PTOLEMY, THE LAST GREAT ASTRONOMER OF ANTIQUITY
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 η μεγιστη
συνταξις,
"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 η μεγι
στη, 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
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,
[73] "he calculated the
longitudes from the Fortunate Islands to Cattigara or the west coast of
Borneo at 180°, conceiving this to be one-half the
circumference of the globe. The real distance is only 125°
or 127°, 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.