ASTRO 
connect together, in reality, thofe different movements 
and effects which the artift has occafion tor. A fyftem is 
an imaginary machine invented to connect together in the 
fancy thofe different movements and effects which are al¬ 
ready in reality performed. The machines that are firft 
invented to perform any particular movement are always 
the mofl complex, and fucceeding artiffs generally difco- 
ver that, with fewer wheels, with fewer principles of mo¬ 
tion, than had originally been employed, the fame effects 
may be more eafily produced. The firft fyftems, in the 
fame manner, are always the moft complex, and a parti¬ 
cular conneding chain or principle'is generally thought 
neceffary to unite every two fcemingly disjointed appear¬ 
ances: but it often happens, that one great conneding 
principle is afterwards found to be fufficient to bind-toge¬ 
ther all the difGbrdant phenomena that occur in a whole 
fpecies of things. How many wheels are neceffary to car¬ 
ry on the movements of this imaginary machine, the fyf- 
-tem of eccentric fpheres ? The weftward diurnal revolu¬ 
tion of the firmament, whofe rapidity carries all the other 
heavenly bodies along with it, requires one. The periodi¬ 
cal eaftward revolutions of the Sun, Moon, and five pla¬ 
nets, require, for each of thofe bodies, another. Their 
differently accelerated and retarded motions require, that 
thofe wheels or circles fhould neither be concentric with 
the firmament, nor with one another; which, more than 
any thing, feems to difturb the harmony of the univerfe. 
The retrograde and ftationary appearance of the five pla¬ 
nets, as well as the extreme inconftancy of the Moon’s 
motion, require, for each of them, an epicycle, (another 
little wheel attached to the circumference of the great 
wheel,) which dill-more interrupts the uniformity of the 
fyftem. The motion of the apogeum of each of thofe 
bodies requires, in each of them, ftill another wheel, to 
carry the centres of their eccentric fpheres round the cen¬ 
tre of the Earth. And thus this imaginary machine, tlio’ 
perhaps more litnple, and certainly better adapted to the 
phenomena, than the fifty-fix planetary fpheres of Arifto¬ 
tle, was ftill too intricate and complex for the imagination 
So reft in it with complete tranquillity and fatisfaftion. 
It maintained its authority, however, without any dimi¬ 
nution of reputation, as long as fcie'nce was at all regard¬ 
ed in the ancient world. After the reign of Antoninus, 
and indeed after the age of Hipparchus, who lived almoft 
3 oo years before Antoninus, the great reputation which 
the earlier philofophers had acquired fo impofed upon the 
imaginations of mankind, that they feem to have defpaired 
of e°ver equalling their renown. All human wifdom, they 
fuppofed, was comprehended in the writings of thofe elder 
lattes. To abridge, to explain, and to comment upon, them, 
and thus Iliew themfelves, at lead, capable of underftand- 
ing fome of their fublime myft'eries, became now the only 
probable road to reputation. Proclus and Theon wrote 
commentaries upon the Syftem of Ptolemy; but, to have 
attempted to invent a new one, would then have been re- 
' carded, not only as preemption, but as impiety to the 
memory of their fo much revered prcdecetfors. 
The ruin of the empire of the Romans, and, alongwith 
it, the fubverfion of all law and order, which happened a 
few centuries afterwards, produced the entire negleft of 
that ftudy of the connecting principles of nature, to which 
leifure and fecurity can alone give occafion. After the 
fall of thofe great conquerors and civilizers of mankind, 
the empire of the khalifs feems to have been the firft date 
under which the world enjoyed that degree of tranquillity 
which the cultivation of the fciences requires. It was tin¬ 
ker the protection of thofe generous and magnificent prin¬ 
ces, that the ancient philofophy and aftronomy of the 
Greeks were reftored and eftablifhed in the eaft ; that tran¬ 
quillity, which their mild, juft, and religious, government 
diffufed over their vaft empire, revived the curiolity of 
mankind, to inquire into the connedting principles of na¬ 
ture. Tht fame of the Greek and Roman learning, which 
was tiien recent in the memories of men, made them de- 
' jjre to know, concerning thefe abftrufe fubjeCts, what were 
N O M Y. 
the doctrines of the fo much renowned fages of thofe two 
nations. 
They tranflated, therefore, into the Arabian language, 
and ftudied, with great eagernefs, the works of many Greek 
philofophers, particularly of Ariftotle, Ptolemy, Hippo¬ 
crates, and Galen. The fuperiority which they eafily dif- 
covered in them, above the rude effays which their own 
nation had yet had time-to produce, and which were fuch, 
we may fuppofe, as arife every where in the firft infancy of 
fcierice, neceffarily determined them to embrace their fyf¬ 
tems, particularly that of aftronomy : neither were they 
ever afterwards able to throw off their authority. For, 
though the munificence of the Abafiides, the fecond race 
of khalifs, is faid to have fupplied the Arabian aftrono- 
mers with larger and better inftruments than any that were 
known to Ptolemy and Hipparchus, the ftudy of the fci¬ 
ences feems, in that mighty empire, to have been either 
of too ftiort or too interrupted a continuance, to allow 
them to make any confiderable correction in the doCtrines 
of thofe old mathematicians. The imaginations of man¬ 
kind had not yet got time to grow fo familiar witli the an¬ 
cient fyftems, as to regard them^vithout fome degree of 
that aftoniftiment which their grandeur and novelty exci¬ 
ted ; a novelty of a peculiar kind, which had at once the 
grace of what was new, and the authority of what was 
ancient. They were ftill, therefore, too much enftaved 
to thofe fyftems, to dare to depart from them, when thofe 
confufions which (hook, and at laft overturned, the peace¬ 
ful throne of the khalifts, bamlhed the ftudy of the fci¬ 
ences from that empire. They had, however, before this, 
made fome confiderable improvements: they had mea- 
fured the obliquity of the ecliptic with more accuracy than 
had been done before. The tables of Ptolemy had, by the 
length of time, and by the inaccuracy of the obfervations 
upon which they were founded, become altogether wide 
of what was the real fituation of the heavenly bodies, as 
he himfelf indeed had foretold they would do. It became 
neceffary, therefore, to form newones, which was accord¬ 
ingly executed by the orders of the khalif AJmamon, un¬ 
der whom was alfo made the firft menfuration of the Earth 
that we know of, after the commencement of the Chriftian 
era, by two Arabian aftronomers, who, in the plain of 
Sennaar, meafured two degrees of its circumference. 
The victorious arms of the Saracens carried into Spain 
the learning, as well as the gallantry, of the eaft; and 
along with it the tables of Almamon, and the Arabian 
tranfiations of Ptolemy and Ariftotle ; and thus Europe 
received a fecond time, from Babylon, the rudiments of 
the fcience of the heavens. The writings of Ptolemy were 
tranflated from Arabic into Latin; and the Peripatetic 
philofophy was ftudied in Averroes and Avicenna with as 
much eagernefs, and witli as much fnbmiflion to its doc¬ 
trines in the weft, as it had been in the eaft. 
The doCtrine of the (olid fpheres hud originally been 
invented in order to give a phyfical account of the revo¬ 
lutions of the heavenly bodies, according to the fyftem of 
concentric circles, to which that doctrine was very eafily 
accommodated. Thofe mathematicians who invented the 
doctrine of eccentric circles and epicycles, contented them¬ 
felves with fhewing how, by fuppofing the heavenly bo¬ 
dies to revolve in fuch orbits, the phenomena might be 
connected together, and fome fort of uniformity and co¬ 
herence he beftowed upon their real motions. The phy¬ 
fical caufes of thofe motions they left to the confideration 
of the philofophers; though, as appears from fome paf- 
fages of Ptolemy, they had fome general apprelteniion, 
that they were to be explained by a like hypothefis. Bur, 
though the fyftem of Hipparchus was adopted by all aftro¬ 
nomers and mathematicians, it never was received, as we 
have already obferved, by any one feCt of philofophers 
among the ancients. No attempt, therefore, feems to have 
been made amongft them, to accommodate to it any fuch 
hypothefis. 
The fchoolmen, who received at once from the Ara¬ 
bians the philofophy of Ariftotle and the aftronomy of 
Hipparchus, 
