Not only the sun but the moon and planets as well move through the zodiacal constellations. In fact a limit for the zodiac of 8° on either side of the ecliptic was chosen because it marks the extent of the excursions of the moon and planets from the ecliptic. Neither moon nor planets will be found at a greater distance than 8° on either side of the ecliptic.
For convenience in determining the positions of the heavenly bodies the astronomer assumes that they lie upon the surface of a celestial sphere that has its center at the center of the earth.
The north pole of the celestial sphere lies directly above the north pole of the earth and the south pole of the celestial sphere directly above the south pole of the earth. The celestial equator is the great circle of the celestial sphere that lies midway between its north and south poles and directly above the earth's equator. The ecliptic is also a great circle of the celestial sphere and cuts the celestial equator at an angle of 23½° in the two points 180° apart known as the equinoctial points, of which we have already spoken.
The zodiacal constellations lie nearly overhead within the tropics and can be seen to advantage all over the world except in polar regions.
For every position of the earth's surface except at the equator we have also our circumpolar constellations which are the ones that never pass below the horizon for the place of observation.
In 40° N. Latitude the Big Dipper is a circumpolar constellation for it is above the horizon at all hours of the day and night and all times of the year. If our latitude is 40° N., all stars within 40° of the north pole of the heavens are circumpolar and never set, while stars within 40° of the south pole of the heavens never rise. All other stars rise and set daily.
If we were at the north pole all stars within 90° of the north pole of the heavens would be circumpolar and would describe daily circuits of the pole parallel to the horizon remaining always above it.
If we were at the equator all stars within zero degrees of either pole would be circumpolar, that is no stars would be circumpolar, all stars rising and setting daily.
As a general rule, then, we may say that stars within an angular distance of the nearest pole of the heavens equal to the latitude never set and stars within an equal distance of the opposite pole never rise while all stars outside of these limits rise and set daily.
The beginner who attempts to make the acquaintance of the principal stars and constellations occasionally may find a bright star in a constellation that is not noted in the diagrams. In this case he has probably happened upon one of the bright planets.