The elements of astronomy; a textbook . that the planet can-not exceed, depending upon the diameter of its orbit. 290. Motion of a Planet in Eight Ascension and Longitude. — Starting from the line of superior conjunction, the planet whethersuperior or inferior, moves eastward or direct for a time, but at a §290] MOTION OF THE PLANETS. 205 rate continually slackening until the planet becomes stationary;then it reverses its course and moves westward, the middle of the arcof retrogression nearly coinciding with the point where it comes toopposition or inferior conjunction. After a time it becomes
The elements of astronomy; a textbook . that the planet can-not exceed, depending upon the diameter of its orbit. 290. Motion of a Planet in Eight Ascension and Longitude. — Starting from the line of superior conjunction, the planet whethersuperior or inferior, moves eastward or direct for a time, but at a §290] MOTION OF THE PLANETS. 205 rate continually slackening until the planet becomes stationary;then it reverses its course and moves westward, the middle of the arcof retrogression nearly coinciding with the point where it comes toopposition or inferior conjunction. After a time it becomes oncemore stationary, and then resumes its eastward motion until it againarrives at superior conjunction, having completed a synodic time, as well as in degrees, the advance always exceeds theretrogression. As observed with a sidereal clock, all the planets come later to themeridian each night when moving direct, since their right ascension isthen increasing ; vice versa, of course, when they are retrograding. Conjunction. OppositionFig. 75. — Planetary Configurations and Aspects. 291. Motion of the Planets with respect to the Suns Place inthe Sky; Change of Elongation. — The visibility of a planetdepends mainly on its elongation (, its angular distancefrom the sun), because when near the sun, the planet will beabove the horizon only by day. As regards their motion, con- 206 MOTION OF THE PLANETS. [§ 291 sidered from this point of view, there is a marked differencebetween the inferior planets and the superior. I. The superior planets drop always steadily westward withrespect to the suns place in the heavens, continually increasingtheir western elongation or decreasing their eastern. As ob-served by an ordinary time-piece (keeping solar time), theytherefore invariably come earlier to the meridian every succes-sive night, never moving eastward among the stars as rapidlyas the sun does. Beginning at conjunction, the planet is then behind the sun, at itsgreate
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