ORIGIN OF GREEK ASTRONOMY
While the Greeks laid the foundations of modern scientific astronomy,
they were not as a whole observers: rather philosophers, we should say.
The later representatives of the Greek School, however, saw the
necessity of observation as a basis of true induction; and they
discovered that real progress was not possible unless their speculative
ideas were sufficiently developed and made definite by the aid of
geometry, so that they became capable of detailed comparison with
observation. This was the necessary and ultimate test with them, and the
same is true to-day. The early Greek philosophers were, however, mainly
interested, not in observations, but in guessing the causes of
phenomena.
Thales of Miletus, founder of the Ionian School, introduced the system
of Egyptian astronomy into Greece, about the end of the seventh century
B. C. He is universally known as the first astronomer who ever predicted
a total eclipse of the sun that happened when he said it would: the
eclipse of B. C. 585. This he did by means of the Chaldean eclipse cycle
of 18 years known as the Saros.
Aristarchus of Samos was the first and most eminent of the Alexandrian
astronomers, and his treatise "On the Magnitudes and Distances of the
Sun and Moon" is still extant. This method of ascertaining how many
times farther the sun is than the moon is very simple, and
geometrically exact. Unfortunately it is impossible, even to-day, to
observe with accuracy the precise time when the moon "quarters," (an
observation essential to his method), because the moon's terminal, or
line between day and night, is not a straight line as required by
theory, but a jagged one. By his observation, the sun was only twenty
times farther away than the moon, a distance which we know to be nearly
twenty times too small.
His views regarding other astronomical questions were right, although
they found little favor among contemporaries. Not only was the earth
spherical, he said, but it rotated on its axis and also traveled round
the sun. Aristarchus was, indeed, the true originator of the modern
doctrine of motions in the solar system, and not Copernicus, seventeen
centuries later; but Seleucus appears to have been his only follower in
these very advanced conceptions. Aristarchus made out the apparent
diameters of sun and moon as practically equal to one another, and
inferred correctly that their real diameters are in proportion to their
distances from the earth. Also he estimated, from observations during an
eclipse of the moon, that the moon's diameter is about one-third that of
the earth. Aristarchus appears to have been one of the clearest and most
accurate thinkers among the ancient astronomers; even his views
concerning the distances of the stars were in accord with the fact that
they are immeasurably distant as compared with the distances of the sun,
moon, and planets.
Practically contemporary with Aristarchus were Timocharis and
Aristillus, who were excellent observers, and left records of position
of sun and planets which were exceedingly useful to their successors,
Hipparchus and Ptolemy in particular. Indeed their observations of star
positions were such that, in a way, they deserve the fame of having made
the first catalogue, rather than Hipparchus, to whom is universally
accorded that honor.
Spherical astronomy had its origin with the Alexandrian school, many
famous geometers, and in particular Euclid, pointing the way. Spherics,
or the doctrine of the sphere, was the subject of numerous treatises,
and the foundations were securely laid for that department of
astronomical research which was absolutely essential to farther advance.
The artisans of that day began to build rude mechanical adaptations of
the geometric conceptions as concrete constructions in wood and metal,
and it became the epoch of the origin of astrolabes and armillary
spheres.