CELESTIAL MEASURINGS AND WEIGHINGS. 2O5 



the situation in space of the planes in which they revolve ; 

 and the number of years required to complete their re- 

 volutions. Among them occurs every variety of form 

 (always elliptic), from the nearly circular one of the 

 planetary, to the long ellipsis of the cometary orbits ; 

 every variety of oblique presentation, from a plane pass- 

 ing nearly or quite edgeways through the eye of the 

 spectator to one nearly perpendicular to the visual line ; 

 and every length of period, from thirty years up to many 

 centuries. The only element about which in the great 

 majority of cases we are left in complete uncertainty, is 

 the actual size of the orbit, which cannot become known 

 till the distance of the star is ascertained. For our pre- 

 sent purpose then we must confine our attention to those 

 of which at present the distance is known. The two 

 just spoken of present a striking contrast. The revolu- 

 tion of the two stars of a Centauri is performed in about 

 seventy-eight years. Their orbit is a very elongated 

 ellipse, decidedly cometary in its character ; and its pre- 

 sentation to our sight so nearly edgeways, that the two 

 stars at present almost occult or cover one another; 

 though when at their greatest distance from each other, 

 they would appear, if viewed perpendicularly, nearly 

 thirty seconds apart. The other requires about 514 

 years for a complete revolution. Its orbit is nearly 

 circular, and its presentation to our view nearly perpen- 

 dicular : so that we see the distance between the two 

 stars unforeshortened ; and so seen it measures almost 

 exactly sixteen seconds, or a little less than the average 

 apparent diameter of the globe of Saturn. Now we have 



