Encyclopaedia Britannica > Volume 3, Anatomy-Astronomy

790 ASTRONOMY.
History, zeal and encouraged their inquiries. This noble institu-
tion, which survived all the vicissitudes of nine centuries,
was the means of conferring incalculable benefits on the
human race ; and the name of its founder, Ptolemy Phi-
ladelphus, will be gratefully remembered while science
and learning occupy a place in the estimation of man¬
kind.
Aristillus The first astronomers of the Alexandrian school were
and Timo- Aristillus and Timocharis, who flourished under the first
charis, 300pto]einy? about 300 years before Christ. The chief ob-
C‘ ject of their labours was the determination of the rela¬
tive positions of the principal stars of the zodiac, instead
of merely announcing their risings and settings, as had
been the practice of the orientals and the ancient Greeks.
The observations of these two astronomers conducted
Hipparchus to the important discovery of the precession
of the equinoxes, and served as the basis of the theory
which Ptolemy, some centuries afterwards, gave of that
phenomenon.
Aristar- Aristarchus of Samos, the next in order of the Alexan-
chus, B. C.drian astronomers, composed a treatise on the Magnitudes
28b and Distances of the sun and moon, which has been pre¬
served to our times. In this treatise he describes an
ingenious method which he employed to obtain the re¬
lative distances of the two luminaries. At the instant
when the moon is dichotomized, that is, when the ex¬
act half of her disk appears to a spectator on the earth
to be illuminated by the sun’s light, the visual ray pass¬
ing from the centre of the moon to the eye of the ob¬
server is perpendicular to the line which joins the cen¬
tre of the moon and sun. At that instant, therefore, he
measured the angular distance of the two bodies, and
finding it to be 87 degrees, he concluded, by the resolu¬
tion of a right-angled triangle, that the distance of the
sun is between eighteen and nineteen times greater than
that of the moon. This method is perfectly correct in
theory, but it is difficult to be assured of the exact instant
of the moon’s dichotomy, and in an angle of such mag¬
nitude a very small error greatly affects the result. The
error of Aristarchus is very considerable, the true angle
being about 87° 50'. The estimated distance of the sun
is by consequence far too small; yet the determination,
faulty as it was, contributed to expand greatly the exist¬
ing notions relative to the boundaries of the universe, for
the Pythagoreans had taught that the sun is only three,
or at most three and a half times more distant than the
moon. Another delicate observation made by Aristarchus
was that of the magnitude of the sun’s diameter, which,
as we learn from Archimedes, he determined to be the
720th part of the circumference of the circle which the
sun describes in his diurnal revolution. This estimate is
not very far from the truth, and the observation is by no
means an easy one. He embraced the doctrine of Py¬
thagoras respecting the earth’s motion, and appears to
have entertained juster notions than any of the astrono¬
mers who preceded him, on the magnitude and extent of
the universe. The treatise on the Magnitudes and Dis¬
tances is published in the third volume of the works of
Dr Wallis, with a Latin translation by Commandine, and
some notes.
Eratos- Eratosthenes, the successor of Aristarchus, was a na-
tlienes, tive of Gyrene, and invited to Alexandria by Ptolemy
born 276 Evergetes, who appointed him keeper of the royal libra-
B- C. ry# He is supposed to have been the inventor of armil¬
lary spheres, a species of instrument extensively used by
the ancient astronomers. By means of an instrument of
this kind he observed the distance between the tropics to
be to the whole circumference of a great circle as 11 to 83;
a ratio equivalent to 47° 42' 39", half of which gives 23°
51' 19*5" for the obliquity of the ecliptic. This is a very History,
important observation, and confirms the gradual diminu-
tion of the obliquity as indicated by theory. Eratos¬
thenes is celebrated for being the first who attempted, on
correct principles, to determine the magnitude of the
earth. Having remarked, by some means with which we
are unacquainted, that Syene, the most southern of the
cities of ancient Egypt, is situated nearly on the same
meridian with Alexandria, he conceived the idea of de¬
termining the amplitude of the celestial arch intercepted
between the zeniths of the two places, and of measuring
at the same time their distance on the ground; operations
which would afford data for the determination of the
whole length of the terrestrial meridian. Syene was
known to be situated exactly under the tropic; for at the
summer solstice the gnomon had no shadow, and the sun's
rays illumined the bottom of a deep well in that city.
On the day of the solstice he found the meridional dis¬
tance of the sun from the zenith of Alexandria to be 7°
12', or a fiftieth part of the circumference. It had also
been ascertained by the bematists or surveyors of Alex¬
ander and the Ptolemies, that the itinerary distance be¬
tween Alexandria and Syene was 5000 stadia; therefore
5000 x 50 = 250,000 stadia form the circumference of
a great circle of the earth, or the length of the ter¬
restrial meridian. Unfortunately, on account of the un¬
certainty respecting the length of the stadium here em¬
ployed, we possess no means of estimating the degree of
approximation afforded by this rude though ingenious
attempt; but the idea does immortal honour to Eratos¬
thenes, and the moderns have added nothing to his
method: their better success is owing solely to the pro¬
gress of the arts and the perfection of astronomical in¬
struments.
About this time the science of astronomy was enriched
by the discoveries of some of the distinguished geometri¬
cians whose labours have so greatly extended the glory
of the Alexandrian school. Euclid, the celebrated author Euclid,
of the Elements, lived in the reign of the first Ptolemy.
He composed a book on the sphere, which probably served
as a model for future works of the same kind, and was the
first who treated in a geometrical manner of the pheno¬
mena of the different inclinations of the sphere. Conon Conon.
of Samos, the friend of Archimedes, collected the records
of eclipses, which had been observed by the ancient
Egyptians ; and Callimachus ascribes to him the constella¬
tion of Berenice’s hair. Archimedes, whose profound Archi-
genius and deep knowledge of geometry and mechanics medes.
entitle him to the appellation of the Newton of the an¬
cients, also claims a high rank among the cultivators of
astronomy. His celebrated planetarium, which repre¬
sented the motions of the sun, moon, planets, and starry
sphere, has been a frequent theme of the admiration and
praises of the poets :
Jura poll, rerumque fidem, legesque deorum,
Ecce Syracusius transtulit arte senex.
Apollonius of Perga solved the important problem of the Apollo,
stations and retrogradations of the planets by means ofmus.
epicyles and deferents; and he is entitled to the glory of
having formed the alliance between the two sciences of
geometry and astronomy, which has been productive of
the greatest advantages to both.
Astronomy, which had as yet only consisted of a know- Hippar.
ledge of isolated facts, acquired a systematic form, anddius, ^
almost a new existence, from the genius and assiduity of * '
Hipparchus, one of the most astonishing men of antiquity,
and perhaps the greatest of all in the sciences which are
not purely speculative. This illustrious founder of astro¬
nomical science was born at Nice in Bythinia, and ob-