3o6 



THE GUIDE TO NATURE 



The second or third weeks of the pre- 

 sent month will be especially favorable 

 for the observation, especially the 

 third week, when the moon changes 

 from last quarter to new. 



Fig. 2 — The southern heavens at midnight, showing 

 the motion of the Counter-Glow during the mor.th of 

 March. 



The observer will then clearly see the 

 faint pyramid of light ; it will be seen 

 to be brightest nearest the sun and 

 along its axis, and to fade away toward 

 its edges. Its brightest portions may 

 be nearly or quite as bright as the aver- 

 age area of the Milky Way. It may 

 perhaps be traced at its apex up to, or 

 even beyond, the Pleiades. At first the 

 observer can best locate it by turning 

 his eye to the right and left of the eclip- 

 tic and noting how much darker the 

 sky appears in these adjacant regions. 

 It may also be more clearly seen by 

 what is called "Averted Vision," the 

 eye not being directed exactly toward 

 the object itself, but a little to one side 

 of it. But when its position has been 

 clearly ascertained by these methods it 

 will bear more direct scrutiny. 



It is practically certain that this il- 

 lumination is caused by a great, flatten- 

 ed cloud of little particles which sur- 

 round the sun and extend outward in 

 all directions beyond the orbit of the 

 earth. The earth and the two inner- 

 most planets are, in fact, immersed in 

 the cloud. It is the sunlight reflected 

 from each of the innumerable little par- 

 ticles of the cloud that causes the faint 

 illumination that we see. 



The Counter-Glow. 



A very much more difficult object 

 than the Zodiacal Light is a very faint 

 little area of illumination which moves 

 entirely around the heavens among the 

 constellations in the course of the year, 

 its center always being found in the 

 ecliptic and exactly opposite the sun. 



This is k.:iown as the Counter-Glow, an 

 object so faint that it eluded detection 

 until the year 1854, and which when 

 again noticed many years later was 

 thought to be a new discovery. 



The Counter-Glow can only be seen 

 Vv'hen the general background of the 

 heavens is very dark ; when it is passing 

 through the Milky Way, and even when 

 it approaches the vicinity of a bright 

 star or planet, its observation is render- 

 ed very difficult. During the present 

 month its center will move over a com- 

 paratively vacant region of the heavens 

 from Leo into Virgo. Since it is always 

 opposite the sun, it will be found high- 

 est in the heavens at midnight. During 

 the last week or ten days of the present 

 month, while the moon is absent from 

 the midnight heavens, the observer by 

 using proper precautions in regard to 

 getting away from all artificial light 

 and employing averted vision, may very 

 possibly detect this exceedingly inter- 

 esting but little known object. He will 

 then have the satisfaction of knowing 

 that he has accomplished what is re- 

 garded as one of the most difficult of 

 naked eye observations. 



Is There a "Tail" to the Earth? 



We do not definitely know what the 

 source of the Counter-Glow is. It is 

 one view that the innumerable par- 

 ticles of matter forming the cloud of 

 the Zodiacal Light are crowded to- 

 gether most closely in a region exact- 

 ly opposite the sun so that here the 

 great cloud has an unusual density. 

 Each particle in this position must turn 

 its illuminated half directly toward us 

 and indeed shine as a full moon, so that 

 from this region we should receive an 

 unusual amount of light. A profound 

 and beautiful mathematical research 

 shows, in fact, that under the combined 

 pulls of the sun and earth the particles 

 of the zodiacal cloud will tend to accu- 

 mulate in a region which is exactly op- 

 posite the sun as viewed from the earth. 

 The analysis shows that particles which 

 would normally revolve regularly in 

 their orl)its about the sun, as the earth 

 does, will upon reaching this region be 

 arrested. Here their motion will be- 

 come very irregular ; they will oscillate 

 backward and forward for a greater or 

 less length of time, finally escaping to 

 pursue their paths about the sun, their 

 places being taken by other newly ar- 



