Sky Study 921 



And because of no layers of air to make an aerial perspective, a mountain a 

 hundred miles away would seem as close to us as one a mile away. 



Since there is no atmosphere on the moon to act as a buffer between the 

 cold of outer space, which is estimated to be 250° below zero, and the heat 

 of the sun, which is 500° above zero, the temperature of the moon would 

 vary 750° between day and night, or between sunshine and shadow, because 

 there is no air to carry the heat over into the shadow or to blanket the world 

 at night. But this great change of temperature between sunlight and dark- 

 ness is the only force on the moon to change the shape of its rocks, for the 

 expansion under heat and contraction under cold must break and crumble 

 even the firmest rock more or less. Our rocks are broken by the freezing of 

 water that creeps into every crevice, but there is no water to act on the 

 moon's mountains in this fashion or to wear them away by dashing over 

 their surface. However, the rocks and mountains of the moon may be 

 changed in shape by the battering of meteorites, which pelt into the moon 

 by the million, since the moon has no air to set them afire and make them 

 into harmless shooting stars, burning up before they strike. But though a 

 meteorite weighing thousands of tons should crash into a moon mountain 

 and shatter it to atoms there would be no sound, since sound is carried 

 only by the atmosphere. 



Imagine this barren, dead world, chained to our earth by links forged 

 from unbreakable gravity, with never a breath of air, a drop of rain or flake 

 of snow, with no streams, nor seas, nor graced by any green thing — not even 

 a blade of grass — a tree, nor by the presence of any living creature ! Out 

 there in space it whirls its dreary round, with its stupendous mountains 

 cutting the black skies with their jagged peaks above, and casting their 

 inky shadows below ; heated by the sun's rays until hotter than the flame of 

 a blast furnace, then suddenly immersed into cold that would freeze our air 

 into solid ice, its only companion the terrific rain of meteoric stones driven 

 against it with a force far beyond that of cannon balls, and yet with never a 

 sound as loud as a whisper to break the terrible stillness which envelops it. 



LESSON CCXXXIV 

 The Moon 



Leading thought — The moon always has the same side turned toward us 

 so we do not know what is on the other side. The moon shines by re- 

 flected light from the sun, and is always half in light and half in shadow. 

 The moon has neither air nor water on its surface and what we call the 

 moon phases depend on how much of the lighted surface we see. 



Method — Have the pupils observe the moon as often as possible for a 

 month, beginning with the full moon. After the suggested experiment, the 

 questions which follow may be given a few at a time. 



Experiment for recess — Darken the room as much as possible; use a 

 lighted lamp or gas jet or electric light for the sun, which is, of course, 

 stationary. Take a large apple to represent the earth and a small one to 

 represent the moon. Thrust a hat pin through the big apple to represent 

 the axis of the earth and also the axis about which the moon revolves. Tie 

 a string about a foot long to the stem of the moon apple and make fast the 

 other end to the hat pin just above the earth apple. Hold the hat pin in 

 one hand and revolve the apple representing the moon slowly with the other 



