4 zz ASTRO 
nothing. Alfo the fine of the latitude of the ftar is to ra¬ 
dius, as the cotangent of A the angle of polition, to the 
tangent of C. 
Hence Dr. Bradley finally deduced the following con- 
clufions. i. That the light of all the fixed ftars arrives 
at the Earth with equal velocities ; for the major axis ot 
the ellipfe is the fame in all the ftars, that is 40" accord¬ 
ing to his laft determination. 2. That unlefs their di(- 
tances from us are all equal, which is very improbable, 
their lights are propagated uniformly to all diftances from 
them. 3. That light moves from the Sun to the Earth 
in 8' 7-5", and its velocity is to the velocity of the Earth 
in its orbit as 1031410.1. 4. That the time thus deter¬ 
mined can fcarcely err from the truth by above 5 or 10" at 
molt, which is fuch a degree of exactnefs as can never be 
expected from the eclipfes of Jupiter’s fatellites. 5. That, 
as this velocity of ftar-light comes out about a mean of 
the feveral velocities found from the eclipfes of Jupiter’s 
fatellites, we may reafonably conclude that the velocities 
of thefe reflected lights are equal to the velocity of diredt 
light. 6. And, as it is highly probable that the velocity 
of the.Sun’s emitted light is equal to that of ftar-light, it 
follows that its velocity is not altered by reflection into the 
fame medium. 
The aberration of the planets is greateft in longitude, 
and very fmall in latitude, becaufe they deviate very lit¬ 
tle from the plane of the ecliptic, or path of the Earth ; 
fio that the aberration in the latitudes of the planets is 
commonly neglected, as infenfible; the greateft in Mer¬ 
cury being only 45", and much lefs in the other planets. 
As to the aberrations in declination and right afcenfion, 
they nuift depend on the fituation of the planet in the zo¬ 
diac. The aberration in longitude, being equal to the 
geocentric motion, will be more or lefs according as that 
motion is ; it will therefore be leaft, or nothing at all, 
when the planet is ftationary ; and greateft in the luperior 
planets Mars, Jupiter, Saturn, &c. when they are in 
oppofition to the Sun ; but in the inferior planets Venus 
and Mercury, the aberration is greateft at the time of their 
fuperior conjunction. Thefe maxima of aberration for 
the feveral planets, when their diftance from the Sun is 
leaft, are as follow : viz. for 
Sa’urn - - 27-0" 
Jupiter - - - 29-8 
Mars - - 37-8 
Venus - - - 43'2 
Mercury - 59-0 
The Moon - § 
And between thefe numbers and nothing the aberrations 
of the planets, in longitude, vary according to their fitu- 
ations. But that of the Sun varies not, being conftantly 
20". And this may alter his declination by a quantity, 
which varies from o to near 8" ; being greateft or 8" about 
the equinoxes, and vanifhing in the folftices. See the ar¬ 
ticles Aberration and Ottics. 
Of PRACTICAL AST RO NO MY. 
This title comprehends all kinds of aftronomical calcu¬ 
lations ; the admeafurement of inacceffible objects; the 
mode of afcertaining the periodical times, eccentricities, 
pofitions, ■ motions, and other properties, of the celeftial 
bodies, by means of tables, and aftronomical inftruments, 
whereby the theoretical part is elucidated and explained, 
and adapted to the ufes of the practical. The immenfe 
labours of former aftrononters in this department does 
honour to the age and country in which they lived, and 
thews that in lefs enlightened times the moft laborious af¬ 
tronomical tables were calculated, and the utmoft exer¬ 
tions made to bring the fcience of aftronomy into general 
and deferved repute. Thales, the Milefian, is juftly ce¬ 
lebrated for his early prowefs in practical aftronomy. He 
was the firft among the Greeks who obferved the ftars, the 
folftices, the eclipfes of the Sun and Moon, which lie cal¬ 
culated and foretold. And the fame was farther calcu« 
N O M Y. 
lated and extended by his fucceftors Anaximander, Anaxt. 
manes, and Anaxagoras ; but moft materially by Pytha¬ 
goras, who brought from Egypt the learning of that people, 
and taught the fame in Greece and Italy. 
Philolaus, a Pythagorean, who flourilhed about 430 years 
before Chrift, aflerted the annual motion of the Earth 
about the Sun ; and, not long after, the diurnal motion of 
the Earth on her own axis was taught by Hicetas, a Syra- 
citfan. About the fame time flourifhed at Athens, Meton 
and Eucftemon, where they obferved the lummer folfttice 
432 years before Chrift, and obferved the rifings and let¬ 
tings of the ftars, and what feafons they correfponded with* 
Meton alfo invented the cycle of nineteen years, which 
ftill bears his name. Eudoxus the Cnidian, who lived about 
370 years before Chrift, was accounted one of the moft (kil- 
iul practical aftronomers and geometricians of antiquity, 
being accounted the inventor of many of the propofitions 
in Euclid’s Elements, and having introduced geometry 
into the fcience of aftronomy. He travelled into Alia, 
Africa, Sicily, and Italy, for improvements in aftronomy; 
and we are informed by Pliny, that he determined the an¬ 
nual year to contain 365 days 6 hours, that he determined 
alfo the periodical times of the planets, and made other 
important obfervations and difcoveries. 
Calippus flourifhed foon after Eudoxus, and his celeftial 
fphere is mentioned by Ariftotle ; but he is better known 
by a period of feventy-fix which he invented, containing- 
four correct Metonic periods, which commenced at the 
fuimner folftice in the year 330 before Chrift. About his 
time the knowledge of the Pythagorean lyftem was carried 
into Italy, Gaul, and Egypt, by certain colonies of Greeks. 
Vitruvius, however, maintains that Berofus, a Babylo¬ 
nian, opened an aftronomical fc.hool in the ifle of Cos, by 
which the fcience was introduced into Greece. And Pliny 
fays, that in confideration of his wonderful predictions, 
the Athenians ereCted him a ftatue in the gymnaftum, with 
a gilded tongue. But if this Berofus be the lame with the 
author of the Chaldaic hiftories, he mull have lived before 
Alexander. 
After the death of this great conqueror, the fcience flou¬ 
rifhed chiefly in Egypt, under the aufpices of Ptolemy 
Philadelphus and his fucceffors. He founded a fchool, 
which continued to be the grand feminary of learning, till 
the invafion of the Saracens in the year of Chrift 650. 
From the founding of that fchool, practical aftronomy ad¬ 
vanced confiderably. Ariftarchus, about 270 years before 
Chrift, gave a method of determining the Sun’s diftance by 
the dichotomy of the Moon. Eratofthenes, who was born at 
Cyrene in the year 271 before Chrift, meafured the circum¬ 
ference of the Earth by means of a gnomon ; and being invit¬ 
ed to Alexandria, from Athens, by Ptolemy Euergetes, and 
made keeper of the royal library there, he fet up for that 
prince thofe armillary fpheres, which Hipparchus and 
Ptolemy the aftrologer afterwards employed fo fuccefsfully 
in obferving the heavens. He alfo determined the dif¬ 
tance between the tropics to be of the whole meridian 
circle, which makes the obliquity of the ecliptic in his 
time to be 23 0 51^'. The celebrated Archimedes, too, 
cultivated practical aftronomy, as well as geometry and 
mechanics: he determined the diftances of the planets 
from one another, and conftrudted a kind of planetarium 
or orrery, to reprelent the phenomena and motions of the 
heavenly bodies. 
To pafs by feveral others of the ancients, who prattifed 
aftronomy, we find that Hipparchus, who flourilhed about 
140 years before Chrift, was the firft who applied himfelf 
to the ftudy of every part of aftronomy, and, as we are 
informed by Ptolemy, made great improvements in the 
practice of it: he difcovered that the orbits of the planets 
are eccentric, that the Moon moved (lower in the apogee 
than in her perigee, and, that there was a motion of anti¬ 
cipation of the Moon’s nodes : he conftrinfted tables of 
the motions of the Sun and Moon, colledted accounts of 
fuch eclipfes, &c. as had been made by the Egyptians and 
Chaldeans, and calculated all that were to happen for 60o 
years 
