MARS. 



is 27", and it increases so much, that 

 when the planet is in opposition, the appa- 

 rent diameter is 81". Then the parallax 

 of Mars becomes sensible, and about 

 double that of the sun. The disk of Mars 

 changes its form relatively to its position 

 with regard tD the sun, and becomes oval. 

 Its phases show that it derives its light 

 from that luminary. The spots observed 

 on its surface have informed astronomers, 

 that it moves round its axis from West to 

 East in 1.02733 days, and its axis is in- 

 clined to the ecliptic at an angle of about 

 59.70. 



They were first observed in 1666 by 

 Cassini at Bologna, with a telescope 

 about 16 feet long ; and continuing to 

 observe them for a month, he found they 

 came into the same situation in twenty- 

 four hours and forty minutes. The pla- 

 net was observed by some astronomers 

 at Rome, with longer telescopes, but they 

 assigned to it a rotation in thirteen hours 

 only. This, however, was afterwards 

 shewn by M. Cassini to have been a mis- 

 take, and to have arisen from their not 

 distinguishing the opposite sides of the 

 planet, which, it seems, have spots pretty 

 much alike. He made further observa- 

 tions on the spots of this planet in 1670, 

 from whence he drew an additional con- 

 firmation of the time the planet took to 

 revolve. The spots were again observed 

 in subsequent oppositions, particularly 

 for several days in 1704, by Maraldi, who 

 took notice that they were not always well 

 defined, and that they not only changed 

 their shape frequently in the space "be- 

 tween two oppositions, but even in the 

 space of a month. Some of them, how- 

 ever, continued of the same form long 

 enough to ascertain the time of the pla- 

 net's revolution. Among these there ap- 

 peared this year an oblong spot, resem- 

 bling one of the belts of Jupiter when 

 broken. It did not reach quite round 

 the body of the planet, but had, not far 

 from the middle of it, a small protube- 

 rance towards the North, so well defined, 

 that he was thereby enabled to settle the 

 period of its revolution at twenty-four 

 hours thirty-nine minutes, only one minute 

 less than what Cassini had determined it 

 to be. 



Besides these dark spots, former as- 

 tronomers took notice that a segment of 

 his globe about the South pole exceeded 

 the rest of his disk so much in brightness, 

 that it appeared beyond them as if it were 

 the segment of a larger globe. Maraldi 

 informs us, that this bright spot had been 

 taken notice of for sixty years, and was 



more permanent than the other spots on 

 the planet. One part of it is brighter than 

 the rest, and the least bright part is sub- 

 ject to great changes, and has sometimes 

 disappeared. 



A similar brightness about the North 

 pole of Mars was also sometimes ob- 

 served ; and these observations are now 

 confirmed by Dr. Herschel, who has 

 viewed the planet with much better in- 

 struments, and much higher magnifying 

 powers, than any other astronomer ever 

 was in possession of. His observations 

 were made with a view to determine the 

 figure of the planet, the position of his 

 axis, Sec. See Philosophical Transactions, 

 vol. Ixxiv. 



" The analogy," says Dr. Herschel, 

 " between Mars and the earth, is, perhaps, 

 by far the greatest in the whole solar sys- 

 tem. Their diurnal motion is nearly the 

 same; the obliquity of their respective 

 ecliptics not very different. Of all the 

 superior planets, the distance of Mars 

 from the sun is by far the nearest alike 

 to that of the earth ; nor will the length 

 of the Martial year appear very different 

 from what we enjoy, when compared to 

 the surprising duration of the years of 

 Jupiter, Saturn, and the Herschel. If 

 then we find that the globe we inhabit 

 has its polar region frozen and covered 

 with mountains of ice and snow, that 

 only partly melt when alternately exposed 

 to the sun, I may well be permitted to 

 surmise, that the same causes may proba- 

 bly have the same effect on the globe of 

 Mars ; that the bright polar spots are 

 owing to the vivid reflection of light from 

 frozen regions, and that the reduction of 

 those spots is to be ascribed to their being 

 exposed to the sun. In the year 1781, 

 the South polar spot was extremely large, 

 which we might well expect, as that pole 

 had but lately been involved in a whole 

 twelvemonth's darkness and absence ot 

 the sun ; but in 1783, I found it consi- 

 derably smaller than before, and it de- 

 creased continually from the 20th of May 

 till about the middle of September, when 

 it seemed to be at a stand. During this 

 last period the South pole had already 

 been above eight months enjoying the 

 benefit of summer, and still continued to 

 receive the sun-beams, though, towards 

 the latter end, in such an oblique direc- 

 tion, as to be but little benefited by them. 

 On the other hand, in the year 1781, the 

 North polar spot, which had been its 

 twelvemonth in the sun-shine, and was 

 but lately returning into darkness, ap- 

 peared small, though undoubtedly in.- 



