PLANETS. 



their orbits ; but these things belong to the science 

 of astronomy which will open to your view new scene* 

 and fill your miml with new pleasures. Enough how- 

 ever has been said to excite you to extend your in- 

 quiries at a maturer age, and, in the study of Nature 

 and Nature's God, you will always find your admira- 

 tion excited and your pleasure increased in propor- 

 tion to the increase of your knowledge. 



These orbs, of whose motions, magnitudes, and 

 mean distances from the sun I have exhibited a con- 

 'cise sketch, are called primary planets. The moon 

 is only a secondary planet or statellite to our earth, 

 round" which it makes its revolution in twenty-nine 

 days, twelve hours and twenty-four minutes, and is 

 carried along with this globe round the sun in one 

 year. Other planets also have moons or statellitcs 

 revolving round them and carried along with them in, 

 their orbits : Jupiter has four-, Saturn seven, and th 

 Georgium Sidus six. The diameter of the moon is 

 two thousand one hundred and eighty miles, so that 

 you perceive it is far less than the least of the seven 

 primary planets, and its nearness to the earth is th& 

 cause of its greater apparent magnitude. 



Besides these there are another sort of solid bodies 

 like the planets, of which the orbits are exceedingly 

 eccentric, and their motions irregular. They are 

 called comets. Little is known concerning them. 

 Though great rmm be rs have made their appearance 

 at different times, the periods of three only have been 

 ascertained with any degree of certainty by astrono- 

 mers, who have found that they return at intervals of 

 75, 129, and 575 years. Of these the latter, which 

 appeared in 1680, is the most remarkable. This co 

 met, when at its greatest distance, is about eleven 

 thousand two hundred millions of miles from the su$, 

 and at its nearest approach only about four hundred 

 and ninety thousand miles. Sir Isaac Newton com- 

 puted that when nearest the sun it must have acquir- 

 ed a heat, two thousand times greater than that of red 

 hot iron ; and, considering its' size, that it cou^i not 

 become cold ia ttrenty thousand years. 

 B 



