THE STARRY HEAVENS FOR MARCH 



34i 



Veiling the Time by the Stars. 



One familiar with the general ap- 

 pearance of the heavens can thus read 

 from them at a glance what the season 

 of the year is, and he welcomes the ap- 

 pearance of certain stars which herald 

 the near approach of spring or summer 

 or autumn. This, though interesting, is, 

 of course, of but little practical value. 

 Were the observer lost, and without a 

 compass, he could also very easily tell 

 the direction of true north, since the 

 Pole Star is always exactly north of 

 him ; by therefore facing Polaris and 

 stretching out his arms he would easily 

 fix the four points of the compass at all 

 times. 



How to make use of our knowledge 

 of the heavens in these ways is prob- 

 ably known to all of the readers of this 

 article, but a far fewer number of those 

 who study the stars know how easy a 

 matter it is to determine the time from 

 them. 



To do this it is first necessary to know 

 what is called the Right Ascension of 

 a few of the southern stars of the Ce- 

 lestial Sphere. This so-called Right 

 Ascension of a star is measured on the 

 surface of the Celestial Sphere in almost 

 exactly the same way as the longitude of 

 a place is measured on the surface of 

 the earth. It may be found for a se- 

 lected number of the brighter stars from 

 almost any astronomy, or from the 

 American Nautical Almanac, or it may 

 be measured from any star atlas. For 

 example, the right ascensions of several 

 of the bright stars now found in the 

 southern heavens are as follows (the 

 letters refer to Figure 1) : 



Aldebaran, at A, 4 hrs. 31 min. 



Betelgeuze, at B, 5 hrs. 51 min. 



Shins, at C, 6 hrs. 41 min. 



Pollux, at D, 7 hrs. 40 min. 



Delta Hydrae, at E, 8 hrs. 33 min. 



Regulus, at F, 10 hrs 3 min. 



Delta Leonis, at G, 11 hrs. 10 min. 



The right ascension of any star re- 

 mains practically the same for many 

 centuries, but to tell the time of night 

 we need also the right ascension of the 

 sun, and this is continually changing. 

 It may be found for any day of the year 

 in many almanacs, or we may determine 

 it as follows : 



On March 21 the right ascension of 



the sun is o hours, and it increases one 

 hour every fifteen days thereafter; thus 

 on April 20 it is approximately 2 hours, 

 on May 20, 4 hours, and so on, reaching 

 22 hours on February 19 and 23 hours 

 on March 6. 



If now we find the difference between 

 the right ascension of the sun on any 

 date and that of any star, this difference 

 will be the number of hours which will 

 elapse between when the star was seen 

 on the meridian and when the sun will 

 be found there; as we know that 

 the sun is always on the meridian at 

 noon, the true time when the star was 

 on the meridian is at once known. 



This brief description has been in- 

 serted at the request of several different 

 correspondents ; it is hoped that the 

 reader will not be deterred from reading 

 it by the slight reference to mathe- 

 matics which it involves, for the proc- 

 ess is really very simple and easy indeed 

 and the ability to read the time of night 

 from the ever-turning dial above us is 

 always most pleasurable. 



Illustrations of Finding the Time at 

 Night. 



Suppose that on the evening of March 

 21 the observer saw the bright star Reg- 

 ulus due south of him, what would the 

 time be? Since on March 21 the right 

 ascension of the sun is o hours since 

 that of Regulus is always 10 hours and 

 3 minutes, the star will come to the 

 meridian 10 hours 3 minutes later than 

 the sun. It would therefore be 10 hours 

 3 minutes P. M. 



As a second illustration, let us inquire 

 at what time the star at G, Fig. I, will 

 be seen due south of the observer on 

 April 5 and on April 20. The right 

 ascension of the sun on April 5 is I 

 hour.; that of the star is 11 hours 10 

 minutes ; the difference is 10 hours 10 

 minutes, and therefore when the star is 

 seen due south on this date it will be 10 

 hours 10 minutes P. M. And on April 

 20 the right ascension of the sun is 2 

 hours, the difference is 9 hours 10 min- 

 utes, and the time will be 9 hours 10 

 minutes P. M. This illustrates the well- 

 known fact that the stars rise, come to 

 the meridian, and set just one hour 

 earlier every two weeks. 



After acquiring a little practice the 

 observer will not usually wait until the 



