earth seemed stable under his feet and that each morning the sun swept from the east to set at night in the west. Sometimes it rose more to the north or to the south than at other times. How could that be explained if the sun were stationary?
Study of the stars was valuable for navigators and for surveyors, perhaps, but such disturbing theories should not be propounded by philosophers. Cleanthes, according to Plutarch, "advised that the Greeks ought to have prosecuted Aristarchus the Samian for blasphemy against religion, as shaking the very foundations of the world, because this man endeavoring to save appearances, supposed that the heavens remained immovable and that the earth moved through an oblique circle, at the same time turning about its own axis." Few would care to face their fellows as blasphemers and impious thinkers on behalf of an unsupported theory. Eighteen hundred years later Galileo would not do so, even though in his day the theory was by no means unsupported by observation.
Furthermore, one of the weaknesses of the Greek civilization militated strongly against the acceptance of this hypothesis so contrary to the evidence of the senses. Experimentation and the development of applied science was practically an impossibility where the existence of slaves made manual labor degrading and shameful. Men might reason indefinitely; but few, if any, were willing to try to improve the instruments of observation or to test their observations by experiments.
At the same time another astronomical theory was developing which was an adequate explanation for the phenomena observed up to that time. This theory of epicycles and eccentrics worked out by Apollonius of Perga (c. 225 B.C.) and by Hipparchus (c. 160 B.C.) and crystallized for posterity in Ptolemy's great treatise on astronomy, the Almagest, (c. 140 A.D.) became the fundamental principle of the science until within the last three hundred years. The theory of the eccentric was based on the idea that heavenly bodies Following circular orbits revolved around a center that did not coincide with that of the observer on the earth. That would explain why the sun appeared sometimes nearer the earth and sometimes farther away. The epicycle represented the heavenly body as moving along the circumference of one circle (called the epicycle) the center of which moves on another circle (the deferent). With better observations additional epicycles and eccentric were used to represent the newly observed phenomena till in the later Middle Ages the universe became a
With Centric and Eccentric scribbled o'er,
Cycle and Epicycle, Orb in Orb"—
Yet the heliocentric theory was not forgotten. Vitruvius, a famous Roman architect of the Augustan Age, discussing the system of the universe, declared that Mercury and Venus, the planets nearest the sun, moved around it as their center, though the earth was the center of the universe. This same notion recurs in Martianus Capella's book in the fifth century A.D. and again, somewhat modified, in the 16th century in Tycho Brahe's conception of the universe.
Ptolemy devotes a column or two of his Almagest (to use the familiar Arabic name for his Syntaxis Mathematica) to the refutation of the heliocentric theory, thereby preserving it for later ages to ponder on and for a Copernicus to develop. He admits at the outset that such a theory is only tenable for the stars and their phenomena, and he gives at least three reasons why it is ridiculous. If the earth were not at the center, the observed facts of the seasons and of day and night would be disturbed and even upset. If the earth moves, its vastly greater mass would gain in speed upon other bodies, and soon animals and other lighter bodies would be left behind unsupported in the air—a notion "ridiculous to the last degree," as he comments, "even to imagine it." Lastly, if it moves, it would have such tremendous velocity that stones or arrows shot straight up in the air must fall to the ground east of their starting point,—a "laughable supposition" indeed to Ptolemy.
This book became the great text of the Middle Ages; its author's name was given to the geocentric theory it maintained. Astronomy for a thousand years was valuable only to determine the time of Easter and other festivals of the Church, and to serve as a basis for astrology for the mystery-loving people of Europe.
To the Arabians in Syria and in Spain belongs the credit of preserving for Europe during this long period the astronomical works of the Greeks, to which they added their own valuable observations of the heavens—valuable because made with greater skill and better instruments, and because with these observations later scientists could illustrate the permanence or the variability of important elements. They also discovered the so-called "trepidation" or apparent shifting of the fixed stars to explain which they added another sphere to Ptolemy's eight. Early in the sixth century Uranus translated Aristotle's works into Syrian, and this later was translated into Arabic. Albategnius (c. 850-829 A.D.), the Arabian prince who was the greatest of all their astronomers, made his observations from Aracte and Damascus, checking up and in some cases amending Ptolemy's results.
Then the center of astronomical development shifted from Syria to Spain and mainly through this channel passed on into Western Europe. The scientific fame of Alphonse X of Castile (1252-1284 A.D.) called the Wise, rests chiefly upon his encouragement of astronomy. With his support the Alfonsine Tables were calculated. He is