LK.SSON8 IN ASTRONOMY. 



133 



explained, the dark hemisphere U turned toward* the earth, and 

 horefore, totally invisible. Aa it trareU onward* in it- 

 orbit, u hinall portion of tliu illuminated hemisphere come* into 

 view, presenting the appearance of a narrow orescent. At thin 

 : mil- tho whole of the moon may at tituuti bu faintly dUcoruud by 

 :.aked eye. This U 



is refloctc-d from tho earth 

 on to tho moon.ortho earth- 

 thine, a* it is .-allnl. 



A moment's consider** 

 tion of the relative posi- 

 tions of tho three bodies 

 will Hhow that at th> 

 when the moon is new to 

 M I'uith must appear 

 full to tho inhiibiUnta of 

 the moon (should there be 

 any), and thus at tliU 

 d tho most powerful 

 li^'ht would bo refloctod 

 upon her. 



As tho moon travels on 

 in lu-r orbit, more and more 

 of her illuminated hemi- 

 sphere comes into sight, 

 iill she arrives at n, when 

 exactly one-half of her disc 

 is illuminated, and it is 

 called her first quarter. 

 Still continuing to move on, 



.she at length arrives, in the course of her circuit round tho earth, 

 at c, when the sun and the earth are on the same side of her, 

 and accordingly the illuminated hemisphere is directed towards 

 us, presenting the entire disc of the full moon. After passing 

 c, tho same series of changes is repeated, but in the inverse 

 order ; the cusps, however, 

 are turned in the other 

 direction. 



The phenomena of lunar 

 and solar eclipses are 

 closely connected with the 

 motions of the moon : we 

 must, however, defer tho 

 consideration of the causes 

 which produce them till we 

 have completed our account 

 of the solar system. 



There is one very remark- 

 able fact in connection with 

 the moon, and that is that 

 we have never seen more 

 than about one-half of its 

 surface. It rotates on its 

 own axis in exactly the 

 same time that it takes to 

 revolve around the earth, 

 and hence the same side is 

 always turned towards us. 

 Owing to the fact that the 

 moon's axis is inclined 

 about 1^ to the plane of 

 its orbit, we occasionally 

 see a little beyond tho 

 north pole, and then a 

 similar distance beyond the 

 south. This variation, how- 

 ever, which is known as its 

 libration in latitude, is 

 very slight. It has also a 

 slight libration in longitude, 

 but both these motions are 



so small, being in fact almost imperceptible, that they do not 

 sensibly interfere with the accuracy of the above statement. 

 A good deal of fruitless controversy has frequently arisen as 

 to whether the moon does actually rotate or not. The fact 

 that she always presents one side towards the earth is not at 

 all disputed ; the only question is whether this motion ought to 



be called rotation or not. A very little consideration, however, 

 will BOOD convince the student that when he compares it with 

 tho nun, or *orae fixed body, it does actually rotate on its axi*. 



By mean* of a goo teleioope a very distinct view may be 

 obtmuud of tUo uioon. With a power of 1000 we are, an it were, 



brought within 239 milM 

 of it* surface, and on very 

 favourable occasion* a 

 power even higher than 

 this has been applied 

 With the highest power, 

 however, yet employed, no 

 trace of any inhabitant* ha* 

 been discovered, though 

 any large town mnut hare 

 been seen, did snch exist 

 on the visible side. 



Even to the naked eye 

 the moon present* the ap- 

 pearance of a ragged and 

 uneven surface, and tele- 

 scopic observation confirms 

 this opinion. We find that 

 in many parts of it* sur- 

 face very high mountains 

 exist, and the elevation* of 

 many of these have been 

 measured by observing the 

 shadows cast by them when 

 the sun shine* obliquely. 

 At the time of full moou 

 these shadows, that have hitherto been so conspicuous, disap- 

 pear, as the suu then shines vertically upon them. Very accu- 

 rate maps have now been drawn of the moon's surface on 

 a large scale, and the principal mountains have received names, 

 usually those of celebrated astronomers. One peak, named 



Newton, is found to have 

 an elevation of nearly 

 24,000 feet, and several 

 others are very lofty. 

 Figure 23 presents a good 

 general idea of its appear- 

 ance ; it is, however, a 

 much more beautiful object 

 for telescopic observatior 

 previously to its first quar- 

 ter, as at that time the 

 shadows impart a greater 

 relief to its surface. 



The most careful tele- 

 scopic observations hare 

 shown that our satellite is 

 devoid of an atmosphere, 

 or, at least, that if one be 

 present at all. its density 

 must be less than that of 

 the most perfect vacuum 

 which can be obtained by 

 the air-pump. On account 

 of this, twilight does not 

 exist on its surface, and a 

 sharp line divides the illu- 

 minated portions from tht 

 dork parts. When we gat* 

 on it through a powerful 

 glass, the dork black sha- 

 dow may be seen gradually 

 diminishing in length a* 

 the sun shines more verti- 

 cally on the mountains. 



The elevated peaks of 

 the mountains of the moon 



likewise catch and reflect the rays of the sun long before the 

 plains around them, and shine out brilliantly against the dark 

 ground. The most remarkable feature, however, in the lunar 

 scenery is the number of ring-craters which exist on its surface. 

 These resemble huge volcanic craters. In some a large plain, 

 somewhat circular in shape, is surrounded by a lofty and rugged 



