PTOLEMY AND HIS GREAT BOOK
Ptolemy was an observer of the heavens, though not of the highest order;
but he had all the work of his predecessors, best of all Hipparchus, to
build upon. Ptolemy's greatest work was the "Megale Syntaxis," generally
known as the Almagest. It forms a nearly complete compendium of the
ancient astronomy, and although it embodies much error, because built on
a wrong theory, the Almagest nevertheless is competent to follow the
motions of all the bodies in the sky with a close approach to accuracy,
even at the present day. This marvelous work written at this critical
epoch became as authoritative as the philosophy of Aristotle, and for
many centuries it was the last word in the science. The old astrology
held full sway, and the Ptolemaic theory of the universe supplied
everything necessary: further progress, indeed, was deemed impossible.
The Almagest comprises in all thirteen books, the first two of which
deal with the simpler observations of the celestial sphere, its own
motion and the apparent motions of sun, moon, and planets upon it. He
discusses, too, the postulates of his system and exhibits great skill as
an original geometer and mathematician. In the third book he takes up
the length of the year, and in the fourth book similarly the moon and
the length of the month. Here his mathematical powers are at their best,
and he made a discovery of an inequality in the moon's motion known as
the evection. Book five describes the construction and use of the
astrolabe, a combination of graduated circles with which Ptolemy made
most of his observations. In the sixth book he follows mainly Hipparchus
in dealing with eclipses of sun and moon. In the seventh and eighth
books he discusses the motion of the equinox, and embodies a catalogue
of 1,028 stars, substantially as in Hipparchus. The five remaining books
of the Almagest deal with the planetary motions, and are the most
important of all of Ptolemy's original contributions to astronomy.
Ptolemy's fundamental doctrines were that the heavens are spherical in
form, all the heavenly motions being in circles. In his view, the earth
too is spherical, and it is located at the center of the universe, being
only a point, as it were, in comparison. All was founded on mere
appearance combined with the philosophical notion that the circle being
the only perfect curve, all motions of heavenly bodies must take place
in earth-centered circles. For fourteen or fifteen centuries this false
theory persisted, on the authority of Ptolemy and the Almagest,
rendering progress toward the development of the true theory impossible.
Ptolemy correctly argued that the earth itself is a sphere that is
curved from east to west, and from north to south as well, clinching his
argument, as we do to-day, by the visibility of objects at sea, the
lower portions of which are at first concealed from our view by the
curved surface of the water which intervenes. To Ptolemy also the earth
is at the center of the celestial sphere, and it has no motion of
translation from that point; but his argument fails to prove this. Truth
and error, indeed, are so deftly intermingled that one is led to wonder
why the keen intelligence of this great philosopher permitted him to
reject the simple doctrine of the earth's rotation on its axis. But if
we reflect that there was then no science of natural philosophy or
physics proper, and that the age was wholly undeveloped along the lines
of practical mechanics, we shall see why the astronomers of Ptolemy's
time and subsequent centuries were content to accept the doctrines of
the heavens as formulated by him.
When it came to explaining the movements of the "wandering stars," or
planets, as we term them, the Ptolemaic theory was very happy in so far
as accuracy was concerned, but very unhappy when it had to account for
the actual mechanics of the cosmos in space. Sun and moon were the only
bodies that went steadily onward, easterly: whereas all the others,
Mercury, Venus, Mars, Jupiter, Saturn, although they moved easterly most
of the time, nevertheless would at intervals slow down to stationary
points, where for a time they did not move at all, and then actually go
backward to the west, or retrograde, then become stationary again,
finally resuming their regular onward motion to the east.
To help out of this difficulty, the worst possible mechanical scheme was
invented, that known as the epicycle. Each of the five planets was
supposed to have a fictitious "double," which traveled eastward with
uniformity, attached to the end of a huge but mechanically impossible
bar. The earth-centered circle in which this traveled round was called
the "deferent." What this bar was made of, what stresses it would be
subjected to, or what its size would have to be in order to keep from
breaking--none of these questions seems to have agitated the ancient and
medieval astronomers, any more than the flat-earth astronomy of the
Hindu is troubled by the necessity of something to hold up the tortoise
that holds up the elephant that holds up the earth.
But at the end of this bar is jointed or swiveled another shorter bar,
to the revolving end of which is attached the actual planet itself; and
the second bar, by swinging once round the end of the primary advancing
bar, would account for the backward or retrograde motion of the planet
as seen in the sky. For every new irregularity that was found, in the
motion of Mars, for instance, a new and additional bar was
requisitioned, until interplanetary space was hopelessly filled with
revolving bars, each producing one of the epicycles, some large, some
small, that were needed to take up the vagaries of the several planets.
The Arabic astronomers who kept the science alive through the Middle
Ages added epicycle to epicycle, until there was every justification for
Milton's verses descriptive of the sphere:
With Centric and Eccentric scribbled o'er,
Cycle and Epicycle, Orb in Orb.