SECT, xxxvn.] MASSES OF THE STAES. 429 



more time to come to us in proportion as the star moves 

 round to the most distant point in its orbit. On that ac- 

 count the star will appear to us to take more time in moving 

 through that half of its orbit than it really does. Exactly 

 the contrary takes place on the other half; for the light 

 will take less and less time to arrive at the earth in propor- 

 tion as the star approaches nearer to us ; and therefore it 

 will seem to move through this half of its orbit in less time 

 than it really does. This circumstance furnishes the means 

 of finding the absolute breadth of the orbit in miles, and 

 from that the true distance of the star from the earth. For, 

 since the greatest and least distances of the satellite star 

 from the earth differ by the breadth of its orbit, the time 

 which the star takes to move from the nearest to the 

 remotest point of its orbit is greater than it ought to be by 

 the whole time its light takes to cross the orbit, and 

 the period of moving through the other half is exactly as 

 much less. Hence the difference between the observed 

 times of these two semi-revolutions of the star is equal to 

 twice the time that its light employs to cross its orbit ; and, 

 as we know the velocity of light, the diameter of the orbit 

 may be found in miles, and from that its whole dimensions ; 

 for the position of the orbit with regard to us is known by 

 observation, as well as the place, inclination, and apparent 

 magnitude of its major axis, or, which is the same thing, 

 the angle under which it is seen from the earth. Since, 

 then, three things are known in this great triangle, namely, 

 the base or major axis of the orbit in miles, the angle 

 opposite to it at the earth, and the angle it makes with the 

 visual ray, the distance of the satellite star from the earth 

 may be found by the most simple of calculations. The 

 merit of having first proposed this very ingenious method 

 of finding the distance of the stars is due to M. Savary ; 

 but, unfortunately, it is not of general application, as it 

 depends upon the position of the orbit, and a long time 

 must elapse before observation can furnish data, since the 



