2 9 z THE POPULAR SCIENCE MONTHLY. 



Merak, Phegda, Megrez, Alioth, Mizar, and Acka'ir. The last is the 

 name given to the foremost horse. Persons possessed of good visual 

 powers can discern above the second horse, Mizar, a small star which 

 is called the Postilion. But these names are seldom employed in our 

 times, the usual custom being to designate the seven principal stars 

 of Ursa Major by the first seven letters of the Greek alphabet, as 

 shown in the diagram. All these stars are of the second magnitude, 

 except Delta (Fig. 1), which is of the third. 



In the diagram, the arrows show the mean direction in which each 

 of the seven stars moves. It will be seen that, of the seven, the first 



Fig. 1. 



The Seven Stars of Ursa Major in their Present Positions. 



and the last, Alpha and El a, are moving in a direction contrary to 

 that of the other five. It must be added that they have not all the 

 same velocity. Eta, for example, moves rapidly ; Epsilon slowly ; 

 and so on with the others. 



The quantity of their annual proper movements in right ascension, 

 and in polar distance, is given for each of the seven stars in the fol- 

 lowing table : 



e. a. p. D. 



Alpha (a) — 0".013 + 0".09 



Beta (/?) + 0".015 — 0".03 



Gamma (y) + 0".016 + 0".02 



Delta (J) + 0".019 + 0".06 



Epsilon (e) + 0".017 + 0".06 



Zeta (C) + 0".020 + 0".04* 



Eta 0/) - 0".033 + 0".03 



In consequence of these proper movements, the relative distances 

 between the stars of the constellation are ever changing. But, as this 

 change only amounts to a few seconds in a century, many centuries 

 must elapse before it is perceptible to the naked eye. Our human 

 generations, our dynasties, nay, even our nations, are not sufficiently 

 lonor-lived to measure this chancre. 



We have here to deal with astronomical quantities, and, to appre- 



