The smaller arcs represent the orbits of the superior planet, Mars, and the earth around the sun, S. At the point of opposition of Mars (when Mars and the sun are at opposite points in the heavens to an observer on the earth) we should see Mars projected on the zodiac at M{1}. After a month Mars will be at M' and the earth at E', so that in its apparent motion Mars will have retrograded to M{2}. After three months from opposition Mars will be at M" and the earth at E", making Mars appear at M{3} on the celestial sphere, its motion having changed from retrograde to direct. [Illustration: Fig. 5.] Both Figures 4 and 5 take no account of the fact that the earth's orbit and those of the planets are not in exactly the same planes. Remembering this fact we see at once that the apparent oscillations of the planets are not back and forth in a straight line but in curves and spirals. It is easy to see why the apparent motions of the planets were accounted for by deferents and epicycles, before the Copernican system revealed the true nature of the solar system as heliocentric and not geocentric. SELECTED BIBLIOGRAPHY BERRY, ARTHUR, _A Short History of Astronomy_. New York. 1899. BRYANT, W. W., _A History of Astronomy_. London. 1907. CUMONT, FRANZ, _Astrology and Religion among the Greeks and Romans_. New York. 1912. CUSHMAN, H. E., _A Beginner's History of Philosophy_. Boston. 1910. DREYER, J. L. E., _History of the Planetary Systems from Thales to Kepler_. Cambridge. 1906. EVERSHED, M. A., _Dante and the Early Astronomers_. London. 1913. GOMPERZ, T., _Greek Thinkers, A History of Ancient Philosophy_. New York. 1901. GORE J. ELLARD, _Astronomical Essays, Historical and Descriptive_. London. 1907. HINKS, A. R., _Astronomy_. London. 1911. JACOBY, HAROLD, _Astronomy_. New York. 1913. JASTROW, MORRIS, "Astrology," _Encyclopaedia Britannica_ ii, 795-800. LEA, H. C., _History of the Inquisition of the Middle Ages_. New York. 1906. III. 409-549. ORCHARD, T. N., _Milton's Astronomy_. New York. 1913. TAYLOR, H. O., _The Mediaeval Mind_. 2 vols. New York. 1911. TODD, MABEL L., _Steele's Popular Astronomy_. New York. 1884. TRAILL, H. D., _Social England_. New York and London. 1902. WALLACE, A. R., _Man's Place in the Universe_. London. 1903. WHITE, A. D., _Warfare of Science with Theology_. New York and London. 1909. I. 381. * * * * * CHAUCER, _The Complete Works of Geoffrey Chaucer_. W. W. Skeat, edit. Clarendon Press. 1894. CHAUCER, _Treatise on the Astrolabe_, A. E. Brae, edit. London. 1870. _Cambridge History of English Literature, The_, ed. by A. W. Ward and A. R. Waller. Vol. II. 1908. TEN BRINK, BERNARD, _History of English Literature_. Vol. II. New York. 1893. COURTHOPE, W. J., _Literary History of the English People_. Vol. I. New York. 1898. HADOW, GRACE E., _Chaucer and His Times_. New York. 1914. HAMMOND, ELEANOR P., _Chaucer: A Bibliographical Manual_. New York. 1908.