LB880N8 IN ASTRONOMY. 



10.1 



LESSONS IN ASTRONOMY.- M 1 



MABH THB MINOR PLANKT8 JUPITKB. 



WK now pass on to notice the auperior planet*, concerning 

 the nearer of which our inf.n -HMD- e.nnpleto than it i* 



uliout t!i.- inferior ones, an the lattor are usually too much over- 

 powered by the. brightness of the sun'B rays to be dintim-tly 

 observed. The superior ones, howurer, since their orbits are out- 

 fit of the earth, are at times in opposition t<> tli.) nan j at 

 .ey are in periyce, that is, at thuir leant diatanoe 

 tn'in tho oarth, ami therefore in all respects moat favourably 

 ited for obaorrution. 



Tho nearest to us of 

 Iheso bodies is Mars, re- 

 presented by the sign <J. 

 This planet revolves around 

 t)ii> nun, at a mean distance 

 ..f 1H9,312,000 miles, in an 

 orbit of considerable eccen- 

 tricity, the difference be- 

 tween its greatest and 

 least distances being about 

 2<i, 000,000 miles. When 

 tho planet in in opposition, 

 both it and the Earth are 

 on the same side of the 

 Sun, and the mean distance 

 .( -en them then ia about 

 48,000,000 miles. At this 

 period the planet shines 

 with a brilliancy almost 

 equal to that of Jupiter; 

 this happens about once 

 in two years, its synodic 

 period being 780 days. 

 When it happens to be in 

 its j* rihflion at tho same 

 time its brilliancy is still 

 greater, and consequently 

 this is the most favourable 

 opportunity for telescopic 

 observations. 



When thus examined 

 with powerful telescopes 

 Mars is found to exhibit 

 an appearance similar to 

 that which tho Earth would 

 probably present to the in- 

 habitants of that planet, 

 should there bo any. The 

 surface is diversified by 

 dark portions which repre- 

 sent water, and lighter 

 parts which arc tho conti- 

 nents. These markings 

 vary a little at times, pro- 

 bablyowingto largo masses 

 of clouds in tho planet's 

 atmosphere : tho broad fea- 

 tures are, however, suffi- 

 ciently permanent to ena- 

 ble maps to bo constructed 

 showing the configuration of its surface ; and by observations 

 made on the more distinct markings, its period of revolution 

 has been ascertained to be 24 hours, 37 minutes, 23 seconds. 

 Figs. 24, 25, 26 give a general idea of the appearance of tho 

 planet when seen through a large telescope. When its atmo- 

 sphere is clear, the land appears to bo of a ruddy hue, while the 

 water is somewhat greenish. Fig. 25 represents the southern 

 and northern hemispheres of tho planet, as drawn by MM. Beer 

 and MiUller, who devoted many years to the examination of 

 Mars ; Fig. 26 is taken from the observations of Secchi, the 

 eminent Roman observer, at tho opposition of 1858. 



Near the south polo a remarkably bright spot is observed, as 

 shown at a. The reason why it appears at the upper part of the 

 figure instead of the lower is that tho sketch is taken with an 

 astronomical telescope, which always inverts the object. Tho 

 size of this spot is found to increase at the period of winter in 



117-N-E. 



the aonthera hemisphere of the pit***, awl to diminish again 

 during the summer, and henee it h* been aumiueaJ to be pro- 

 duoed by the accumulation of now around thepole and ia 

 uanally known a* the note MMU. 



The question may naturally be asked, why, if this pot be pro. 

 dnoed by mow, it in not M*n at the north pole Ukewiee. The 

 (Treat eccentricity of the planet ' orbit folly explain, this an 

 from thia oaoae the winter in the Booth is moon longer and more 

 severe than in the northern bemiaphere. The atnalleat iliaaitar 

 of thia tone oooon about thirty days after the rammer "Wft, 

 a period corresponding to that which on the Earth is the that 



of i/:.-.a-,t Mat 



TI..- t,m,. oeeopW -v 

 Mara in aooompluhing H 

 journey round the aon is 

 nearly 687 of oar daya. 

 A, however, ita day U a 

 little longer than onra. 

 there will not be quite thia 

 number of Haitian daya 

 inita year. 



The inclination of the 

 ax in of Mara to the plane 

 of ita orbit U 28 51', or a 

 little greater than that of 

 the Earth, consequently ita 

 easons will reaemble oora 

 to a conaiderable extent. 

 The .final '!..LI:.' -t. r <>l 



the planet ia 4,920 miles, 

 ao that ita bulk ia about 

 i that of the Earth. The 

 polar compreaaion ia be- 

 lieved to amount to about 

 95 milea, or of ite dia- 

 meter ; thia measurement ie 

 however somewhat doubt- 

 ful. In 1877 two moona 

 were discovered accom- 

 panying Man in ita orbit. 



THE MINOR PLANETS. 



Aa already mentioned in 

 our fourth lesson, a wide 

 gap waa found to exiat be- 

 tween the orbits of Man 

 and Jupiter, which, accord- 

 ing to Bode'a law, ought 

 to be occupied by a planet. 

 In the year 1800 six astro- 

 nomers determined to esta- 

 blish a society of twenty- 

 four observer*, who should 

 divide the zodiac between 

 them, each taking 15, and 

 should search for the BUS- 

 pected planet Thia plan 

 soon succeeded, for on the 

 lat of January, 1801, Piani 

 discovered a moving body, 

 which he at first supposed 

 to be a comet, but which 



soon proved to be a planet, afterwards named Ceres, whose 

 position coincided very nearly with that pointed out by Bode'a 

 law. When this fact became generally known, the search 

 was discontinued, as the system appeared now to be complete ; 

 in the course of the following year, however, Olbera disco- 

 vered a second planet, revolving almost in the same period, 

 and at the same distance aa Ceres. This planet waa named 

 Pallas, and its discovery excited great attention among astro- 

 nomers, such a thing having hitherto been quite nn thought of, aa 

 that there should be two planets revolving at almost the same 

 distance from the sun. After some time Olbera ventured a 

 speculation that these two planets might be the remains of a 

 single one that had by some means become shattered, and sug- 

 gested that in thia case other fragments might not improbably b 

 discovered. The search waa accordingly renewed, and Juno and 

 Vesta were discovered in 1804 and 1807. For many years no 



