In the _Astrolabe_[112] Chaucer explains a method of determining whether a planet's motion is retrograde or direct.[113] The altitude of the planet and of any fixed star, is taken, and several nights later at the time when the fixed star has the same altitude as at the previous observation, the planet's altitude is again observed. If the planet is on the right or east side of the meridian, and its second altitude is less than its first, then the planet's motion is direct. If the planet is on the left or west side of the meridian, and has a smaller altitude at the second observation than at the first, then the planet's motion is retrograde. If the planet is on the east side of the meridional line when its altitude is taken and the second altitude is greater than the first, it is retrograde; and if it is on the west side and its second altitude is greater, it is direct. This method would be correct were it not that a change in the planet's declination or angular distance from the celestial equator might render the conclusions incorrect. Chaucer mentions the irregularity of planetary movements in _Boethius_ also when he says: "and whiche sterre in hevene useth wandering recourses, y-flit by dyverse speres."[114] The expression "y-flit by dyverse speres" may have reference only to the one motion of the planets, that is, their motion concentric to the star-sphere; or it may be used to include also their epicyclic motion. Skeat interprets the expression in the former way; but the context, it seems, would justify interpreting the words "dyverse speres" as meaning the various spheres of the planets to-gether with their epicycles; i. e., both deferents and epicycles. Of all the planets, that most often mentioned by Chaucer is Venus, partly, no doubt, because of her greater brilliance, but probably in the main because of her greater astrological importance; for few of Chaucer's references to Venus, or to any other planet, indeed, are without astrological significance. Chaucer refers to Venus, in the classical manner, as Hesperus when she appears as evening[115] star and as Lucifer when she is seen as the morning star: "and that the eve-sterre Hesperus, which that in the firste tyme of the night bringeth forth hir colde arysinges, cometh eft ayein hir used cours, and is pale _by the morwe_ at the rysing of the sonne, and is thanne cleped Lucifer."[116] Her appearance as morning star is again mentioned in the same work: "and after that Lucifer the day-sterre hath chased awey the derke night, the day the fairere ledeth the rosene hors _of the sonne_,"[117] and in _Troilus and Criseyde_ where it is said that "Lucifer, the dayes messager, Gan for to ryse, and out hir bemes throwe;"[118] Elsewhere in the same poem her appearance as evening star is mentioned but she is not this time called Hesperus: "The brighte Venus folwede and ay taughte The wey, ther brode Phebus doun alighte;"[119]