ASTRONOMY. 



89 



most distant planet from the sun, that has yet 

 been discovered ; being removed at no less than 

 1800 millions of.niles from that luminary, which 

 is nineteen tmes farther than the earth is from 

 the sun a distance so great, that a cannon ball, 

 flying at the rate of 480 miles an hour, would 

 not reach it in 400 years. Its diameter is about 

 35,000 miles ; and, of course, it is about eighty 

 times larger than the earth. It appears like a 

 star of the sixth magnitude ; but can seldom be 

 distinguished by the naked eye. It takes about 

 83 years and a half to complete its revolution 

 round the sun; and, though it is the slowest 

 moving body in the system, it moves at the rate 

 of 15,000 miles an hour. As the degree of sen 

 sible heat in any planet does not appear to de 

 pend altogether on its nearness to the sun, the 

 temperature of this planet may be as mild as 

 that which obtains in the most genial climate of 

 our globe.* The diameter of the sun, as seen 

 from Herschel, is little more than the apparent 

 diameter of Venus, as seen by the naked eye ; 

 and the light which it receives from that lumi 

 nary, is 360 times less than what we experience ; 

 yet this proportion is found by calculation to be 

 equal to the effect which would be produced by 

 248 of our full moons ; and, in the absence of 

 the sun, there are si* moons which reflect light 

 upon this distant planet, all of which were dis 

 covered likewise by Dr. Herschel. Small as the 

 proportion of light is, which this planet receives 

 from the sun, it is easy to conceive, that beings 

 similar to man, placed on the surface of this 

 globe, with a slight modification of their organs 

 of vision, might be made to perceive objects with 

 a clearness and distinctness even superior to 

 what we can do. We have only to suppose, 

 that the Creator has formed their eyes with pupils 

 capable of a much larger expansion than ours ; 

 and has endued their retina with a much greater 

 degree of nervous sensibility. At all events, we 

 may rest assured, that He who has placed sen 

 tient beings in any region, has, by laws with 

 which we are partly unacquainted, adapted the 

 constitution of the inhabitant to the nature of 

 the habitation. 



&quot; Strange and amazing must the difference be, 

 Twixt this dull planet and bright Mercury ; 

 Vet reason says, nor can we doubt at all, 

 Millions of beings dwell on either ball, 

 &quot;With constitutions fitted for that spot 

 Where Providence, all -wise, has fixed their lot.&quot; 

 Baker s Universe. 



The celestial globes which I have now de 

 scribed, are all the planets which are at present 

 known to belong to the solar system. It is pro 

 bable that other planetary bodies may yet be dis 

 covered between the orbits of Saturn and Her 

 schel, and even far beyond the orbit of the latter ; 

 n&amp;lt;* it is also not improbable that planets may 



See Note, page 82. 



exist in the immense interval of 37 millions of 

 miles between Mercury and the Sun.f These 

 (if any exist) can be detected only by a series of 

 day observations, made with equatorial telescopes ; 

 as they could not be supposed to be seen, after 

 sunset, on account of their proximity to the sun. 

 Five primary^ planets, and eight secondaries, 

 have been discovered within the last 42 years , 

 and, therefore, we have no reason to conclude, 

 that all the bodies belonging to our system have 

 yet been detected, till every region of the heavens 

 be more fully explored. 



Comets. Besides the planetary globes to 

 which I have now adverted, there is a class of 

 celestial bodies which occasionally appear in the 

 heavens, to which the name of comets has been 

 given. They are distinguished from the other 

 celestial bodies, by their ruddy appearance, and 

 by a long train of light, called the tail, which 

 sometimes extends over a considerable portion of 

 the heavens, and which is so transparent, that 

 the stars may be seen through it. The tail is 

 always directed to that part of the neavens which 

 is opposite to the sun, and increases in size as it 

 approaches him, and is again gradually dimi 

 nished, as the comet flies off to the more distant 

 regions of space. Their apparent magnitude is 

 very different : sometimes they appear only of 

 the bigness of the fixed stars ; at other times 

 they equal the diameter of Venus ; and some 

 times they have appeared nearly as large as the 

 moon. They traverse the heavens in all direc 

 tions, and cross the orbits of the planets. When 

 examined through a telescope, they appear to 

 consist of a dark central nucleus, surrounded by 

 a dense atmosphere, or mass of vapours. They 

 have been ascertained to move in long narrow 

 ellipses or ovals, around the sun ; some of them, 

 on their nearest approach to him, having been 

 within a million of miles of his centre ; and then 

 fly off to a region several thousands of millions 

 of miles distant. When near the sun, they move 

 with amazing velocity. The velocity of the 

 comet which appeared in 1680, according to Sir 

 Isaac Newton s calculation, was eight hundred 

 and eighty thousand miles an hour. They ap 

 pear to be bodies of no great density, and their 



t The Author, some years ago, described a method 

 by which the planets (if any) within the orbit of 

 Mercury, may he discovered in the day-time, by 

 means of a simple contrivance for intercepting the 

 solar rays, and by the frequent application, by a num 

 ber of observers, of powerful telescopes, to a certain 

 portion of the sky, in the vicinity of the sun. The 

 details of this plan have not yet been published ; but 

 the reader will see them alluded to in No. V. of the 

 Edinburgh Philosophical Journal, for July 1820, p. 



I A primary planet is that which revolves roui.d 

 the sun as a centre ; as Mars, Jupiter, and Saturn. 

 A secondary planet is one which revolves round a 

 primary planet as its centre ; as the Moon, and the 

 satellites of Jupiter and Saturn. The primary pla 

 nets are distinguished from the fixed stars by the 

 steadiness of their light ; not having a twinkling 

 appearance, as the stars exhibit. 



