288 PHYSICAL SCIENCE 



distant stars as the earth travels in its orbit 

 round the sun, and observation confirms this 

 prediction. The first successful measurements 

 were made telescopically in 1838, but much 

 more accurate results can now be obtained by 

 photography. If three photographs be taken of 

 Sirius, for instance, at intervals of six months, 

 and the distance of the image from those of 

 three surrounding distant stars be measured with 

 a micrometer, it will be found that Sirius moves. 

 The small distance between his first position 

 and that he occupies a year later gives his own 

 proper motion compared with the earth and the 

 very distant stars taken as '' fixed." This 

 motion is 1.32 seconds of arc in the year. Half 

 this distance gives the position Sirius would 

 occupy at the intermediate six months' interval 

 if viewed from the same spot in space. The 

 angle, 0.38 second, between this position and 

 that actually observed is called the parallax ; it 

 shows the effect of changing the point of 

 observation to the opposite side of the earth's 

 orbit, that is by 185 million miles. From this 

 it is clear that the distance of Sirius from the 

 earth may be calculated. It proves to be about 

 50 million million miles, a distance it would take 

 light 8.6 years to travel at its speed of 186,000 

 miles a second. 



Beyond the range within which parallax is 

 appreciable, stellar distances can only be esti- 

 mated by indirect means. As examples we may 

 cite the following methods. 



The mean distance of a connected group 

 of stars of given type may be estimated from 

 their average magnitude, that is, their apparent 



