great predecessor
without due credit, and indeed it seems difficult to clear him of
this charge. Yet it is at least open to doubt whether he intended any
impropriety, inasmuch as he all along is sedulous in his references to
his predecessor. Indeed, his work might almost be called an exposition
of the astronomical doctrines of Hipparchus. No one pretends that
Ptolemy is to be compared with the Rhodesian observer as an original
investigator, but as a popular expounder his superiority is evidenced
in the fact that the writings of Ptolemy became practically the sole
astronomical text-book of the Middle Ages both in the East and in the
West, while the writings of Hipparchus were allowed to perish.

The most noted of all the writings of Ptolemy is the work which became
famous under the Arabic name of Almagest. This word is curiously
derived from the Greek title (gr h megisth suntazis), "the greatest
construction," a name given the book to distinguish it from a work on
astrology in four books by the same author. For convenience of reference
it came to be spoken of merely as (gr h megisth), from which the Arabs
form the title Tabair al Magisthi, under which title the book was
published in the year 827. From this it derived the word Almagest,
by which Ptolemy's work continued to be known among the Arabs, and
subsequently among Europeans when the book again became known in the
West. Ptolemy's book, as has been said, is virtually an elaboration
of the doctrines of Hipparchus. It assumes that the earth is the fixed
centre of the solar system, and that the stars and planets revolve about
it in twenty-four hours, the earth being, of course, spherical. It was
not to be expected that Ptolemy should have adopted the heliocentric
idea of Aristarchus. Yet it is much to be regretted that he failed to do
so, since the deference which was accorded his authority throughout the
Middle Ages would doubtless have been extended in some measure at
least to this theory as well, had he championed it. Contrariwise, his
unqualified acceptance of the geocentric doctrine sufficed to place that
doctrine beyond the range of challenge.

The Almagest treats of all manner of astronomical problems, but the
feature of it which gained it widest celebrity was perhaps that which
has to do with eccentrics and epicycles. This theory was, of course, but
an elaboration of the ideas of Hipparchus; but, owing to the celebrity
of the expositor, it has come to be spoke