364 



THE GUIDE TO NATURE 



the beautiful Saturn. During the 

 month this world will move steadily 

 eastward among the stars, emerging 

 from Gemini and crossing into the 

 borders of Cancer on May 27. The 

 observer will easily note that Saturn 

 is now far more nearly to the east of 

 a straight line joining the twin stars, 

 Castor and Pollux, than it was a few 



Figure 2. — The path of the new comet in space. 



weeks ago. When, during next No- 

 vember, the planet again enters our 

 evening sky we will see that it has 

 entirely crossed the latter constellation 

 and entered the borders of Leo. 



;}; ^ % ^ ;[; 



The New Comet. 



This object, which has been attract- 

 ing the attention of astronomers for 

 many months, was discovered a full 

 year ago, but since throughout all of 

 1916 it was so far away that it was 

 only visible in large telescopes, and 

 since it will shine in the heavens 

 throughout all of the present year, it 

 may well be thought of as the comet 

 of 1917. 



The comet was first seen on April 2-, 

 1916, when it appeared as an exces- 

 sively faint (12.8 magnitude) star-like 

 object having a nebulous envelope. It 

 was thought probable that it was one 

 of the numerous little bodies called 

 asteroids, which revolve about the 

 sun between the orbits of Mars and of 

 Jupiter, and it was given the provision- 

 al asteroid designation ZK. The nature 

 of its motion soon showed, however, 

 that it could only be a comet, but a 

 comet so far away that it must have 

 been nearly five hundred millions of 

 miles distant when it was first dis- 

 covered. 



The first uncertainty as to the nature 

 of this new body recalls to an astrono- 

 mer the similar uncertainty connected 



with the discovery of the great planet 

 Uranus in the year 1781. When Her- 

 schel saw this new object he believed 

 it to be a peculiar kind of comet ; it was 

 not until its motion had been studied 

 for nearly a year that the presence of 

 a new world in the Solar System was 

 revealed. 



Figure 2 shows the situation of the 

 path of the new comet with reference 

 to the position of the earth's orbit and 

 to that of the sun. The comet is mov- 

 ing about the sun in the same direction 

 as the earth is, but its path lies in a 

 dift'erent plane, the comet plane being 

 inclined twenty-five degrees to the 

 plane in which the earth moves. The 

 comet passed above the latter plane at 

 the point A, but it was not discovered 

 until it had reached the position B. It 

 will pass the point of its orbit which is 

 nearest the sun on June 16, and will be 

 at its least distance from the earth on 

 August 21. At this time it will be 

 91,700,000 miles away from us. 



As the great cloud moves along its 

 orbit, it should appear to us to grow 

 constantly brighter, both because it is 

 drawing nearer to us and because it is 

 drawing nearer to the sun. The latter 

 source of increase acts in two ways, 

 both by increasing the illumination by 

 sunlight of its opaque matter and by 

 intensifying the excitation of its self- 

 luminous material, either through elec- 

 trical disturbance or otherwise. It is 

 easy to show that if the comet thus 

 brightens in a normal manner it should 

 during the next summer become fully 

 two hundred times as bright as when 

 first discovered and hence should be 

 easily visible to the naked eye. 



Unfortunately, however, these bodies 

 frequently grow inherently brighter or 

 fainter in a cpiite capricious manner, 

 and we cannot therefore at all predict 

 how conspicuous the present comet 

 will grow. Observations made toward 

 the end of March indicated that the 

 comet is not brightening so rapidly as 

 it normally should. 



If the reader will imagine himself 

 standing on the moving earth of Fig. 

 2 and continuously watching the comet 

 as it moves along the path ACE he can 

 readily picture roughly to himself what 

 its apparent motion among the stars 

 will be. Evidently it will mount con- 



