led astronomy to be
regarded as a true theoretical science. The positions of the fixed stars
and the paths of the planets were determined with greater accuracy, and
irregularities of the motions of the Sun and Moon were investigated with
greater precision. Attempts were made to ascertain the distance of the
Sun from the Earth, and also the dimensions of the terrestrial sphere.
The obliquity of the ecliptic was accurately determined, and an arc of
the meridian was measured between Syene and Alexandria. The names of
Aristarchus, Eratosthenes, Aristyllus, Timocharis, and Autolycus, are
familiarly known in association with the advancement of the astronomy of
this period.

We now reach the name of HIPPARCHUS of Bithynia (140 B.C.), the most
illustrious astronomer of antiquity, who did much to raise astronomy to
the position of a true science, and who has also left behind him ample
evidence of his genius 'as a mathematician, an observer, and a
theorist.' We are indebted to him for the earliest star catalogue, in
which he included 1,081 stars. He discovered the Precession of the
Equinoxes, and determined the motions of the Sun and Moon, and also the
length of the year, with greater precision than any of his predecessors.
He invented the sciences of plane and spherical trigonometry, and was
the first to use right ascensions and declinations.

The next astronomer of eminence after Hipparchus was PTOLEMY (130 A.D.),
who resided at Alexandria. He was skilled as a mathematician and
geographer, and also excelled as a musician. His chief discovery was an
irregularity of the lunar motion, called the '_evection_.' He was also
the first to observe the effect of the refraction of light in causing
the apparent displacement of a heavenly body from its true position.
Ptolemy devoted much of his time to extending and improving the theories
of Hipparchus, and compiled a great treatise, called the 'Almagest,'
which contains nearly all the knowledge we possess of ancient astronomy.
Ptolemy's name is, however, most widely known in association with what
is called the Ptolemaic theory. This system, which originated long
before his time, but of which he was one of the ablest expounders, was
an attempt to establish on a scientific basis the conclusions and
results arrived at by early astronomers who studied and observed the
motions of the heavenly bodies. Ptolemy regarded the Earth as the
immovable centre of the universe, round which the Sun, Moon,