MARS. 
Ecliptic, 1)ut deviates from it several de- 
grees. His apparent diameter vai’ies ex- 
ceedingly. His mean apparent diameter is 
27 ", and it increases so much, that when 
the planet is in opposition, the apparent 
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 
to the sun, and becomes oval. Its phases 
show that it derives its light from that lumi- 
nary. 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 inclined to the ecliptic 
at an angle of about 59.70. 
They were first observed in 1666 by Cas- 
sini at Bologna, with a telescope about 
161 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 planet was observed by 
some astronomers at Rome, with longer te- 
lescopes, but they assigned to it a rotation 
in thirteen hours only. This, how'ever, was 
afterwards shewn by M. Cassini to have 
been a mistake, 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 observations 
on the spots of this planet in 1670, from 
whence he dpevv an additional confirmation 
of the time the planet took to revolve. 
The spots w'ere again observed in subse- 
quent 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 fre- 
quently in the space between two opposi- 
tions, but even in the space of a month. 
Some of them, however, continued of the 
same form long enough to ascertain the time 
of the planet’s revolution. Among these 
there appeared this year an oblong spot, 
resembling one of the belts of Jupiter when 
broken. It did not reach quite round the 
body of the planet, but had, not far from 
die middle of it, a small protuberance to- 
wards the North, so well defined, that he 
was thereby enabled to settle the pe- 
riod of its, revolution at twenty-four hours 
thirty-nine minutes, only one minute less 
tlian what Cassini had determined it to be. 
Besides these dark spots, former astrono- 
mers took notice that a segment of his globe 
about the South pole exceeded the rest of 
bis disk so much in brightness, that it ap- 
peared beyond them as if it were the seg- 
ment of a larger globe. Maraldi informs 
us, that this bright spot had been taken no- 
tice of for sixty years, and was more per- 
manent than the other spots on the planet. 
One part of it is brighter than the rest, and 
the least bright part is subject to great 
changes, and has sometimes disappeared. 
A similar brightness about the North 
pole of Mars was also sometimes observed ; 
and these observations are now confirmed 
by Dr. Herschel, who has viewed the pla- 
net with much better instruments, 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 po- 
sition of his axis, &c. See Philosophical 
Transactions, vol. Ixxiv. 
“ The analogy,” says Dr. Herschel, “ be- 
tween Mars and the earth, is, perhaps, by far 
the greatest in the whole solar systeni. Ibeir 
diurnal motion is nearly the same ; the ob- 
liquity of their respective ecliptics not vei-y 
different. Of all the sqperior 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 com- 
pared to the sinprising 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 probably have the 
same effect on the globe of Mars ; that the 
bright polar spots are owing to the vivid re- 
flection 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 ex- 
pect, as that pole had but lately been in- 
volved in a whole twelvemonth’s darkness 
and absence of the sun • but in 1783, I 
found it considerably smaller than before, 
and it decreased 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 be- 
nefit of summer, and still continued to re- 
ceive the sun-beams, though, towards the 
latter end, in such an oblique direction as 
to be but little benefited by them. On tlie 
other hand, nithe year 1781, the North 
polar spot which had been its twelvemonth 
