yyi/ii and .-;;: are small circles parallel to 

 the ecliptic, and each distant from it in the heavens by an angle of 

 5" 8' 47" 8. The moon may rise 8' 47"'15 above XXXJC,OT fall as much 

 below zzsz ; but these two circles are chosen because they are meant: 

 that is to say, for every convolution which rises above xxxjc there will 

 be another, described at some other time, at which it. fall* i-hort of 

 xxxx; so that in a long series of years the sum of all the arcs by 

 which convolutions rise above xxxx would be equal to the 

 those arcs by which other convolutions do not attain .r.r.i .<. Th. 

 5 8' 47"'9 is, in the astronomer'.* elliptic fiction, the mean incl. 

 of the orbit to the plane of the ecliptic. The dotted line \ 

 is one complete convolution of the orbit and the greater p 

 another. We suppose the moon to set off from its highest point (high 

 and low have reference to the ecliptic) A, at or very n ; hence 



it falls to the descending node n, and continues to descend to it.- If 

 point c, whence it rises to the ascending node n. and thence ascend-' to 

 E, thence to the next descending node F, thence to 

 (at nr near ;zzz\, and to the next ascending node n, ic In this way 

 the whole of the lunar zodiac U interlaced with the convolutions of its 

 orbit, which go on for ever ; nor have we any reason to suppose that 

 the cycle of convolutions is ever complete, so as to begin again. 



The first thing we have to notice is what is called the regressive 

 motion [MOTION] of the node*. The first node we meet with in it, and 

 the next, i>, is not exactly opposite to n.but a little behind the opposite 

 Iiint; the next, F, is still more behind B. The \\..r.ls I., lore and 

 behind have reference to the direction of the motion. Thie recession 

 of the node amounts, one year of 365 days with anotl 

 19* 19' 42"-816, and the node makes a complete retrograde revolution 

 in 6798-39108 mean solar days, or 18'6 years nearly. The point in 

 wliieh the moon ascend* through the ec.iptic falls Kick more than 

 twice tho moon's diameter in each revolution. The amount, ho, 

 is subject to some variation ; that given above is its average. 



Again, tin- apparent diameter of the moon is observed to vary, owing 

 to an alteration of her distance from the earth. When least, it is 

 20' 2"-91 ; when greatest, 83' 31" -"7. Hut it is observed thA the least 

 and greatest diameters of a single revolution ore not exactly the same 

 as those of another revolution; and also that the place win i 

 diameter is least U not exactly opposite to that in which it is greatest, 

 but always in advance. Thus the diameter, being greatest at A, becomes 

 least at r, in advance of the point opposite to A, greatest again at g (in 

 advance of A), and least again at B. Now the apparent diameter niiut 

 be least when the distance is greatest, and Woe vtrtd ; the |.int of n 

 convolution most distant from the earth is called the apo-gee, that 

 nearest to the earth the peri-gee. There is, then, a progression of the 

 apogee, and its average quantity i no less than 8' 41" for each solar 

 day, or 40 3V 45 ''3ti in 865 days, which is equivalent to a complete 

 revolution in :ii!:i-2-&75348 mean solar davs, or about nine years. The 

 quantities above given are averages, for the actual program 

 irregular. 



We may notice, then, five distinct species of months : 1. Th.- 

 average sidereal month, or complete circuit of the heavens. 2. The 

 average lunation, common month, or interval between two conjunctions 



