All the planets that are easily visible to the unaided eye were known in Chaucer's time and are mentioned in his writings, some of them many times. These planets are Mercury, Venus, Mars, Jupiter, and Saturn. According to the Ptolemaic system, which as we have seen, held sway in the world of learning during Chaucer's century, the sun and moon were also held to be planets, and all were supposed to revolve around the earth in concentric rings, the moon being nearest the earth, and the sun between Venus and Mars. The circular orbit of each planet was called its "deferent" and upon the deferent moved, not the planet itself, but an imaginary planet, represented by a point. The real planet moved upon a smaller circle called the "epicycle" whose center was the moving point representing the imaginary planet. The deferent of each planet was supposed to be traced as a great circle upon a transparent separate crystal sphere; and all of the crystal spheres revolved once a day around an axis passing through the poles of the heavens. As the sun and moon did not show the same irregularities[108] of motion as the planets, Ptolemy supposed these two bodies to have deferents but no epicycles. Later investigators complicated the system by adding further secondary imaginary planets, revolving in Ptolemy's epicycles and with the actual planets attached to additional corresponding epicycles. They even supposed the moon to have one, perhaps two epicycles and we shall find this notion reflected in Chaucer. The eighth sphere had neither deferent nor epicycle but to it were attached the fixed stars. This sphere as we have seen earlier, revolved slowly from west to east to account for the precession of the equinoxes, while a ninth sphere, the _primum mobile_, imparted to all the inner spheres their diurnal motion from east to west. Chaucer's poetical references to the planets, as we have found to be true in the case of the sun and moon, do not give us satisfactory evidence of the extent of his knowledge, but occasional passages from his prose works again throw light on these allusions. Chaucer refers to the planets in general as 'the seven stars,' as, for instance, in the lines: "And with hir heed she touched hevene, Ther as shynen sterres sevene."[109] and "To have mo floures, swiche seven As in the welken sterres be."[110] Chaucer was undoubtedly familiar with the irregularities of the planetary movements, and with the theory of epicycles by which these irregular movements were in his day explained, although it is not from his poetry that we can learn the fact. He uses the word 'epicycle' only once in all his works. In the _Astrolabe_ when comparing the moon's motion with that of the other planets, he says: "for sothly, the mone moeveth the contrarie from othere planetes as in hir episicle, but in non other manere."[111]