A S T R O N O M 
years to come : he difcovered that the fixed ftars changed 
their places, having a flow motion of their own from weft 
to eaft : lie correlated the Calippic period, and pointed out 
fome errors in the method of Eratofthenes, for meafuring 
the circumference of the Earth : he computed the Sim’s 
dillance more accurately than any of his predeceflors ; and 
alfo the longitudes, latitudes, and apparent magnitudes, 
of the ftars; which, with mod of his other obfervations, 
are preferved by Ptolemy in his Almageft. 
There was but little progrefs made in practical aftrono- 
my from the time of Hipparchus to that of Ptolemy, who 
made the greateft: part of his oblervations at the celebrated 
fchool of Alexandria in Egypt. Profiting by thofe of 
Hipparchus and other ancient aftronomers, he formed a 
fyltem of his own, which, though erroneous, as we have 
feen, was followed for many ages by all nations. He com¬ 
piled a great work, called the Almageft, which contained 
the oblervations and collections of Hipparchus and others 
his predeceflors in practical aftronomy, on which account 
it will ever be valuable to the profelfors of this fcience. 
That work was preferved from the grievous conflagration 
of the Alexandrine library by the Saracens, and tranflated 
out of Greek into Arabic in the year 827, and from thence 
into Latin in 1230. The Greek original was not known 
in Europe till the beginning of the fifteenth century, when 
it was brought from Conftantinople, then taken by the 
Turks, by George, a monk of Trapezond, by whom it 
was tranflated into Latin ; and various other editions have 
been fince made. 
During the long period from the year 800 till the begin¬ 
ning of the 14th century, while the weftern parts of Eu¬ 
rope were immerfed in ignorance and barbarity, the Ara¬ 
bians, profiting by the books they had preferved from tlie 
wreck of the Alexandrine library, cultivated and improved 
all the fciences, and particularly that of aftronomy, in 
which they had many able profelfors and authors. The 
caliph Ai Manfur firft introduced a tafte for the fciences 
iiito his empire. His grandfon Al Mamun, who alcended 
the throne in 814, was a great encourager and improver of 
the fciences, and efpecially of practical aftronomy. Having 
conftruCted proper inftruments, he made many obfervations; 
determined the obliquity of the ecliptic to be 23 0 35' ; 
and under his aufpices a degree of the circle of the Earth 
was meafured a fecond time in the plain of Singar, on the 
border of the Red Sea. About the fame time Alferganus 
wrote elements of aftronomy ; and the fcience was from 
hence greatly cultivated by the Arabians, but principally 
by Albategnius, who flourillied about the year 880, and 
who greatly reformed aftronomy, by comparing his own 
obfervations with thofe of Ptolemy: hence lie computed 
the motion of the Sun’s apogee from Ptolemy’s time to 
his own ; fettled the precellion of the equinoxes at one 
degree in feventy years ; and fixed the obliquity of the 
ecliptic at 23 0 33'. The tables which he compofed, for the 
meridian of AraCta, were long efteemed by the Arabians. 
After his time, though the Saracens had many eminent 
aftronomers, feveral centuries elapfed without producing 
any very valuable obfervations, excepting thofe of fome 
eclipfes obferved by Ebn Younis, aftronomer to the caliph 
of Egypt, by means of which the quantity of the Moon’s 
acceleration fince that time may be determined. 
Other eminent Arabic aftronomers, were, Arzachel a 
Moor of Spain, who obferved the obliquity of the ecliptic: 
he alfo improved trigonometry by conftrufting tables of 
fines, inftead of chords of arches, dividing the diameter 
into 300 equal parts. And Alhazen, his cotemporary, 
wrote upon the twilight, the height of the clouds, the 
phenomenon of the horizontal moon, and firft ftiewed the 
importance of the theory of refradtions in aftronomy. 
Ulugh Beigh, grandfon of the celebrated Tartar prince 
Tamerlane, was alfo a great proficient in practical aftro¬ 
nomy ; he had very large inftruments, particularly a quad¬ 
rant of about 180 feet high, with which he made good 
obfervations. From thefe he determined the latitude of 
Samarcand, his capital, to be 39 0 37' 23" ; and compofed 
aftronomical tables for the meridian of the fame, fo exaft, 
that they differ very little from thole conftrudied after¬ 
wards by Tycho Brahe. 
During this period almoft all Europe was immerfed in 
ignorance, until the fettlement of the Moors in Spain, 
when the fciences were again introduced, and have fince 
continued to improve, and to be communicated from one 
people to another, to the prefent time. The emperor Fre¬ 
deric II. about 1230, firft began to encourage learning; 
reftoring fome decayed univerfities, and founding a new 
one in Vienna : be alfo caufed the works of Ariftotle, and 
Ptolemy’s Almageft, to be tranflated into Latin ; and from 
the tranflation of this work we may date the revival of 
pradtical aftronomy in Europe. Two years after this, 
John de Sacro Bofco, that is, of Halifax, compiled from 
Ptolemy, Albategnius, Alferganus, and other Arabic af¬ 
tronomers, his work De Sphtera, which was held in the 
greateft eftimation for 300 years after, and was honoured 
with commentaries by Clavius and other learned men. In 
1240, Alphonfo, king of Caftile, not only cultivated prac¬ 
tical aftronomy himfelf, but greatly encouraged others ; 
and by the aftiftance of feveral learned men he corrected 
the tables of Ptolemy, and compofed thofe which, were 
from him denominated the Al'phonline Tables. About 
the fame time alfo, Roger Bacon, an Englifh monk, wrote 
feveral traCts relative to aftronomy, particularly of the lu¬ 
nar afpeCts, the folar rays, and the places of the fixed 
ftars. And, about the year 1270, Vitello, a Polander, 
compofed a treatife on optics, in which he (hewed the ule 
of refractions in aftronomy . 
Little other improvements were made in practical aftro¬ 
nomy till the time of Purbach, who was born in 1423. He 
compofed new tables of fines for every ten minutes, mak¬ 
ing tiie radius 60, with four ciphers annexed. He con- 
ftruCted fpheres and globes, and wrote feveral aftronomical 
traCts. In his folar tables, he placed the Sun’s apogee in 
the beginning of Cancer ; but retained the obliquity of 
the ecliptic 23 0 33V, as determined by the lateft obferva¬ 
tions. He alfo oblerved fome eclipfes, made new tables 
for computing them, a"d had juft finiftied a theory of the 
planets, when he died in 1460. He was fucceeded in his 
aftronomical and mathematical labours by his pupil, John 
Muller, commonly called Regiomontanus, from Monte- 
regio, or Koningfberg, a town of Franconia, where he 
was born. He completed the epitome of Ptolemy’s Alma¬ 
geft:, which Purbach had began ; and was afterwards in¬ 
vited to Rome, where he made many aftronomical obler¬ 
vations. Being returned to Nuremberg in 1471, by the 
encouragement of a wealthy citizen named Bernard Walt her, 
he made feveral inftruments for aftronomical obfervations, 
among which was an armillary aftrolabe, like that tiled at 
Alexandria by Hipparchus and Ptolemy, with which he 
made many obfervations. He made ephenterides for thir¬ 
ty years to come, (hewing the lunations, eclipfes, &c. and 
wrote the Theory of the Planets and Comets, and a trea¬ 
tife on triangles, (till in repute for feveral good theorems ; 
computing the tables of lines for every fmgle minute, to 
the radius 1000000, and introducing the ule of tangents 
alfo into trigonometry. After his death, which happened 
in 1476, Walther collected his papers, and continued the 
aftronomical obfervations till his own death alio. The ob¬ 
fervations of both were collected by order of the fenate 
of Nuremberg, and publiflied in 1544 by John Schooner: 
they were alfo afterwards publilhed in 1618 by Snellius, at 
the end of the obfervations made by the landgrave of 
Helfe ; and laftly with thofe of Tycho Brahe, in 1666. 
Walther was fucceeded as aftronomer at Nuremberg, 
by John Werner a clergyman. He obferved the motion of 
the cornet in 1500; and wrote feveral trafts on geometry 
and aftronomy, in a mafterly manner; the 1110ft remarkable 
of which are thofe concerning the motion of the eighth 
fphere, or of the fixed ftars ; in this traft, by comparing 
his own obfervations with thofe of Ptolemy, Alphonfus, 
and others, he ftiewed that the motion of the fixed ftars, 
fince called the precellion of the. equinoxes,,is i° jo' in 
too 
