NEWTON. 201 



it at each new moon. Nor is it true that the sun is stationary, for, as 

 in the case of the earth and the moon, had there been but one planet, 

 it and the sun would each describe similar orbits round their common 

 centres of gravity. In short, the planets do not properly revolve about 

 the sun, but the planets and the sun also revolve about the common 

 centre of gravity of the whole system, and the sun approaches or re- 

 cedes from this point according to the positions of the planets. The 

 mass of the central luminary is, however, so great in comparison with 

 that of the planets, that even if these were all ranged on one side of 

 him, the common centre of gravity would not be more than half a 

 radius from his surface. 



The elliptical orbit is not the only one compatible with the law of 

 gravitation, for, as already stated, the form of the orbit may be any one 

 of the conic sections, but no other curve. 



Those singular bodies which in ancient times had been ranked 

 as meteors engendered in our atmosphere, but which Galileo had 

 proved to be celestial, had been observed to move in orbits much 

 more curved in one part than at others ; but it was reserved for Newton 

 to show that comets were regulated by the same laws as the planets. 

 This was a result of great importance, for as comets enter the range 

 of the solar system from every quarter of space, and from distances 

 inconceivably vast, it proved that the influence of gravitation pervades 

 all space. 



The connection of the moon with the tides had been observed in all 

 ages, yet no satisfactory explanation of the phenomena had been offered, 

 when Kepler referred the tides to the moon's attraction. But the ex- 

 planation he gave was so unsatisfactory that Galileo rejected it, and put 

 forward an erroneous one of his own. Newton conclusively proved 

 that the tides were due to the attraction of gravitation operating between 

 the water of the ocean and the sun and moon respectively. According 

 to the relative position of the two luminaries, their attractions produce 

 effects which assist or oppose each other so as to produce a greater or 

 less rise of the waters. The action of the sun is small compared with 

 that of the moon, hence the tidal wave appears to follow the latter 

 round the earth ; and high water always occurs about three hours after 

 the moon passes the meridian. When the sun and moon are in a line 

 with the earth, which occurs at full moon and at new moon, then are the 

 highest or spring tides. When the directions of the two luminaries arr 

 at right angles to each other the effect is weakest, and then occur the 

 neap tides. Many persons who have no difficulty in understanding the 

 action of the moon in attracting the water on the side of the earth next 

 to itself, find the existence of an equally high tidal wave on the side 

 opposite to the moon not so easily understood. It seems a contradic- 

 tion to maintain that the moon's attraction draws the waters of the ocean 

 towards herself on one side, and causes them to recede on the oppo- 

 site side. But this recession is only relative : it is not the water which 



