INTRODUCTION TO ASTRONOMY. xxxiil 



journey through one half of its orbit in a shorter time than through the 

 other half; and in fact, it is about seven days longer performing our 

 summer half of its orbit than the winter half. The planets are celestial 

 bodies which revolve round the sun, on the same principle, and they are 

 supposed to resemble the earth also in many other respects ; and we are 

 led by analogy to consider them as inhabited worlds. 



Some of the planets are proved to be larger than the earth : it is only 

 their immense distance from us which renders their apparent dimensions 

 so small. Now, if we consider them as enormous globes, instead of 

 small twinkling spots, we shall find it most consistent with our ideas of 

 the Divine wisdom and beneficence, to suppose that these celestial bodies 

 should be created for the habitation of beings, who are, like us, blessed 

 by His providence. Hence, in a moral, as well as a physical point of view, 

 it is most rational to consider the planets as worlds revolving round the 

 sun ; and the fixed stars as other suns, each of them probably attended 

 by its system of planets, to which they respectively impart their in- 

 fluence. Our telescopes are brought to such a degree of perfection, 

 that from the appearances which the moon exhibits when seen through 

 them, we have probable reason to conclude that it is a habitable globe ; 

 its mountains and valleys are very perceptible, and some astronomers 

 imagine that they have seen volcanoes in it. 



The planets which are supposed to revolve round the fixed stars must 

 of course be much smaller than the suns which give them light ; and 

 the distance which makes these suns appear to us like stars must render 

 their planets quite invisible : besides the light of these planets would 

 be much more feeble than that of the fixed stars ; there would be exactly 

 the same sort of difference as between the light of the sun and that of 

 the moon, the first being a fixed star, the second a planet. 



According to the laws of attraction, the planets belonging to our 

 system all gravitate towards the sun ; and this force, combined with 

 that of projection, occasions their revolution round the sun, in orbits 

 more or less elliptical, according to the proportion which these two 

 forces bear to each other. But the planets have also another motion : 

 they revolve upon their axis. The axis of a planet is an imaginary 

 line which passes through its centre, and on which it turns ; and it 

 is this motion which produces day and night. With that side of the 

 planet facing the sun, it is day ; and with the opposite side, which 

 remains in darkness, it is night. Our earth, which we consider as a 

 planet, is 24 hours in performing one revolution on its axis ; in that 

 period of time, therefore, we have a day and a night. Hence this revo- 

 lution is called the earth's diurnal or daily motion ; and it is this revo- 

 lution of the earth from west to east which produces an apparent motion 

 of the sun, moon, and stars in the contrary direction. 



SECTION II. On the Planets. 



THE Planets are distinguished into primary and secondary. Those which 

 revolve immediately about the sun are called primary. Many of these are 

 attended in their course by smaller planets, which revolve round them : 

 these are called secondary planets, satellites, or moons. Such is our 

 moon, which accompanies the earth, and is carried with it round the sun. 

 The sun is the general centre of attraction to our system of planets ; but 

 the satellites revolve round the primary planets, on account of their 

 greater proximity. The force of attraction is not only proportional to 



