doubt whether be intended any impropriety, inasmuch as be 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
"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
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
spoken of as the theory of Ptolemy. We have sufficiently detailed
the theory in speaking of Hipparchus. It should be explained,
however, that, with both Hipparchus and Ptolemy, the theory of
epicycles would appear to have been held rather as a working
hypothesis than as a certainty, so far as the actuality of the
minor spheres or epicycles is concerned. That is to say, these
astronomers probably did not conceive either the epicycles or the
greater spheres as constituting actual solid substances.
Subsequent generations, however, put this interpretation upon the
theory, conceiving the various spheres as actual crystalline
bodies. It is difficult to imagine just how the various epicycles
were supposed to revolve without interfering with the major
spheres, but perhaps this is no greater difficulty than is
presented by the alleged properties of the ether, which
physicists of to-day accept as at least a working hypothesis. We
shall see later on how firmly the conception of concentric
crystalline spheres was held to, and that no real challenge was
ever given that theory until the discovery was made that comets
have an orbit that must necessarily intersect the spheres of the
various planets.
Ptolemy's system of geography in eight books, founded on that of
Marinus of Tyre, was scarcely less celebrated throughout the
Middle Ages than the Almagest. It contained little, however, that
need concern us here, being rather an elaboration of the
doctrines to which we have already sufficiently referred. None of
Ptolemy's original manuscripts has come down to us, but there is
an alleged fifth-century manuscript attributed to Agathadamon of
Alexandria which has peculiar interest because it contains a
series of twenty-seven elaborately colored maps that are supposed
to be derived from maps drawn up by Ptolemy himself. In these
maps the sea is colored green, the mountains red or dark yellow,
and the land white. Ptolemy assumed that a degree at the equator
was 500 stadia instead of 604 stadia in length. We are not
informed as to the grounds on which this assumption was made, but
it has been suggested that the error was at least partially
instrumental in leading to one very curious result. "Taking the
parallel of Rhodes," says Donaldson,[5] "he calculated the
longitudes from the Fortunate Islands to Cattigara or the west
coast of Borneo at 180 degrees, conceiving this to be one-half
the circumference of the globe. The real distance is only 125
degrees or 127 degrees, so that his measurement is wrong by one
third of the whole, one-sixth for the error in the measurement of
a degree and one-sixth for the errors in measuring the distance
geometrically. These errors, owing to the authority attributed to
the geography of Ptolemy in the Middle Ages, produced a
consequence of the greatest importance. They really led to the
discovery of America. For the design of Columbus to sail from the
west of Europe to the east of Asia was founded on the supposition
that the distance was less by one third than it really was." This
view is perhaps a trifle fanciful, since there is nothing to
suggest that the courage of Columbus would have balked at the
greater distance, and since the protests of the sailors, which
nearly thwarted his efforts, were made long before the distance
as estimated by Ptolemy had been covered; nevertheless it is
interesting to recall that the great geographical doctrines, upon
which Columbus must chiefly have based his arguments, had been
before the world in an authoritative form practically unheeded
for more than twelve hundred years, awaiting a champion with
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