344 



THE GUIDE TO NATURE 



the lengths of the corresponding light 

 waves in the ether; that is, upon the 

 number of light waves which pass 

 through the narrow slit in a given time. 

 It is therefore evident that if we are 

 approaching the source of light more 

 waves will fall upon the prism in a 

 given time than otherwise and that the 

 wave lengths will thus become dis- 

 placed toward the violet end, which is 

 the region of shorter wave lengths. 

 And if the distance between us and the 

 source of light is increasing, the lines 

 will be displaced toward the red. By 

 measuring the account of this displace- 

 ment we can even determine how many 

 miles a second the star is approaching 

 or drawing away from us. 



esting example, for this has two dark 

 companions, the revolution about the 

 nearer one being completed in about 

 four days, while that about the more 

 distant requires several years for its 

 completion. 



THE PLANETS IN MAY. 



Though none of the bright planets 

 are now in the evening sky, the ob- 

 server may study three of them to ex- 

 cellent advantage if he will go out for 

 this purpose in the early morning; 

 Mercury and Saturn alone will remain 

 so near the sun throughout the month 

 that they cannot be satisfactorily ob- 

 served at any hour of the night. 



Just before sunrise the bright Jupi- 

 ter may be seen shining out low in the 



SOUTH Eft&f 



Figure 3. The southeastern Heavens at S A. M., May 1 and at 3 A. M., May 31, showing the positions 

 of Venus, Mars and Jupiter. 



The spectrum of Spica is crossed by 

 bright lines whose position shows us 

 that hydrogen and helium are especial- 

 ly prominent in this star. At certain 

 times these are displaced towards the 

 violet so far as to show that Spica is 

 then approaching us with a speed of 57 

 miles a second, two days later they are 

 so displaced towards the red that we 

 know that the star is then receding 

 from us with the same velocity. After 

 four days from the first date we find 

 the star approaching us again and this 

 alternate approach and recession con- 

 tinues with perfect regularity indefi- 

 nitely. It thus becomes evident that this 

 bright star is revolving about the com- 

 mon center of gravity of itself and a 

 dark companion with a speed of at 

 least 57 miles a second, the circuit of 

 its orbit being accomplished in four 

 days. 



Our North Star is a still more inter- 



south, near the eastern border of the 

 constellation Sagittarius. This planet 

 rises about midnight on May 1 and at 

 about 10 P. M. on May 31, so that 

 throughout the morning hours it is an 

 excellent position for observation. 



The fainter, though still conspic- 

 uous, Mars will also attract attention 

 in the southeast. During the month it 

 will move so rapidly across the constella- 

 tion Pisces that although it rises at 3 

 A. M. on May 1 this time will be dimin- 

 ished by only half an hour by May 31. 

 At the latter date Mars will be but a 

 little way below and to the right of the 

 planet Venus, the two objects will then 

 form an interesting figure in the morn- 

 ing sky. 



Throughout the month the planet 

 Venus will be seen running rapidly 

 eastward from the sun, becoming al- 

 most as conspicuous in the morning 

 sky as it has been for so many weeks 



