It is not possible to include the planets in our diagrams for the reason that they are not fixed in position but apparently wander among the stars. The name planet is, in fact, derived from a Greek word meaning "wanderer." The stars shine by their own light but the planets shine only by reflected light from the sun. Of the seven planets in the solar system additional to our own planet earth, there are two, Uranus and Neptune that we need not consider for Neptune is not visible without the aid of a telescope and Uranus is fainter than any of the stars included in our diagrams.
Mercury will never appear except in the morning or evening twilight, when none but the very brightest stars are visible, since it never departs far from the sun. It will only be seen under certain favorable conditions, and usually it will escape our notice altogether unless we know exactly where to look for it although there are but two or three stars in the heavens that surpass it in brightness.
Venus, we will probably never mistake for any star in the heavens for it far surpasses all stars in brightness. It will always be seen in the west after sunset or in the east before sunrise and it is never seen more than three hours before or after the sun.
This leaves us but three planets, Jupiter, Saturn and Mars that we may mistake for bright stars. There is little chance that Jupiter will be thus mistaken for it also is far brighter than all of the stars except Sirius which differs greatly from Jupiter in color. Sirius is a brilliant white and Jupiter is a golden yellow. The planets do not twinkle as the stars do and this is particularly true of Jupiter which is remarkable for the quiet steadiness of its yellow light. This alone would serve to identify it.
Saturn is probably mistaken for a star oftener than any of the other planets. It moves so slowly among the stars that we would have to watch it for a number of successive evenings before we could discover that it is moving with respect to the stars. Saturn is yellowish in color and we can probably best distinguish it by the steadiness of its light. If we find in one of the zodiacal groups of stars—for the planets appear among no other constellations—a bright yellowish star where no bright star is indicated on the diagram we may be reasonably certain that we have found the planet Saturn.
Mars is the only planet that is reddish in color. Once in fifteen or seventeen years, when it is particularly near to the earth, it surpasses even Jupiter in brightness, but ordinarily it appears no more brilliant than one of the brighter stars. There are only two stars with which we are likely to confuse Mars,—Aldebaran and Antares—which are very similar to it in color, and, at times, in brightness. Moreover, both of these stars are zodiacal stars and Mars frequently passes through the constellations to which they belong. There should be no trouble about identifying Aldebaran and Antares, however, from their distinctive positions in the diagrams so that any other reddish star appearing in any of the zodiacal groups we may feel certain is the planet Mars.
In the following diagrams of the constellations the brightest and most conspicuous stars, called first-magnitude stars, are represented by white stars. These are the stars we should all be able to recognize and call by name and in every instance the name of a first-magnitude star is given on the diagram. All other stars are represented by circles, and the size of the circle is an indication of the brightness of the star.
Stars visible without the aid of a telescope are referred to usually as "naked-eye stars." They are classed as first, second, third, four, fifth or sixth magnitude stars, according to their relative brightness. A star of the first magnitude is about two and one-half times brighter than a star of the second magnitude, which in turn is two and one-half times brighter than a star of the third magnitude and so on. A first-magnitude star is, then, one hundred times brighter than a sixth magnitude star which is the faintest star that can be seen without the aid of the telescope.
This ratio between successive magnitudes continues among the telescopic stars. A star of the sixth magnitude is one hundred times brighter than a star of the eleventh magnitude which in turn is one hundred times brighter than a star of the sixteenth magnitude.
The faintest stars that can be seen visually in the greatest telescopes are of the seventeenth or eighteenth magnitude, though stars two or three magnitudes fainter can be photographed.