It is not within the scope of this work to dwell at length on a discussion of the planets. Certain explanatory matter regarding them is necessary, however, to prevent confusion; for the student must bear in mind the fact that from time to time the planets appear in the constellations, and unless identified would lead him to think that the diagrams were inaccurate.
The reader is referred to any one of the four large plates that precede each season. He will observe that a portion of an ellipse has been traced on each of them, and that this line has been designated the Ecliptic, which simply means the sun's apparent pathway across the sky.
This pathway is divided into twelve equal parts of thirty degrees each, and to these twelve divisions are given the names of the constellations of the Zodiac in the following order: Aries (♈), Taurus (♉), Gemini (♊), Cancer (♋), Leo (♌), Virgo (♍), Libra (♎), Scorpio (♏), Sagittarius (♐), Capricornus (♑), Aquarius (♒), Pisces (♓).
The sun, starting from the first degree of Aries, the first day of spring, passes through one constellation a month. The planets follow the same pathway.
Confusion, therefore, respecting their identity can only arise in connection with a study of one of the twelve constellations named above, so that whenever a star of any size is seen in one of these constellations, not accounted for in the diagram, the student may conclude that this is a planet; especially if the unknown star does not twinkle. It now remains to identify the planet.
This can best be done by referring to an almanac, which states what planets are above the horizon, and which are morning and evening stars. By morning star is meant that the planet is east of the sun; by evening star, that it is west of the sun.
If the planet is in the west, and very brilliant, it is safe to assume that it is the planet Venus.
If it is brighter than any of the fixed stars, and is some distance from the sun, it is doubtless the colossal Jupiter.
If it is very red, it will probably be Mars.
Saturn is distinguished because of its pale, steady, yellow light.
As for Mercury, Uranus, and Neptune, the former is very near the sun, and seldom seen; while Uranus and Neptune are so inconspicuous as to lead to no confusion on the part of the novice.
A few notes of interest relative to the planets follow, taking them up in regular order passing outward from the sun: Mercury, Venus, Mars, Jupiter, Saturn, Uranus, Neptune.
Mercury is the nearest to the sun of any of the planets. On this account, and because of its rapid changes, it is seldom seen.
The most favorable time for observing it is just after sunset, or just before sunrise, during the months of March, April, August, and September, when it may be seen for a few successive days.
The greatest distance it ever departs from the sun on either side varies approximately from sixteen to twenty-eight degrees. Its motion resembles a pendulum, swinging from one side of the sun to the other.
Venus approaches nearer to the earth and is more brilliant than any other planet. It is bright enough to cast a shadow at night, and is sometimes visible even at noonday. It is almost as large as the earth, and appears to oscillate, as Mercury does, on either side of the sun.
It never appears more than three hours after sunset, and as long before the sunrise, and is never more than forty-eight degrees from the sun.
Mars is most like the earth of any of the planets, and, although not as interesting an object to view as the more brilliant planets, Venus and Jupiter, it claims our attention chiefly because of the surmises respecting its habitability.
Mars appears to the naked eye as a bright red star, and when at a favorable opposition to the earth (which occurs only once in every fifteen years) it rivals Jupiter in splendor.
The planet may be mistaken for the first magnitude stars, Antares in Scorpius, and Aldebaran in Taurus, near which it frequently passes.
The fixed stars, however, twinkle, while Mars glows steadily. If there is any doubt in the student's mind as to the identity of the planet, a few nights of observation, noting the changes in the planet's position, will decide the point. It takes Mars about fifty-seven days to pass through one constellation in the Zodiac.
Jupiter is the largest of all the planets in the solar system, and it is easily distinguished from the fixed stars because of its brilliancy and splendor, exceeding in brightness all the planets excepting Venus, and casting a perceptible shadow.
It moves slowly and majestically across the sky, advancing through the Zodiac at the rate of one constellation yearly. It is therefore a simple matter to forecast its position, for, in whatever constellation it is seen to-day, one year hence it will be seen equally advanced in the next constellation.
Although Jupiter appears to move slowly, it really travels at the incomprehensible rate of five hundred miles a minute.
The most interesting feature about Jupiter for the amateur astronomer consists in observing four of its moons, which are visible with a small telescope. They appear like mere dots of light, and their transit of or occultation with the planet (that is, their disappearance before or behind its disk) can be watched, and is a never failing source of pleasure. A large telescope alone reveals Jupiter's four other moons.
Saturn is farther removed from the earth than any of the planets in the solar system, visible to the naked eye. It is distinguished from the fixed stars by the steadiness of its light, which is dull and of a yellow hue, though to some it appears to be of a greenish tinge. It seems barely to move, so slow is its motion among the stars, for it takes two and one half years to pass through a single constellation of the Zodiac.
Saturn has eight moons. Titan, its largest one, can be seen with a 3" glass. Its celebrated rings are telescopic objects but a small glass reveals them.
The student will hardly mistake Uranus for a fixed star, as it is only under the most favorable circumstances that it can be seen with the naked eye.
At its nearest approach to the earth, it is as bright as a sixth-magnitude star. Uranus is accompanied by four moons, and takes seven years to pass through a constellation of the Zodiac.
Neptune is the most distant of the planets in the solar system, and is never visible to the naked eye.
The earth comes properly under a discussion of the planets, but a description of it is hardly within the scope of this work.
Confusion in identifying the planets is really confined to Mars and Saturn, for Venus and Jupiter are much brighter than any of the fixed stars, and their position in the heavens identifies them, as we have seen before.
The following table of first-magnitude stars in the Zodiacal constellations confines the question of identifying the planets to a comparison of the unknown star with the following-named stars:
|Castor and Pollux|
The first four stars named above are white in color, so that either Mars or Saturn is readily distinguished from them.
As for Aldebaran and Antares, which are both red stars, not unlike Mars and Saturn in color and magnitude, the fact that the latter do not twinkle, and that they do not appear in the diagrams, should satisfy the observer of their identity. Reference to an almanac, or a few nights of observation, will in any case set at rest any doubt in the matter.
THE PLANETARY ORBITS
COMPARATIVE SIZE OF THE PLANETS.
the Milky Way.