CUBILLOS] THE MOON 9 



around the earth in an orbit whose circumference is about 1,500,680 

 miles, at an average velocity of 2,290 miles per hour, and completes 

 a circuit in 27 days, 7 hours, 43 minutes and 11.5 seconds, which 

 constitutes a sidereal month, the average length of the common 

 month being 29 days, 12 hours, 44 minutes and 2.8 seconds. 



The mean density of the moon is about three-fifths that of the 

 earth and its mass rather less than one-eightieth, while the force 

 of gravity at its surface is about one-sixth that at the earth's 

 surface. 



The lunar globe is fotmd to be devoid of any bodies of water and 

 probably without any sensible atmosphere. Having no atmo- 

 sphere, there is nothing to temper the alternate changes there, 

 either from light to darkness or from heat to cold. The day side 

 of the moon is exposed to the sun's intense heat for about two 

 weeks, the temperatures probably reaching the boiling point, 

 while thru the long lunar night, the surface freezes and the tem- 

 perature falls very low, perhaps 200-250 degrees below zero. 



The most striking phenomena connected with the moon are its 

 series of changes or phases which are repeated every 29.5 days. 

 Being an opaque body it can only be seen as the light of the sun 

 illuminates it. When the moon is between the earth and the sun, 

 its dark side is turned towards the earth. It is then entirely 

 invisible and this phase is the real new moon. About two to three 

 days after this a thin crescent of silvery light appears, just after 

 sunset, this crescent commonly called new moon. Gradually the 

 crescent broadens out, as the moon moves away from the sun, until 

 on or about the seventh day, it reaches a position known as its 

 first quarter, and is then a bright semi-circle off in the south at 

 sunset. During the next few days, as the moon moves eastward, 

 more and more of its illuminating surface is brought into view, 

 until three-quarters of the disk appears lighted up and is then said 

 to be gibbous. On or about the 14th day, the moon now opposite 

 the sun, shows the whole of its round disc illuminated, this is 

 known as full moon, rising about sunset and setting about sunrise. 

 Passing on in its orbit, its phases recur in reverse order, the full 

 phase giving place to the gibbous, this in turn to the semi-circle, 

 which phase it reaches about the 21st day and is then seen high in 

 the heavens in the early morning hours. Step by step it draws 

 closer to the sun, thinning down to a crescent shape again, until 

 it is lost once more in the solar glare, only to re-emerge, on or about 

 the 28th day, as new moon, and begins again its cycle. 



