PHASES OF THE MOON.] 



ASTRONOMY. 



935 



CHAPTER IV. 

 ON THE MOON. 



THIS object, the most valuable in relation to mankind 

 which serves as a guide to the mariner in the trackless 

 path of the ocean, and whose influence, combined with 

 the sun, produces the tides, will next claim our attention. 

 Viewing its path in the heavens, we find its motion to be 

 the most rapid of any other body ; the earth's changes of 

 position, which, with reference to the sun, amounted in 

 the course of a day to 1, will, for the moon, be more 

 than twelve times that quantity. In consequence of this 

 quick displacement with reference to the stars, its motions 

 have become a most valuable means of determining, at 

 any instant of time, the position of a vessel at sea. 



Her appearance, and the breadth of her illuminated 

 disc, vary every moment, showing that she is an opaque 

 body which receives light from the sun. The disc, how- 

 ever, during each successive lunation, always presents 

 the same physical appearances ; and we have never been 

 able to perceive the other hemisphere, which can only 

 be seen when in conjunction with the sun, or at that 

 time when the darkened hemisphere is directed to the 

 earth. This arises from her rotation on an axis, in a 

 period nearly coincident with the time of her sidereal 

 revolution. 



A most striking proof of the opacity of the body of 

 the moon, is shown by a phenomenon frequently visible 

 viz., the occultation of a star by the moon. In con? 

 sequence of the proper motion of the moon from west to 

 east, she will pass over or occult in her march all the 

 fixed stars in her path. Before full moon, we see the 

 star disappear at her unenlightened hemisphere, and re- 

 appear at the illuminated limb. The time of the occulta- 

 tion will depend on the motion of the moon in her orbit, 

 which can thus be calculated with great precision, and is 

 found to agree exactly with observation. 



PHASES OF THE MOON. The phases are thus explained : 

 If we assume that the moon describes a circle around 

 the earth, and that the sun is infinitely distant, we shall 

 find that the rays of light from the sun will always be 

 parallel, and will only illuminate one-half of the convex 

 hemisphere limited by the line m n (Fig. 41). But to a 



Fig. 41. 



pectator on the earth, at T, the illuminated portion will 

 be limited by the linep q, directed perpendicularly to the 

 radius of the moon's orbit. Thus at new moon, or A, 

 the enlightened hemisphere is turned towards the sun, 

 and the darkened part to the earth, when it is, conse- 

 quently, invisible, except in peculiar circumstances, 

 called " eclipses pf the sun," when its diameter obscures 

 for a time that luminary. At B a small portion of the 



enlightened hemisphere is seen from the earth. At C it 

 is at its "first quarter." The enlightened portion in- 

 creases gradually till it becomes a maximum, or is in 

 opposition to the sun. At G it is again on the wane, or 

 at its last quarter, decreasing gradually, but in a direction 

 opposite to the first appearances. At A it is again found 

 in conjunction. The above successive appearances are 

 shown by the following diagrams. When the moon 

 passes into the region of the heavens where the sun is, it 

 is invisible. At the end of a few days, a careful observer 

 will see the moon after sunset in the form of a crescent, 

 with its convex side to the right, as it is represented in 

 A (Fig. 42) ; this disappears, after a short period, under the 



Fig. 12. 



H I 



horizon. For some days the same appearances present 

 themselves, the crescent becoming broader at its centre, 

 as in B, and remaining longer above the horizon. Six or 

 seven days more and the crescent has nearly disappeared, 

 and a half moon presents itself to us, as in C. At this 

 period it is sufficiently distant from the sun, and crosses 

 the meridian six hours after it. The moon continues to 

 increase, first to three-fourths, and finally to a full circle, 

 as in D and E. In another week, the waning moon, 

 which is seen in the morning, assumes the appearance of 

 F, G, H, and I, the concave side of the crescent being 

 now to the left. 



If the moon did not revolve on its axis during this 

 progress, we should not have the same spots visible on, 

 its surface at each position in its orbit, in nearly the s;ime 

 direction. We are thus led to infer that the moon makes. 

 a corresponding turn on her axis, in the interval from 

 conjunction to conjunction. Thus, if the surface, were, 

 without motion, while she 

 moved in her orbit from L 

 to I/ (Fig. 43), the position 

 L it would take a position 

 L' 6 ; but during this time, 

 she has revolved on her axis 

 through an angle 6L'o, equal 

 to the angle L T L' described 

 in her orbit. Similarly, we 

 find that when she has de- 

 scribed a motion of 180" or 

 270, she has rotated on her 

 ijxis by an equivalent angle.* 



LlBRATION OF THE MoON. 



Seen through a telescope, 



there is another phenomenon called the libration, which 



A somewhat acrimonious discussion -was commenced some yean apo, 

 by Mr. Jellinger Symons, who stoutly maintained that the moon did iwt 

 rotate on her li. The idea, however, was at lilt given up. ED. 



Fig. 43. 



