426 THE POPULAR SCIENCE MONTHLY, 



lished till after the year 1840. The same is to be said of Alpha 

 in Centaur, observed in 1832 and in 1839 on the Cape of Good 

 Hope by Henderson and Maclear ; this is the nearest to us of all 

 the stars. 



There are two ways of determining these parallaxes. The first is, 

 to compare together the positions observed at intervals of six months ; 

 the other, to discover an apparent motion in a star (as compared with 

 a motionless star situated at a far greater distance than that w^hich is 

 studied) : this apparent motion being due to the perspective produced 

 by the annual revolution of the earth in its orbit. This is the method 

 mostly employed now. Galileo, in his " Dialogues ; " Gregory, in the 

 "Proceedings of the Royal Society" (1675) ; Huyghens, in his " Cos- 

 motheoros," published in 1695 ; Condorcet, in his " £loge of Roemer," 

 in 1773 ; and William Herschel, in 1781, have described both methods. 

 Hooke, Flamstead, Cassini, Bradley, Robert Long, Herschel, Piazzi, 

 and Brinkley, strove, from 1674 to 1820, to determine the small quan- 

 tity of the apparent movement of the brightest stars, which used to 

 be regarded as the nearest ; but their efibrts were fruitless, owing to 

 the inconsiderable amount of this motion. There was need of instru- 

 ments of the utmost precision, a rigid spirit of observation, and an in- 

 domitable patience, in order to get at trustworthy results. 



Since 1840 the attention of astronomers has been oftentimes di- 

 rected to this investigation, and thousands of calculations have been 

 made. With great difficulty astronomers have succeeded in deter- 

 mining the j^arallaxes of a few stars. But the inevitable errors of 

 observation often involve the results in obscurit3\ Let the reader 

 only bear in mind that there is not one star that is sufficiently near to 

 give us a parallax of one second! A second is the dimension to 

 which would be reduced a circle one metre (3 ft. 3.37 in.) in diameter 

 carried away to a distance of 206 kilometres (127.72 miles) from the 

 eye. This appears to be less than nothing : it is equal to the thick- 

 ness of a hair stretched at the distance from the eye of 20 metres (74 

 feet). The apparent annual movement of a star, Avhose distance can 

 be known, is performed altogether within this infinitesimal space. 

 For an observer on the star that is nearest to us, this hair would con- 

 ceal the whole space between the earth and the sun. 



As no star ofiers a parallax of one second, it follows that the 

 nearest of the stars is distant from earth no less than 206,265 times 

 92,000,000 miles. The space which surrounds the planetary system is 

 void of stars to that distance at least. 



The star which is nearest to us. Alpha of Centaur, has a parallax 

 of 0."91. Its distance from earth is 226,400 times the radius of the 

 earth's orbit, or 21,000,000,000,000 miles. This is our neighbor star, 

 and its distance is probably the minimum distance between star and 

 star — 21,000,000,000,000 miles. Each of these stars shines with its 

 own light — is a sun like our own. 



