It required no very long course of observation to acquaint men with a truth, which at first sight none would have suspected--namely, that the luminous points whereof the sky was full were of two kinds, some always maintaining the same position relatively to one another, while others were constantly changing their places, and as it were wandering about the sky.

It is certain that the Babylonians at a very early date distinguished from the fixed stars those remarkable five, which, from their wandering propensities, the Greeks called the "planets," and which are the only erratic stars that the naked eye, or that even the telescope, except at a very high power, can discern.

With these five they were soon led to class the Moon, which was easily observed to be a wandering luminary, changing her place among the fixed stars with remarkable rapidity.

Ultimately, it came to be perceived that the Sun too rose and set at different parts of the year in the neighborhood of different constellations, and that consequently the great luminary was itself also a wanderer, having a path in the sky which it was possible, by means of careful observation, to mark out.
But to do this, to mark out with accuracy the courses of the Sun and Moon among the fixed stars, it was necessary, or at least convenient, to arrange the stars themselves into groups.

Thus, too, and thus only, was it possible to give form and order to the chaotic confusion in which the stars seem at first sight to lie, owing to the irregularity of their intervals, the difference in their magnitude, and their apparent countlessness.