(456.) The sun and moon are not the only celestial objects which appear to have a motion independent of that by which the great constellation of the heavens is daily carried round the earth. Among the stars there are several,—and those among the brightest and most conspicuous,—which, when attentively watched from night to night, are found to change their relative situations among the rest; some rapidly, others much more slowly. These are called planets. Four of them—Venus, Mars, Jupiter, and Saturn—are remarkably large and brilliant; another, Mercury, is also visible to the naked eye as a large star, but, for a reason which will presently appear, is seldom conspicuous; a sixth, Uranus, is barely discernible without a telescope; and nine others—Neptune, Ceres, Pallas, Vesta, Juno, Astræa, Hebe, Iris, Flora—are never visible to the naked eye. Besides these fifteen, others yet undiscovered may exist; and it is extremely probable that such is the case,—the multitude of telescopic stars being so great that only a small fraction of their number has been sufficiently noticed to ascertain whether they retain the same places or not, and the ten last-mentioned planets having all been discovered within little more than half a century from the present time.