Practice will quickly enable him to distinguish a planet from the true stars, both by its greater apparent size and by the quality of its light. The planets do not twinkle as do the stars. This arises from the fact that they present measurable disks which reflect the sunlight, but do not shine with a light of their own. No star shows a real disk, even when viewed with a powerful telescope. The stars are mere points, and the larger and better the telescope the smaller they appear. This is not to say that they do not look brighter in a telescope, for the larger stars are dazzling when viewed with a glass of large aperture; but they are so distant that the mightiest of telescopes cannot reveal their real surfaces in the form of disks. The apparent disks which they present are due entirely to irradiation, and the higher the power the smaller these spurious disks appear.
Another way in which the beginner may identify a planet is by observing its motion. No planet remains long in the same position with regard to neighboring stars. They all travel, at varying rates, from west to east through the sky. But this motion is not constant, and at times it is reversed. In the cases of Mars, Jupiter, and Saturn the reversal is due to the fact that when they are in opposition to the sun the earth, being nearer the sun than they are, outfoots them in eastward motion, so that they appear for a time to move backward on their orbits. It is like a fast train passing a slow one on a parallel track; to an observer on the fast train the slow one seems to be either standing still or moving backward. But Mercury and Venus, being nearer the sun than the earth is, have at times a backward motion which is real. Let us consider them only when they appear as “evening stars.” From “superior conjunction” (i. e., the point occupied by the planet when it is on the opposite side of the sun from the earth) to “greatest eastern elongation” (greatest apparent distance from the sun in the evening sky) both Mercury and Venus move eastward among the stars; from “greatest eastern elongation” to “inferior conjunction” (i. e., the point occupied by the planet when it is between the earth and the sun) they move westward among the stars, or, in other words, approach the sun.
The motions of Mercury and Venus are comparatively swift, particularly that of the former. Few persons have ever seen Mercury, because of its nearness to the sun. When well seen it is brighter than any first-magnitude star. As an “evening star” it appears in the west immediately after sunset about once every four months (more precisely once every 116 days). It remains within view about twenty days, but can be easily distinguished only for a week or so when it is nearest eastern elongation. Every almanac gives the dates of its appearances.
Venus, being farther from the sun, travels less rapidly. It reappears in the evening sky once in every 584 days, gradually withdrawing from the sun, and growing brighter until it reaches greatest eastern elongation, which may be as much as forty-seven degrees from the sun, after which it approaches the sun, still becoming brighter for several weeks, until at last it is lost in the glare of the sunlight. During its excursions in the evening sky (and the same is true of its morning apparitions), Venus becomes the most brilliant object in the starry heavens, so brilliant, in fact, that many persons can hardly be persuaded that it is not an artificial light, or some extraordinary phenomenon in space. In the telescope it shows (as does Mercury, also) phases like those of the moon, and when it is seen in the form of a narrow crescent it becomes one of the most charming objects imaginable. For more details about Mercury, Venus, and the other planets, the reader may consult Astronomy with the Naked Eye.
Mars, Jupiter, and Saturn are more likely to cause confusion to the beginner by getting “mixed up” with the stars of the constellations he is studying, because they travel all round the sky, and may appear in turn in each of the zodiacal constellations at any hour of the night. The zodiacal constellations are twelve in number—Aries, Taurus, Gemini, Cancer, Leo, Virgo, Libra, Scorpio, Sagittarius, Capricornus, Aquarius, and Pisces—and they lie in succession along the course of the ecliptic.
Mars is not remarkably brilliant except when it is in opposition to the sun, which happens once every 780 days; but some of the oppositions are much more important than the average, because they occur when Mars is relatively near the earth. This planet is always distinguishable by its ruddy color. In case it is mistaken for a star, the error can be corrected by watching it for a few successive nights, when its motion will become clearly apparent. On the average it moves eastward about half a degree per day.
Jupiter, always very conspicuous when in view, outshines even Sirius, though lacking the scintillation characteristic of that great star. Its light has a slightly yellowish tint, and is remarkably steady. Since it requires nearly twelve years to make a revolution round the sky, Jupiter’s motion is not immediately apparent. It remains for a long time in any constellation in which it may be found, travelling eastward, on the average, about 5′ of arc, or one-sixth of the apparent diameter of the moon, per day. In a month it moves about two and a half degrees.
Saturn is yet more deliberate in its movements. Requiring almost thirty years for a revolution, it may remain more than two years in the same constellation, and its real motion will only become evident upon careful observation continued for several weeks.
The best way to recognize the planets with certainty is to look up their positions with the aid of the American Ephemeris and Nautical Almanac, published annually by the Government at Washington. There the right ascensions and declinations of all the planets are given for any time of the year. Having these, you may find on the large-scale charts the approximate place of the planet sought, and, if you choose, indicate its position with a pencil-mark.
The study of the planets, even without telescopic aid, has a charm hardly less potent than that of the stars. Mercury is fascinating because of the difficulty of seeing him in the light of twilight or dawn. The ancients were greatly puzzled by his dodges, and some of them thought that he was a double personality, and gave him two names, one for his morning and the other for his evening apparitions. With the Egyptians he was respectively Set and Horus, and with the Greeks Apollo and Hermes. The same was true of Venus, who was Phosphorus in the morning and Hesperus in the evening.