ASTRONOMY. 
death of Tycho, and about a hundred after that of Co¬ 
pernicus. It was then that Galileo, by explaining the na¬ 
ture of the compofition of motion, by lhewing, both from 
reafon and experience, that a ball dropped from the mad 
of a Ihip under fail would fall precilely at the toot of the 
mad, and by rendering this doctrine, from a great num¬ 
ber of other indances, quite familiar to the imagination, 
took off, perhaps, the principal objection which had been 
iiiadfe to this hypothefis. Several other adronomical dif¬ 
ficulties, which encumbered this account of things, were 
removed by the fame philofopher. Copernicus, after al¬ 
tering the centre of the world, and making the Earth and 
all the planets revolve round the Sun, was obliged to 
leave the Moon to revolve round the Earth as before. 
But, no example of any fitch fecondary planet having then 
been difeovered in the heavens, there 1‘eemed dill to be 
this irregularity remaining in the fydem. Galileo, who 
fird applied telefcopes to adronomy, difeovered by their 
abidance the fatellites of Jupiter, which, revolving round 
that planet, at the fame time that they were carried along 
with it in its revolution, round either the Earth or the 
Sun, made it feem lefs contrary to the analogy oi nature, 
that the Moon (hould both revolve round the Earth, and 
accompany her in her revolution round the Sun. 
It had been objected to Copernicus, that, if Venus and 
Mercury revolved round the Sun in an orbit comprehend¬ 
ed within the orbit of the Earth, they would fhew all the 
lame phafes with the Moon, prefent fometimes their dark¬ 
ened and fometimes their enlightened fides to the Earth, 
and fometimes part of the one and part of the other. He 
anfwered, that they undoubtedly did all this; but that 
their fmallnefs and diftance hindered us from perceiving 
it. This very bold affertion of Copernicus was confirmed 
by Galileo : his telefcopes rendered the phafes of Venus 
quite feniible, and thus demonftrated, more evidently than 
had been done, even by the obfervations of Tycho Brahe, 
the revolutions of thefe two planets round the Sun, as 
well as fo far deftroyed the fyftem of Ptolemy. The moun¬ 
tains and feas, which by the help of the fame inftrument 
he difeovered, or imagined he had difeovered, in the 
Moon, rendering that planet in every refpeCt fimilar to 
the Earth, made it feem lefs contrary to the analogy of 
nature, that, as the Moon revolved round the Earth, the 
Earth fliould revolve round the Sun. The fpots which, 
in the fame manner, he difeovered in the Sun, demonflra- 
ting by their motion the revolution of the Sun round his 
axis, made it feem lefs improbable that the Earth, a body 
fo much fmaller than the Sun, (liould revolve round her 
axis in the fame manner. 
Succeeding telefcopical obfervations, difeovered in each 
of the five planets, fpots not unlike thofe which Galileo 
had obferved in the Moon, and thereby feemed to demon- 
firate what Copernicus had only conjectured, that the 
planets were naturally opaque, enlightened only by the 
rays of the Sun, habitable, diverlified by feas and moun¬ 
tains, and in every refpeCt bodies of the fame kind with 
the Earth; and thus added one other probability to this 
fydem. By difeovering, too, that each of the planets re¬ 
solved round its own axis, at the fame time that it was 
carried round either the Earth or the Sun, they made it 
feem quite agreeable to the analogy of nature, that the 
Earth, which in every other refpeCt refembled the pla¬ 
nets, ftiould like them too revolve round its own axis, 
and at the fame time perform its periodical motion round 
the Sun. 
While in Italy the unfortunate Galileo was adding fo 
many probabilities to the fydem of Copernicus, there was 
another philofopher employing himfelf in Germany, to 
afeertain, correct, and improve, it: Kepler, with great 
genius, but without the fade or the order and method of 
Galileo, poffeffed, like all his other countrymen, the mod 
laborious indudry, joined to that paflion for difeovering 
proportions and relemblances betwixt the different parts 
of nature, which, though common to all philofophers, 
feems in him to have been exceflive. He had been in. 
Vol. II. No. 75. 
drafted by Maefllinus in the fydem of Copernicus ; and 
his fird curiodty was, as he tells us, to find out why tile 
planets, the Earth being counted for one, were fix in 
number ; why they were placed at fuch irregular didan¬ 
ces from the Sun; and whether there was any uniform 
proportion betwixt their feveral didances, and the times 
employed in their periodical revolutions. Tillfome reafon 
or proportion of this kiud could be difeovered, the fydem 
did not appear to him to be completely coherent. He en¬ 
deavoured fird to find it in the proportions of numbers and 
plain figures; afterwards in thofe of the regular folids ; 
and lad of all in thofe of the mufical divifions of the 
octave. Whatever was the fcience which Kepler Was 
dudying, he feems condantly to have pleafed himfelf with 
finding fome analogy betwixt it and the fydem of the uni- 
verle; and thus arithmetic and mufic, plain and folid 
geometry, came all of them by turns to illudrate the doc¬ 
trine of the fphere, in the explaining of which he was 
by his profellion principally employed. Tycho Brahe, to 
whom he had preiented one of his books, though he could 
not but difapprove of his fydem, was pleafed however 
with his genius, and with his indefatigable diligence in 
making the mod laborious calculations. That generous 
and magnificent Dane invited the obfeure and indigent 
Kepler to come and live with him, and communicated to 
him, as foon as he arrived, his obfervations upon Mars, 
in the arranging and methodizing of which his difciples 
were at that time employed. Kepler, upon comparing them 
with one another, found that the orbit of Mars was not a 
perfect circle ; that one of its diameters was fomewhat 
longer than the other ; and that it approached to an oval, 
or an ellipfe, which had the Sun placed in one of its foci. 
He found too, that the motion of the planet was not equa¬ 
ble ; that it was fwifted when neared the Sun, and flowed 
when farthed from him ; and that its velocity gradually 
encreafed or diminiflied, according as it approached or re¬ 
ceded from him. The obfervations of the fame adronomer 
difeovered to him, though not fo evidently, that the fame 
things were true of all the other planets ; that their orbits 
were elliptical, and that their motions were fwifted when 
neared the Sun, and flowed when farthed from him. They 
Ihewed the fame things too of the Sun, if fuppofed to 
revolve round the Earth ; and confequently of the Earth, 
if fuppofed to revolve round the Sun. 
That the motions of all the heavenly bodies were per¬ 
fectly circular, had been the fundamental idea upon which 
every adronomical hypothefis, except the irregular one of 
the Stoics, had been built. A circle, as the degree of its 
curvature is every where the fame, is of all curve lines 
the limpled and the mod eafily conceived. Since it was 
evident, therefore, that the heavenly bodies did not move 
in flraight lines, the indolent imagination found, that it 
could moft eafily attend to their motions if they were fup¬ 
pofed to revolve in perfect circles. It had, upon this 
account, determined that a circular motion was the mod 
perfeCt of all motions, and that none but the mod perfect 
motioa could be worthy of fuch beautiful and divine ob¬ 
jects ; and it had upon this account, fo often, in vain, en¬ 
deavoured to adjufl to the appearances, fo many different 
fyflems, which all fuppofed them to revolve in this manner. 
The equality of their motions was another fundamental 
idea, which, in the fame manner, and for the fame reafon, 
was fuppofed by all the founders of aftronomical fyflems. 
For an equal motion can be more eafily attended to, than 
one that is continually either accelerated or retarded. All 
incondancy, therefore, was declared to be unworthy thofe 
bodies which revolved in the celeflial regions, and to be 
fit only for inferior and fublunary things. The calcula¬ 
tions of Kepler overturned, with regard to the planets, 
both thefe natural prejudices of the imagination ; deflroy. 
ed their circular orbits, and introduced into their real mo¬ 
tions fuch an inequality as no equalizing circle would re¬ 
medy. It was, however, to render their motions perfectly 
equable, without even the affidance of an equalizing circle, 
that Coperoicus, as lie himfelf allures us, had originally 
4 R. invented 
