This is a matter of convenience in arrangement, based upon the fact that the ecliptic, which, and not the equator, marks the center of the zodiac, indicates the position of the tracks of the planets among the stars; and the ecliptic, being inclined 23 deg.

to the plane of the equator, lies half to the north and half to the south of the latter.
Those who, after all, may not care to consult the ephemeris in order to find the planets, may be able to locate them, simply from a knowledge of their situation among the constellations.

Some ordinary almanacs tell in what constellations the principal planets are to be found at various times of the year.

Having once found them in this way, it is comparatively easy to keep track of them thereafter through a general knowledge of their movements.

Jupiter, for instance, requiring a period of nearly twelve years to make a single journey around the sun, moves about 30 deg.