66 The Realm of Nature CHAP. 



30' ; that is to say, 1 80 full moons, one above another, 

 would extend from the horizon to the zenith. The diameter 

 of a body subtending this angle at a distance of 240,000 

 miles must be about 2000 miles, or, to be exact, 2153 miles. 

 The mass of the Moon has been estimated to be -^ of that 

 of the Earth ; its mean density is about 3 times that of water. 



10 1. The Moon's Surface. The Full Moon appears 

 to be diversified with patches of unequal brightness, but 

 observations with powerful telescopes prove that it is simply 

 a lithosphere surrounded by neither water nor air. Ring- 

 shaped mountains closely resembling volcanic craters may 

 be easily seen by using an ordinary field-glass, especially 

 when the Moon is so placed that sunlight illuminates only 

 part of the surface. The Moon shines by reflecting sunlight, 

 and even when most brilliant its light is so feeble that if the 

 whole visible sky (a surface equal to 105,000 moons) were 

 to shine as brightly the effect on the Earth would only be 

 equal to one-fifth that of the Sun. As the Moon revolves 

 round the Earth we see the side turned toward us wholly lit 

 by the Sun once a month and call it Full Moon ; a fortnight 

 later the Sun is shining only on the side turned from us and 

 we see the Moon dark, calling it New Moon. Between these 

 periods the illuminated area wanes or dwindles down to a 

 crescent, and again waxes or grows into the full round. 



102. Period of the Moon. The Moon revolves round 

 the Earth in 27 days, 7 hours, 43 minutes; but the in- 

 terval of time between successive new moons or full moons 

 (the lunar month) is rather more than two days longer. The 

 Moon always presents the same aspect to the Earth only 

 one half, and always the same half, is to be seen, although 

 now and again slight irregularities in its motion reveal a 

 narrow additional strip at one edge or another. The fact that 

 no one has seen the other half proves that the Moon rotates 

 on its axis in exactly the same time as it revolves round the 

 Earth. If it had no rotation we should see all round it. To 

 prove this, pass a loop of thread over a drawing-pin fixed in 

 a horizontal board or table and the other end of the loop 

 round a pencil. Keep the cord stretched, and, holding the 

 pencil between the finger and thumb facing in the direction 



