THE UNIVERSE IN WHICH WE LIVE 13 



the easy recognition of the new moon, the first quarter, the full 

 moon or second quarter, and the third quarter. 



The ancients classed the sun and moon as planets, for they, 

 like the true planets which they knew, Mercury, Venus, Mars, 

 Jupiter, and Saturn, seemed to move about among the stars. 

 The names of the seven days of the week were given in honor of 

 these seven planets. Sunday, Monday, and Saturday are evi- 

 dently names from sun, moon, and Saturn. The French names 

 for the other days of the week show plainly their derivation from 

 the Greek or Latin gods. Mardi is Tuesday; merer edi, Wednes- 

 day; jeudi, a contraction of Joms dies, is Thursday; and vendredi 

 is Friday. Our English names have come to us through the 

 substitution of the corresponding Norse deities, Tyr's or Tiwes' 

 day, Woden's day, Thor's day, Freya's day. Thus we are 

 reminded daily of the old myths that were blended with the 

 early astronomical lore. 



The moon exerts two very potent influences on the earth. It 

 is the chief cause both of the tides and of the precession of the 

 equinoxes. The sun is only a secondary cause, for, though it is 

 immeasurably larger, its greater distance makes it play the minor 

 role. We say that the moon revolves about the earth. As a 

 matter of fact, earth and moon revolve about a point that is 

 relatively near the earth's center. It is as if we should balance 

 on a point a rod with a large and very heavy ball at one end and a 

 small light one at the other (see Fig. 5, p. 14), then set it to whirling. 

 The small ball would move about the big one, but still the big 

 one would travel in a circular path about the balancing point. 

 So the earth constantly moves straight ahead and at the same 

 time toward the moon, making a nearly circular path around the 

 center of gravity of the pair. This path is not jerky as indicated 

 in the diagram, where first one movement is shown and then the 

 other, but quite smooth, since both movements occur simultane- 

 ously. Now, that part of the ocean near the moon moves toward 

 the moon most rapidly, the solid earth next most rapidly, and the 

 waters on the side opposite the moon least rapidly, since the pull 



