LKSSONS IN ASTKON'nMY 



m 



LESSONS IN ASTRONOM Y. X 1 1 1 



JUPITER (' -BATDBN URAMD8 KXPTUKt. 



WHEN we examine Ju; '7) with tho aid of a (food tele- 



scope, the moat remarkable feature that strike* us is the nnmbor 

 of almost parallel belts which mark it* surface, a* shown in the 

 accompanying view of tho planet. Bonn t 



changes take plaeo in tin- number and appoaranoo of those belts j 



<!-, they remain for a long time almost unchanged. 



They seem to consist of large 



mosses of cloud, with which tho 



atmosphere is so charged that tho 



surface of the planet itaolf can 



In- MTU. and are arranged 



i -i l>y tin- rapid rotation of 

 tlir planet, coupled with its im- 

 mense size the equatorial portion 

 of its surface travelling about 4CO 

 mill-* a minute, while tho speed of 

 tho corresponding portion of tho 



is only 17 miles in tho same 

 time. 



Distinct markings or spots are 

 at times visible on these belt*, and 

 remain constant sufficiently long 

 to enable us to ascertain the time 

 the planet occupies in rotating on 

 its axis, which, as a result of many 

 such observations, we find to be 

 9 hours 55 J minutes. This is, as 

 will bo observed, a much shorter 

 period than that of tho planets 

 already considered, and is the 

 more remarkable when we re- 

 member tho great size of Jupiter. 

 By observing the attractive in- 

 fluences of tho planets on one another, astronomers are enabled 

 to calculate their respective densities, and they find that the 

 density of Jupiter is less than a quarter that of the Earth, or, in 

 other wordc, Jupiter, taken bulk for bulk, weighs less than a 

 quarter as much as the Earth does. The density of the latter is 

 about five and a-half times as great as that of water. Jupiter, 

 therefore (and Saturn also), has a density little greater than that 

 of water. From thia 

 and other causes 

 some astronomers 

 have supposed that 

 the planet may con- 

 sist of a largo en- 

 velope of a dense 

 atmosphere charged 

 with clouds, and 

 within it a planet of 

 much smaller dimen- 

 sions, and approach- 

 ing more nearly to 

 the density of the 

 earth. This is, how- 

 ever, mere conjecture. 

 In consequence of 

 its rapid rotation, f<lg 



Jupiter is very much 



flattened at the poles, tho difference between its polar and 

 equatorial diameters amounting to about ^, or nearly 5,000 

 miles. 



SATURN. 



The interval between the planets is now becoming wider and 

 wider, and we have to pass nearly 400 millions of miles beyond 

 the orbit of Jupiter before we reach that of Saturn (h). The 

 mean distance of this body from the sun is 872,135,000 miles, 

 but owing to the eccentricity of its orbit, the real distance may 

 he greater or less than this by nearly 50,000,000 miles. In this 

 immense orbit Saturn revolves, accomplishing his journey round 

 it in 10,729'2 days, or nearly 294 years. In size it comes next 

 to Jupiter, having an equatorial diameter of 71,904 miles. Its 

 polar compression is, however, greater than that planet's, being 

 set down by different observers at J or ^ of the diameter. The 

 flattening is not so much observed as it otherwise would 



121-N.E. 



be, on account nl the ring with which the planet is 



Thi* ring U the moet remarkable peculiarity of flatom, and 

 appear* to be quite unique in the whole *y*t*m. To the eariy 

 observers it caused a considerable degree of wonder. Galileo, 

 when he ftnrt directed hi* magia tab* to the planet, obeerred that 

 it appeared somewhat elongated, a* if it were oval fa shape 

 instead of round, the power of the eariy telescope* not being 

 afiajent, nor their definition good enough, to show the real eaoee 

 of the appearance. After a time 

 he advanced the opinion that the 

 planet WM in reality triple, having 

 a small attendant on each nde of 

 it. Tbl* was believed for acme 

 time, till gradually the Hag began 

 to be preeented edgewiee to the 

 earth, and then disappeared alto- 

 gether. Thi* the aetrouomef* of 

 tho day were unable to explain, 

 and were for a time much per- 

 plexed; but, after the lapse of 

 about half a century, Huygens 

 discovered the real cause of them 

 appearances, and announced that 

 Saturn was surrounded by a 

 slender flat ring, nowhere touch- 

 ing it. He also predicted the 

 period when it would again be- 

 come invisible, and proved to be 

 very nearly right in hi* date*. 



After a short time it was dis- 

 covered that, in place of one ring, 

 there were two concentric one* ; 

 and numerous recent obaerra- 



2* tions have shown that these again 



are divided, so that we mn*t 

 look upon the whole as a compound or multiple ring, made up 

 of several distinct and separate ones. Three are, however, well 

 marked, the innermost of which is commonly known a* the dusty 

 ring, and is partially transparent (Fig. 28). From several pheno- 

 mena that have been observed, there seems ground for supposing 

 that one or more of these rings may be fluid rather than solid. 

 The outer bright ring has an exterior diameter of nearly 



170,000 miles, and it 

 U upwards of 10,000 

 miles broad. Th3 

 interval between this 

 and the inner bright 

 ring is but slight, but 

 the Utter ha* a 

 breadth of 17,000 

 mile*, and that of the 

 dar k ring is estimated 

 at about half this 

 amount, so that the 

 united breadth of the 

 whole U very great. 

 Their thiokne** is, 

 however, but small, 

 being variously esti- 



2g mated at from 4. 



to 250 mile*. 



seen through a good telescope, the appearance of this planet U 

 very beautiful. At times the ring is presented with it* edge t j 

 us, and is then almost invisible, being just discernible a* a thread 

 of light, along which some of the satellites appear to be travel- 

 ling. As the earth moves out of the plane of the planet'* equator, 

 the ring opens out wider and wider, the projecting sides hiring 

 the appearance of handles, whence their technical name OHM*. 



The opening of the ring attained it* maximum in August, 

 1869, and after that date slowly closed up again. It* edge 

 was directed to the earth again about the close of the year 

 1876. Sometimes the sun is on one side of the plane of these 

 rings, and the earth on the other. The dark side is then tamed 

 towards us, and the rings are invisible. Fig. 28 show* the 

 appearance of the rings in 1852, a* Been by Mr. Da we*. 



Continued observations have revealed to us two more remark- 

 able facts about these ring*. The first is that the planet is not 



