Lesson n.] VIEW OF THE HEAVENS. 7 



themselves are frequently called primaries., and 

 their satellites secondaries. Tfiere are two methods 

 of discovering which are planets, and which are 

 fixed stars : every fixed star twinkles, but a planet 

 does not ; for the stars are to appearance only lu- 

 cid poinis, and therefore any opaque particle float- 

 ing in the air is sufficient to cause a momentary 

 eclipse of them f; but ihe planets, though smaller, 

 suffer very little apparent diminution. The pro- 

 per criterion, however, is this : the planets are al- 

 ways in motion from one part of the heavens to 

 the other, whereas the other stars keep constantly 

 the same relative distance. 



The names of the planets, beginning with that 

 nearest the sun, are as follow : Mercury, Penus, 

 the Earthy Mars, Jupiter, Saturn, and Georgium 

 Sidus, or Herschel. These are all that are yet 

 discovered, though there very probably may be 



signifying any thing that wanders. Agreeably to this accep- 

 tation, the Sun itself is a Planet; as it, like the others, has a 

 two-fold motion. And indeed, to complete the similarity, 

 Dr. Herschel has (in a paper which will be presently more 

 largely spoken of) asserted^ that his body is opaque, and that 

 it is diversified with hills and valleys. 



t Although the above is the generally adopted method! of 

 explaining the twinkling of the fixed stars, yet it is now 

 thought to be inaccurate. Mr. Michell justly observes (in 

 the " Philosophical Transactions,") that no object can hide & 

 star from us, that is not large enough to exceed the apparent 

 diameter of the star, by the diameter of the pupil of the eye> 

 nay, it must be large enough to hide the star from both eyes 

 at the same time. 



Perhaps the principal cause of this twinkling, is the 

 unequal refraction of light, in consequence of inequalities 

 and undulations in the atmosphere. 



* more : 



