ASTKUNOMY. 



[THI PLANETS. 



arc. C represent* the moon in vat-ion* positions in re- 

 f.-n-noo tn the ,-.u t!i :in<l tlio sun. In tlu- first ami second 

 figure* she i* in omjuii liroctly between the 



earth and the sun. In tliu next figures tiio oarth U in- 



terposed botweon the sun and the moon. D shows how 



tlio olliptio.il orliit of |>l;uioti is pro 1., M co-opera- 



tion of attractive an.l Tlio middle cut 



represents tlio phases of the m 



I ..-. M. 



CHAPTER V. 

 ON THE PLANETS. 



Axoxo the more lustrous objects which arrest our ob- 

 servation in the heavens, are those which, since the 

 earliest times, have been known by the name of Planets, 

 from their erratic course among the fixed stars. The 

 variable brightness of Venus, the morning and evening 

 star, as well as those which appear at all times of the 

 night, claim our attention equally with the sun and 

 moon. Five of those objects were known to the ancients : 

 Mercury, Venus, Mars, Jupiter, and Saturn ; the first 

 of which is but rarely seen with the naked eye in our 

 latitude*, in consequence of its constant proximity to 

 the sun, although a sparkling object, and much brighter 

 than any of the fixed stars. These bodies, it will be 

 perceived, do not, like the sun and moon, move in one 

 constant direction from west to east, but sometimes 

 direct, sometimes retrograde, and are at other tin..-- 

 tationary. It was noticed, however, by the ancients, 

 that whatever might be their movements in longitude, 

 their latitudes did not depart much from the ecliptic ; 

 and a cone of 1<J in breadth, or 8 north and south of 

 the ecliptic, contained all the ancient planets. This 

 was termed by them the zodiac, and was divided into 

 twelve equal portions, the signs at which it cuts the 

 equator, or the lino of the equinoxes, being in the con- 

 stellations Arii x and ( '.ipriconius. 



If Venus, the most brilliant of all the planets, be ob- 

 served in iU paiuage tlirou^h tin- heavens, it will be seen 

 that, in the course of a few mouths, it describes in the 

 sky a very irregular circle, and that it oscillates to a 

 certain distance on each side of the sun, sometimes rising 

 before it in great brilliancy in the morning, and subse- 

 quently netting nft.T it in the evening, and then for a 

 length of time U-mi' altogether invisible to the naked 

 ere (Fig. 55). It thu* appears at an epoch nearly in 

 the direction of the sun S ; it then passes from 8 to A, 

 and then returns from A t<> S ; and having passed the 

 sun, it gnej from 8 to It, and finally returns to it in the 

 contrary direction, or from B to S. When an evening 



star, it is seen between S and A, and between S and B 



iu the morning. Its latitude varies in the same manner. 



Fig. ss. 



The greatest elongation from the sun never ex. < 

 :ind in never less than 46 ; and at thoM- point..* its motion 

 is considerably slower than when near the sun. In order 

 to explain the oscillatory motion of Venus in respect to 

 the sun, the ancients imagined an epicycle similar to 

 that already mentioned in the case of the sun. If T 

 (Fig. 55) be the position of the earth, it was lUppOMd 

 that V, the planet, described an orbit round the centre C, 

 which had a motion itself around the earth, the tlim: 

 points, TCS, being always in a straight line. At the 

 times of greatest elongation, the planet was in tliu 

 direction of T A and T B ; and in the intermediate times, 

 its apparent motion was sometimes from west to east, 

 and at others from east to west, or retrograde. If Venus 



