MOON 



earth's attractive force, whereby tin- IIIIMIII'H angular 

 xclocity is increased. This |.urt will, however, 



compensate itself in tli urse of ages. 1 1 is partly 



*lo due to a slow leBgtheeJBg of the da\ i .. the 

 period of tin- earth's inflation, which arises limn the 

 Irictional action of the tides, th:it art like a brake 

 ii|M)ii th(* earth's surface. Tin- |H>rtion remain* 



UIICom|H>l|sj|tl, of Course. 



The IIIIMMI 's distance fnun tin 1 earth is obtained I'.v 

 observation* "I ii- place fnun two wUdj^OpUBtM 

 stations, such as the oliscnaliiiics at (Ireenwich 

 and the <'a|>e of (iood Hope. If simultaneously 

 observed from them-, the moon will not a|>|iear to 

 both observers in the MIMIC ]M>sition among the 

 stars: the amount of dillerence in apparent |Mwi- 

 tion depending "ii it" distance from the earth at 

 the time. From this dillercnce is deduced the 

 mi MIU'S hurizontal parallax. This is the change in 

 the iniHin's place which would lie noted by an 

 observer on shifting his place from the centre of 

 the earth to a point on it* surface where the moon 

 would be seen on the horizon. The moon's mass 

 being very nearly ,",th of the earth's, the force of 

 lunar gravity at the moon's surface is then ouch 

 that any object would weigh there only (V15 of tte 

 weight at the earth's surface, and a falling body 

 ould there only traverse 2'48 feet in the first 

 second of it course. The moon's rotation on its 

 axis agrees in |>criod with its revolution round 

 the earth, HO that, as has been said, we have 



Fig 2. The Moon 



tion. The delicate colouring and shade of terrestrial 

 scenery is entirely ali-eni. All i- marked in white 

 and black, or in various shades of yellowish gin\. 

 Nothing like mist, cloud, or vapour ha* ever been 

 -'!. except ill Mime doiilitful instance* mi the Moor 



of tl T.-itci Plato, or other deep depraeiMMi There 



is neither water to furnish va|Kinr, nor atmosphere 

 lit to liear clouils. ( >li~ervation of the stars occulted 

 hi the moon (aeeOcci'l.TA'i KINS i confirms this, and, 

 if there In- even an attenuated atmosphere, it cannot 

 have more than T loth of the surface-density of our 

 o ii. liessel's maximum value for thin of m'oit has 

 Ix-eii shown by Xeison to In- too small, anil it in not 

 improbable that the moon possesses an atmo-pheie 

 of extreme rarity, having .1 surface density of prob- 

 ably aliout jjjtli that of il ailli. Veg lation and 



animal life appear to lie eijually alieent from tin* 

 moon, and the best modern theories of its state 

 require us to regard the surface as either bare rock 

 and sand, or as ice and snow. These theories have 

 arisen in the at tempt to explain the strange forms 

 of the lunar surface. These foniix have IK-CM 

 classine<l, and the arrangement commonly in uxe is 

 followed here as convenient. But it must not be 

 regarded as a really scientific one. For some 

 formations, while in their general aspect liclonging 

 to one class, might really be assignee! to other 

 classes in other respect*. 



The term Mare ( Lat. ) has been applied to the 

 large dark plains, an example of which i- the .!//< 

 Crixivm, easily seen as an oval dark 

 spot near the edge of the new moon. 

 There are also large level areas which 

 are brighter, and to which no special 

 name lia- lieen attached. To one lai^-e 

 irregular dark plain the title of On mi a* 

 Prorellarttm has Iwen given. The terms 

 Pains (marsh), l.arta (lake), and Xinii 

 (gulf) have been somewhat fancifully 

 used to denote smaller dark areas. 



Under the broad title craters have 

 liecn gr<iu|ied many formations, so dif- 

 ferent from one another that seleno- 

 grajihcrs now divide them into walled 

 plains, mountain-rings, ring-plains, 

 crater-plains, craters, craterlete, crater- 

 pite, crater-cones, and depressions 

 names expressive enough of more or 

 ]e>s circular ramparts varying in si/e 

 from 150 miles in diameter to a few 

 hundred yards, and in depth, or height 

 of walls, ranging from 18,000 feet do\\ n 

 wnrds. In some parts of the lunar sur- 

 face these literally swarm, crossing and 

 interrupt ing one another, smaller ones 

 |>crched on the edge or sides of larger 

 ones, and, generally, in the flat bottom 

 of the larger ones several of the smaller 

 kinds arc sure to be seen. Any moder- 

 ately good telescope will show the larger 

 nrt .|iiart.i ( inverted, MBeen through telescope ). kinds. Besides these there are the line 



(Fnmi Photograph tl.niuuli ti.eOmit Lick Telaicope, by Pr-.f. sTw. Biirnluiia) mountain ranges, called the Lunar Alps. 



nines, Cordilleras, &<., similar in 



always the same side presented to our view. 

 Occasionally, however, we see a little round one 

 or other edge owing to I.ilirnlinn (ij.v.|. 



From thi-w millions of size, density, and mas 



we should ex|iect that, while presenting some 

 features of agreement, in most rrapecto the moon 

 would ililler widely from the e.-uih in />/iyvi-'"' 

 /linn. Kven to the naked eye some peculiar- 

 me* arc olniou-. Attentively watching the full 

 moon, we soon become familiar with ito irregularly- 

 sinittcd surface which nrnr r/miiiir.i. It cannot 

 ilieii be like that of the I'arlh. which is often 

 obscured by clouds ami mist. The tclc-copc con 

 linn- tin- imprcioii. All the . Ictaiis of the lunar 

 vurlacc an- hard, cold, and glaring in their delinea- 



Apennines, 



most n--|,e, t- to terrestrial chains. These 

 from 20,000 feet in height downwards, and where 

 their profile is seen at the edge of the lunar .li-c 

 they form distinct nntrJies. The lunar rill* (so 

 named by Schrtiter, their discoverer, in 1787) are 

 clefts or cracks in the surface, passing often right, 

 through mountains and valleys, sometimes for a 



distal if :KI miles, their brciidth licing rchitiicly 



so small as to gi\e th the appearance of true 



ov<'7..v. 



Most striking of all lunar appearances are the, 

 broad while m//v. which diverge from some of the 

 principal lunar ring-plains. Those proceeding from 

 I'lfliii extend, in one cse at lea.-t. nearly 2<KX) 

 miles. There are hundreds of them, and they 



