||Almost the same
thing may be said of Ptolemy, an even more celebrated writer, who was born
not very long after the death of Pliny. The exact dates of Ptolemy's life
are not known, but his recorded observations extend to the year 151 A.D.
He was a working astronomer, and he made at least one original discovery
of some significance - namely, the observation of a hitherto unrecorded
irregularity of the moon's motion, which came to be spoken of as the moon's
evection. This consists of periodical aberrations from the moon's regular
motion in its orbit, which, as we now know, are due to the gravitation
pull of the sun, but which remained unexplained until the time of Newton.
Ptolemy also made original observations as to the motions of the planets.
He is, therefore, entitled to a respectable place as an observing astronomer;
but his chief fame rests on his writings.
His great works have to do with geography
and astronomy. In the former field he makes an advance upon Strabo, citing
the latitude of no fewer than five thousand places. In the field of astronomy,
his great service was to have made known to the world the labors of Hipparchus.
Ptolemy has been accused of taking the star-chart of his 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 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 , 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
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, "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 courage enough to put them to the test.