44 GEORGIUM siDUs. [Lesson x. 





II. Applying a similar mode of reasoning, we may conclude^ 

 that Mercury moves round the Sun in a less orbit than that of 

 I'tnus; because it puts on the same phases with Venus, and 

 is never seen so far from the sun as that planet is. 



III. The orbit of Mars includes both the Earth and the 

 Sun ; and the Earth is not in the centre thereof. For this 

 planet is capable of appearing opposite to the Sun, or in any 

 other situation with respect to that luminary ; which could 

 not be, unless it moved round the Earth: and it always appears 

 full, or nearly so ; which it could not do if it ever came be- 

 tween the Sun and the Earth ; hence, it moves also round the 

 Sun. And farther, when Mars is in the opposite part of the 

 Heavens to the Sun, it appears about five times larger than 

 when it is near the Sun ; which shews that it is much nearer 

 the Earth in one situation than in the other : the Earth, there- 

 fore, is not in the centre of its motion. 



IV. The like being observable of Jupiter, Saturn, and Her- 

 schel, (though, on account of their greater distance from the 

 Sun and us, the diversity in their apparent magnitudes in dif- 

 ferent parts of their orbits, is not so great as in Mars), it is 

 reasonable to conclude, that these planets also have both the 

 Earth and the Sun within their orbits, and that the Sun, rather 

 than the Earth, is in the centre of the same. 



V. Since then, the Earth is placed within the orbits of Mars 

 Jupiter, Saturn, and Hersche I, these planets cannot appear to us 

 to stand still, or to go backward, as observations shew they 

 sometimes do appear, unless the earth moves : and since, as 

 has been shewn, Venus and Mercury revolve about the Sun, 

 and not about the Earth ; since also, the Earth is placed be- 

 tween the orbits of Mars and Venus, and the periodical time 

 of the Earth, if it does move, is, in point of magnitude, be- 

 tween the periodical times of those two, it seems quite rea- 

 sonable to suppose that the Earth revolves about the Sun in 

 the same manner as the .planets do : it is therefore reckoned 

 in the number of them. 



VI. But what very much confirms this conclusion is, that 

 astonishing harmony,which,upon this supposition, runs through 

 the whole solar system: for it is known that the motions of all 

 the planets, both primary and secondary, are in conformity 



to 



