HEAVENLY BODIES 477 



of 0".06. Dividing 19,000,000,000,000 by 0".06 we get about 

 316,666,666,000,000 miles. 



Below is a list of parallaxes of the most important stars. Work 

 some of them out and determine their distance from us. Those 

 which have all zeros signify that the distance is so great that no 

 angle can be obtained. 



Polaris (the north Star), 0".06; Aldebaran, 0".ll; Capella, 

 0".09; Rigen, 0".00; Betelgeuse, 0".02; Canopus, 0".00; Sirius, 

 0".37; Castor, 0".20; Pollux, 0".06; Procyon, 0".30; Regulus, 

 0".02; Arcturus, 0".03; Alpha Centauri, 0".75; Antares, 0".02; 

 Vega, 0". 11; Altair, 0".23; Deneb, 0".00; Fomalhaut, 0".13. 



The figure 19,000,000,000,000 is obtained by multiplying 93,000,000,368=- the 

 radius of the earth's orbit, by 206,265, the distance from us of an object having 

 one second of arc as its parallax. The number equals 19,172,645,000,000, but 

 for convenience just 19,000,000,000,000 is chosen as an adopted standard of 

 measurement for the distance of stars. The name applied to this number 

 is the " parsec." 



Comets. Comets occasionally visit the solar system. The 

 comet usually has a long tail millions of miles in length. The 

 comet and tail are supposed to be made up of little particles covered 

 with luminous gas. The particles are composed of carbon, sodium, 

 iron and magnesium. Occasionally the comet, breaks up and the 

 fragments continue on their way as meteors. Sometimes the earth 

 plunges through a swarm of meteors. At such a time the sky seems 

 to be full of shooting stars, so called. 



Comets pass into space and sometimes return a great many years 

 afterwards. Othe s go near the sun and after passing very close 

 to it rush off into space probably never to return. These strangers 

 of space are very interesting. In olden times people thought that 

 they foretold war, famine and many other sufferings. 



Constellations. None of us can look at the heavens on a clear 

 night without being overwhelmed by an almost endless vista of 

 suns beyond suns and systems upon systems. The mind of the 

 ancients could not grasp the vastness of the great universe. In a 

 childlike way they traced the outlines of men and beasts among 

 the stars and invented a pleasing story about each. The groups 

 of stars in these imaginary pictures are called constellations. 



