I. Most of the terms at present used to describe the movements of the heavenly bodies were used in Chaucer's time and occur very frequently in his writings. The significance of Chaucer's references will then be perfectly clear, if we keep in mind that the modern astronomer's description of the _apparent_ movements of the star-sphere and of the heavenly bodies individually would have been to Chaucer a description of _real_ movements. When we look up into the sky on a clear night the stars and planets appear to be a host of bright dots on the concave surface, unimaginably distant, of a vast hollow sphere at the canter of which we seem to be. Astronomers call this expanse of the heavens with its myriad bright stars the _celestial sphere_ or the _star sphere_, and have imagined upon its surface various systems of circles. In descriptions of the earth's relation to the celestial sphere it is customary to disregard altogether the earth's diameter which is comparatively infinitesimal. If we stand on a high spot in the open country and look about us in all directions the earth seems to meet the sky in a circle which we call the _terrestrial horizon_. Now if we imagine a plane passing through the center of the earth and parallel to the plane in which the terrestrial horizon lies, and if we imagine this plane through the earth's center extended outward in all directions to an infinite distance, it would cut the celestial sphere in a great circle which astronomers call the _celestial horizon_. On the celestial horizon are the north, east, south and west points. The plane of the celestial horizon is, of course, different for different positions of the observer on the earth.