ASTRONOMY. 



the difference is thus 26 miles, and the mean 

 diameter 7912 miles. 



To ascertain the density of the earth, is a deli- 

 cate problem, to be more particularly described 

 under the head of Mechanical Astronomy. It is 

 found to be from 5^ to 6 times that of water. 

 The densities of the other heavenly bodies are 

 always given as compared with that of the earth, 

 which is stated as I. The motions of the earth, 

 and the appearances caused by them, as well as 

 the motions of its accompanying satellite, the 

 moon, will be described under separate heads (see 

 also PHYSICAL GEOGRAPHY). 



Mars (<$), the fourth in order, is the nearest to 

 us of the superior planets. In many respects it 

 resembles our earth. It is distinctly ascertained 

 to turn on its axis in 24 hours 37 minutes, the 

 axis being inclined to the plane of its orbit at an 

 angle of 28 27'. Its days, then, are nearly the 

 same as ours ; and its year, which contains 668 

 Martial days, is varied by seasons like the ter- 

 restrial year. Mars has a reddish aspect. Around 

 each pole is a region of dazzling white, conjec- 

 tured to be snow. In 1877 it was discovered to 

 be attended by two satellites. 



The asteroids or small planets circulate in a 

 region lying between the orbits of Mars and 

 Jupiter, but on the whole nearer to the former. 

 They form a distinct group of themselves. It is 

 only recently that the existence of the small 

 planets has become known, the first, Ceres, having 

 been discovered in 1801. From observing, how- 

 ever, that the distances at which the planets 

 generally succeed one another, form a kind of 

 progression, that of each orbit, counting from 

 Mercury, being nearly double of the one preced- 

 ing, it had long been conjectured that a planet or 

 planets might yet be discovered in the interval 

 between Mars and Jupiter, which formed a break, 

 as it were, in the regularity of the progression. 

 The discovery of Ceres by Piazzi at Palermo in 

 Sicily (Jan. i, 1801), was speedily followed by that 

 of Pallas, Vesta, and Juno ; and up to the present 

 time more than 200 have been catalogued and 

 named. 



In addition to their comparative smallness, the 

 planets of this group are distinguished by their 

 orbits being much more elliptical or elongated 

 than those of the others, and also having a greater 

 inclination that is, rising and sinking much 

 farther from the plane of the ecliptic. The in- 

 clination of Pallas is as much as 34 37'. The 

 diameter of the largest of the minor planets is 

 only 228 miles, and many of the smaller ones 

 are less than 50. They are invisible to the naked 

 eye, except occasionally Ceres and Vesta. 



From various observed facts, cosmogonists 

 presume that the matter which in other cases has 

 gone to form one planet of the first rank, has 

 in their case been separated into several parts, 

 assuming various but connected orbits. 



Jupiter (2f). The largest of all the planets is ' 

 Jupiter. The diameter of Jupiter being upwards 

 of eleven times that of the earth, his volume is '' 

 1400 times the volume of the earth. To the ' 

 inhabitants of Jupiter the sun must appear less 

 than one-fifth of the breadth he presents to us. 

 By means of permanent marks, it is ascertained 

 that the planet rotates on an axis inclined to 

 its orbit at the small angle of 3 4', and in the 

 short space of 9 hours 55 minutes. This makes | 



the rotary velocity of Jupiter's surface twenty- 

 seven times greater than that of the earth. 

 Viewed through .a telescope, Jupiter appears tra- 

 versed by dusky streaks parallel to the planet's 

 equator. Both Jupiter and Saturn have atmos- 

 pheres so densely laden with clouds that the 

 surfaces of the planets are, it is believed, never 

 seen ; and the parallel streaks, which both ex- 

 | hibit, are supposed to arise from the rapid rotation 

 of those planets disposing the clouds in belts, on 

 the principle of our trade-winds and calm-belts. 

 The density of Jupiter, taking his bulk as we see 

 it, is little more than that of water ; but if he is 

 surrounded by a thick envelope of cloudy atmos- 

 phere, the kernel of the planet may approach that 

 of the earth. The same may be said of Saturn, 

 whose density as a whole is about half that of 

 water. 



Jupiter is attended by four satellites, which 

 revolve round it as the moon revolves round the 

 earth, but in much shorter periods the nearest 

 requiring only forty-two hours. One of these 

 satellites is of the same size as our moon ; the 

 others, larger. Their density is very small ; so 

 that the mass of the whole is only a 6oooth part 

 of that of Jupiter itself. The satellites of Jupiter 

 were discovered by Galileo, being among the first 

 results of the invention of the telescope. 



Saturn (h) with its ring, or rather rings, and 

 eight moons, is the most remarkable member of 

 the solar system. It turns on its axis in 10 hours 

 29 minutes. 



The ring of Saturn, which surrounds the planet 

 in the plane of its equator, is found, on closer 

 examination, to consist of a series of rings one 

 within the other, the two outer being bright, and 

 the innermost dark and transparent. The distance 

 from the body of the planet to the dark ring is 

 9760 miles. The whole ring system, inclusive of the 

 interval between the two bright rings, is 37,570 

 miles broad, while its thickness appears not to 

 exceed 100 miles. In certain positions of the 

 planet, we can see its surface at a considerable 

 angle, and the openings or loops which it forms at 

 the sides of the planet. At other times, we see its 

 dark side, or only its edge. It is now generally 

 believed that these rings are composed of innumer- 

 able small satellites, moving each in its own orbit 

 round the planet, and presenting a bright appear- 

 ance where they are densely packed, but a dim 

 appearance when scattered. 



The eight satellites of Saturn revolve around it, 

 on the exterior of the ring, and almost all of them 

 in nearly the same plane. They are so small as 

 not to be visible without a powerful telescope. 



Uranus ($) is invisible to the naked eye, and 

 was discovered by Sir William Herschel in 1781. 

 Owing to its enormous distance, little is known 

 regarding it. 



Uranus is attended by at least four satellites. 

 In two respects, these satellites are quite singular: 

 their orbits are nearly perpendicular to the plane 

 of the ecliptic ; and their motions in the orbits are 

 retrograde that is, from east to west, instead of 

 being from west to east, like those of all other 

 planets both primary and secondary. 



The discovery of Neptune ( ^ ) is one of the 

 greatest triumphs of scientific astronomy. From 

 irregularities observed in the motion of the planet 

 Uranus, it had been conjectured that some dis- 

 turbing cause, not yet discovered, was acting upon 



