ASTRONOMY. 4^£ 



The planets are evidently opaque bodies, and 

 they shine only by reflecting the light which they 

 receive from the sun ; for Mercury and Venus, 

 when viewed by a telescope, often appear to be 

 only partly illuminated, and have the appearance 

 of our moon when she is cusped or homed, having 

 the illumined part always turned towards the sun. 

 From the appearance of the boundary of light and 

 shadow upon their surfaces, we conclude that they 

 are spherical, which is confirmed by most of them 

 having been found to turn periodically on their 

 axes. 



Venus and Mercury, being nearer to the sun 

 than our earth, are called inferior planets ; and all 

 the rest, which are without the earth's orbit, are 

 called superior planets. That the first go round 

 the sun is certain, because they are seen sometimes 

 passing between us and the sun, and sometimes 

 they go behind it. That their orbits are within 

 that of the earth is evident ; because they are 

 never seen in opposition to the sun, that is, appear- 

 ing to rise from the horizon when the sun is set- 

 ting. On the contrary, the orbits of all the other 

 planets surround that of the earth ; or they some- 

 times are seen in opposition to the sun ; and they 

 never appear to be horned, but always nearly or 

 quite full, though sometimes they appear a little 

 gibbous, or somewhat deficient from full. 



We mentioned above, that all the planets move 

 round the sun in elliptical orbits. The sun is si- 

 tuated in one of the foci of each of them. That 

 focus is called the lower focus. If we suppose the 

 plane of the earth's orbit, which passes through the 

 centre of the sun, to be extended in every direc- 

 tion, as far as the fixed stars, it will mark out 

 among them a great circle, which is the ecliptic ; 



