THE CENTURY'S PROGRESS IN ASTRONOMY 



before the Christian era, we of to-day should still re- 

 ceive their light and seem to see them just as we do. 

 When we look up to the sky, we study ancient history ; 

 we do not see the stars as they are, but as they were 

 years, centuries, even millennia ago. 



The information derived from the parallax of a star 

 by no means halts with the disclosure of the distance of 

 that body. Distance known, the proper motion of the 

 star, hitherto only to be reckoned as so many seconds of 

 arc, may readily be translated into actual speed of prog- 

 ress ; relative brightness. becomes absolute lustre, as com- 

 pared with the sun * and in the case of the double stars 

 the absolute mass of the components may be computed 

 from the laws of gravitation. It is found that stars 

 differ enormously among themselves in all these regards. 

 As to speed, some, like our sun, barely creep through 

 space compassing ten or twenty miles a second, it is 

 true, yet even at that rate only passing through the 

 equivalent of their own diameter in a day. At the 

 other extreme, among measured stars, is one that 

 moves two hundred miles a second ; yet even this "fly- 

 ing star," as seen from the earth, seems to change its 

 place by only about three and a half lunar diameters 

 in a thousand years. In brightness, some stars yield to 

 the sun, while others surpass him as the arc-light sur- 

 passes a candle. Arcturus, the brightest measured star, 

 shines like two hundred suns ; and even this giant orb 

 is dim beside those other stars which are so distant that 

 their parallax cannot be measured, yet which greet our 

 eyes at first magnitude. As to actual bulk, of which 

 apparent lustre furnishes no adequate test, some stars 

 are smaller than the sun, while others exceed him hun- 

 dreds or perhaps thousands of times. Yet one and all, 



