ASTRONOMY. 
Hipparchus, were neceffinnly obliged to reconcile them to 
one another, and to connect together the revolutions of 
the eccentric circles and epicycles of the one, by the folia 
fpheres of the other. Many different attempts of this 
kind were made by many different philofophers: but, of 
them all, that of Purbach, in the fifteenth century, was 
the happieft and the mod elieemed. Though his liypo- 
thefis is the fimp'.eft «f any of them, it would be in vain 
to deferibe it ; for, if the fyftern of eccentric circles and 
epicycles was before too perplexed and intricate tor the 
imagination to reft in with tranquillity and fat is fa cl i'on, it 
became much more fo, when this addition had been made 
to it. The world, juftly indeed, applauded the ingenuity 
of that philofopher, who could unite, fo happily, two fuel! 
feemingly incohfiftent f'yftems. His labours, however, feem 
rather to have increafed than to have diminifhed the caufes 
of that difiktisfaction, which the learned began to feel with 
the f'yftem of Ptolemy. He, as well as all thofe who had 
worked upon the fame plan before him, by rendering this 
account of things more complex, rendered it (till more e:n- 
barrafling than it had been before. Neither was the com- 
plexnefs of this fyftern the foie cattfe of the diffatisfaClion, 
w hich the world in general began, foon after the days of 
Purbach, to exprefs for it. The tables of Ptolemy hav¬ 
ing, upon account of the inaccuracy of the observations 
on wh:ch they were founded, become altogether wide of 
the real fituation of the heavenly bodies, thofe of Alma- 
mon, in the ninth century, were, upon the fame hypo- 
thelis, compofed to cor reft their deviations. Thefe again, 
a few ages afterwards, became, for the fame reafon, equal¬ 
ly ufelefs. In the thirteenth century, Alphonfus, the phi- 
lofophical king of Caftile, found it necetfary to give orders 
for the compofition of thofe tables which bear his name. 
It is he, who is fo well known for the whimlical impiety of 
uftng to fay, that, had he been confulted at the creation 
of the univerfe, he could have given better advice; an 
apophthegm which is fuppofed to have proceeded from 
his diflike to the intricate fyftern of Ptolemy. In the fif¬ 
teenth century, the deviation of the Alphonfine tables be¬ 
gan to be as fenfible as thofe of Ptolemy and Almamon 
had been before. It appeared evident, therefore, that, 
though the fyftern of Ptolemy might, in the main, be true, 
certain corrections were neceffary to be made in it before 
it could be brought to correfpond with exact precifion to 
the phenomena. For the revolution of his eccentric cir¬ 
cles and epicycles, fuppofing them to exilt, could not, it 
was evident, be precifely fuch as he reprefented them ; 
fince the revolutions of the heavenly bodies deviated, in a 
lhort time, fo widely from what the moft exaCt calcula¬ 
tions, that were founded upon his hypotbefis, reprefented 
them. It had plainly, therefore, become necetfary to cor¬ 
rect, by more accurate obfervations, both the velocities 
and directions of all the wheels and circles of which his 
hypotbefis is compofed. This, accordingly, was begun 
by Purbach, and carried on by John Muller, commonly 
called Regiomontanus, the difciple, the continliator, and 
the perfecter, of the fyftern of Purbach; and one, whole 
untimely death, ainidlt innumerable projects for the re¬ 
covery of old, and the invention and advancement of new, 
fciences, is, even at this day, to be regretted. 
When once the world have been convinced that an efta- 
blifhed fyltem ought to be corrected, it is not very difficult 
to perfuade them that it Ihould be deftroyed. Not long, 
theiefore, after the death of Regiomontanus, Copernicus 
began to meditate a new fyltem, which Ihould conned to¬ 
gether the celeftial appearances, in a more limple as well 
as a more accurate manner than that of Ptolemy. The 
confulion, in which the old hypotbefis reprefented the 
motions of the heavenly bodies, was, be tells us, what firft 
fuggefted to him the defign of forming a new fyftern, that 
thefe, the nobleft works of nature, might no longer appear 
devoid of that harmony and proportion which dilcover 
themfelves in her meaneft productions. What moft of all 
dilfatisfied him was, the notion of the equalizing circle, 
which, by reprefenting the revolutions of the celeftial 
Voi. II. No. 34. 
fpheres, as equable Only, when furveyea from a point 
that was different from their centres, introduced a real in¬ 
equality into their motions ; contrary to that molt natu¬ 
ral, and indeed fundamental, idea, with which all the au¬ 
thors of aftronomical fyftems, Plato, Eudoxus, Ariftotle, 
even Hipparchus and Ptolemy themfelves, had hitherto 
fet out, that the real motions of fuch beautiful and divine 
objeCfs mull neceflarily be perfectly regular, and go qn in 
a manner as agreeable to the imagination as the objeds 
themfelves are to the fenfes. He began to confider, there¬ 
fore, whether, by fuppofing the heavenly bodies to be ar¬ 
ranged in a different order from that in which Ariftotle 
and Hipparchus had placed them, this fo much fought 
for uniformity might not be beltowed upon their motions. 
To dilcover this arrangement, he examined all the obfeure 
traditions delivered down to 11s, concerning every other 
hypotbefis which the ancients had invented for the fame 
purpofe. He found, in Plutarch, that fome old Pytha¬ 
goreans had reprefented the Earth as revolving in the cen¬ 
tre of the univerfe, like a wheel round its own axis; and 
that others’,- of the fame fed, had removed it from the 
centre, and reprefented it as revolving in the ecliptic like 
a ftar round the central fire. By this central fire, he fup¬ 
pofed they meant the Sun; and, though in this he was 
very widely miftaken, it was,’it feems, upon this interpre¬ 
tation that he began to conlider bow fuch an hypotbefis 
might be made to correfpond to the appearances. The 
luppofed authority of thofe old philofophers, if it did not 
originally fuggeft to him his fyftern, feems, at leaft, to 
have confirmed him in an opinion, which it is not impro¬ 
bable that he bad before-hand other reafons for em¬ 
bracing, notwithftanding what he himlelf would affirm to 
the contrary. 
It then occurred to him, that, if the Earth was fup¬ 
pofed to revolve every day round its axis, from w'eft to 
eaft, all the heavenly bodies would appear to revolve, in 
a contrary direction, from eaft to weft. The diurnal re¬ 
volution of the heavens, upon this hypotbefis, might be 
only apparent; the firmament, which has no other lenlible 
motion, might be perfedly at reft ; while the Sun, the 
Moon, and the five planets, might have no other move¬ 
ment belide that eaftward revolution, which is peculiar 
to themfelves. That, by fuppofing the Earth to revolve 
with the planets, round the Sun, in an orbit, which com¬ 
prehended within it the orbits of Venus and Mercury, but 
was comprehended within thofe of Mars, Jupiter, and Sa¬ 
turn, he could, without the embarraffment of epicycles, 
connect together the apparent annual revolutions of the 
Sun, and the direct, retrograde, and ftationary, appear¬ 
ances of the planets: that, while the Earth really revolved 
round the Sun on one fide of the heavens, the Sun would 
appear to revolve round the Earth on the other; that, 
while Ihe really advanced in her annual courfe, he would 
appear to advance eaftward in that movement which is pe¬ 
culiar to himlelf. That, by fuppofing the axis of the Earth 
to be always parallel to itfelf, not to be quite perpendi¬ 
cular, but fomewhat inclined to the plane of her orbit, 
and confequently to prefent to the Sun, the one pole when 
011 the one fide of him, and the other when on the other, 
he would account for the obliquity of the ecliptic; the 
Sun’s feemingly alternate progrellion from north to fouth, 
and from fouth to north, the confequent change of the 
feafons, and different lengths of days and nights in the 
different feafons. 
If this new hypotbefis thus connected together all thefe 
appearances as happily as that of Ptolemy, there were 
others which it conneifted together much better. The 
three fuperior planets, when nearly in conjunction with 
the Sun, appear always at the greateft di(lance from the 
Earth, are ('mailed, and lead fenfible to the eye, and feeni! 
to revolve forward in their direft motion with the greateft 
rapidity. On the contrary, when inoppolition to the Sun, 
that is, when in their meridian about midnight, they ap¬ 
pear neareft the Earth, are largeft, and moft lenlible to 
the eye, and feem to revolve backwards in their retro- 
4 Qw grade 
