THE SMITHSONIAN INSTITUTION. 189 



the lecture-room, and .a great fact in astronomy, are explained on the 

 same principles. 



If there were but two bodies in the system, their mutual orbits 

 would be undisturbed. Some conic section would be exactly and for- 

 ever described by each about their conmion centre of gravity. But 

 the introduction of a third body disturbs both these orbits, and its 

 own is disturbed by them. In the solar system, therefore, in which 

 hundreds of bodies are attracting each other, the disturbances are al- 

 most numberless ; though multitudes of them are too minute to be 

 perceived. The two which have now been noticed — namely, the ret- 

 rogradation of nodes, and the advance of apsides— are among the 

 most prominent. And though in some instances they are exceedingly 

 minute, they at length become apparent, because they go on accumu- 

 lating for ages instead of oscillating back and forth. The equinoxes, 

 though they have an oscillatory inequality in their motion, are yet 

 perpetually receding on the ecliptic, and must continue to do so while 

 the earth exists. And the apsides, in like manner, are always moving 

 fonoard in the same direction in which the planet moves. 



It is worthy of notice, that while the mutual attractions of the 

 planets disturb the orbits, they do not derange them. When the 

 learner first considers the fact, that the sun and moon are perpetually 

 pressing the equator of the earth towards the ecliptic, he is almost com- 

 pelled to infer that it will be brought nearer and still nearer, until at 

 length the two planes will coincide, and all distinction of seasons 

 win disappear in every latitude of the earth. The sun will always 

 culminate vertically at the equator ; at the poles he will always be 

 seen circulating about the horizon. But this calamitous derangement 

 never can occur ; the revolution' on the axis prevents it. The combi- 

 nation of the ^liJO movements is, as we have seen, a simple retrocession 

 of the equinoxes, which involves no change in the succession of sea- 

 sons. 



So, too, when the student of astronomy learns that the outer planets 

 draw the earth away from the sun most of all at the aphelion, where 

 it is already at the greatest distance, he seems to see this aphelion 

 distance becoming greater and greater, as ages pass on, and the peri- 

 helion, of necessity, during the same ages, drawing nearer and nearer 

 (as I move this ball in more and more eccentric ellipses about the 

 lamp) until the condition of the earth's climate becomes fatal to every 

 living thing. At the perihelion, the earth is subjected to an intolerable 

 heat ; at the aphelion, to a cold equally intolerable. But calculation 

 and experiment both show, that the aphelion point, instead of being 

 removed from the sun, by the attractions of the outer planets, will 

 simply slide around, keeping its distance from the sun the same as 

 ever. The planets have too much stability to be seriously deranged 

 in respect to their orbits by the influence of outsiders. 



This preservation of safe relations among the planets, in the midst 

 of unceasing changes and disturbances, is one of the most interesting 

 facts presented to the mind of the pupil in astronomy. He who made 

 the countless spheres, ordained the laws of their motion ; and those 

 laws, by their perfect operation, secure the utmost peace and harmony, 

 though worlds, thousands of miles in diameter, are rushing through 



