There is no reason why everybody should not know the principal planets at sight nearly as well as everybody knows the moon. It only requires a little intelligent application to become acquainted with the other worlds that have been discussed in the foregoing chapters, and to be able to follow their courses through the sky and recognize them wherever they appear. No telescope, or any other instrument whatever, is required for the purpose. There is but one preliminary requirement, just as every branch of human knowledge presupposes its A B C. This is an acquaintance with the constellations and the principal stars—not a difficult thing to obtain.

Almost everybody knows the "Great Dipper" from childhood's days, except, perhaps, those who have had the misfortune to spend their youth under the glare of city lights. Some know Orion when he shines gloriously in the winter heavens. Many are able to point out the north star, or pole star, as everybody should be able to do. All this forms a good beginning, and may serve as the basis for the rapid acquirement of a general knowledge of the geography of the heavens.

If you are fortunate enough to number an astronomer among your acquaintance—an amateur will do as well as a professor—you may, with his aid, make a short cut to a knowledge of the stars. Otherwise you must depend upon books and charts. My Astronomy with an Opera-Glass was prepared for this very purpose. For simply learning the constellations and the chief stars you need no opera-glass or other instrument. With the aid of the charts, familiarize yourself with the appearance of the constellations by noticing the characteristic arrangements of their chief stars. You need pay no attention to any except the bright stars, and those that are conspicuous enough to thrust themselves upon your attention.

Learn by observation at what seasons particular constellations are on, or near, the meridian—i.e., the north and south line through the middle of the heavens. Make yourself especially familiar with the so-called zodiacal constellations, which are, in their order, running around the heavens from west to east: Aries, Taurus, Gemini, Cancer, Leo, Virgo, Libra, Scorpio, Sagittarius, Capricornus, Aquarius, and Pisces. The importance of these particular constellations arises from the fact that it is across them that the tracks of the planets lie, and when you are familiar with the fixed stars belonging to them you will be able immediately to recognize a stranger appearing among them, and will correctly conclude that it is one of the planets.[21] How to tell which planet it may be, it is the object of this chapter to show you. As an indispensable aid—unless you happen already to possess a complete star atlas on a larger scale—I have drawn the six charts of the zodiacal constellations and their neighbors that are included in this chapter.

Chart No. 1.—From Right Ascension 0 Hours to 4 Hours; Declination 30° North to 10° South.

Chart No. 1.—From Right Ascension 0 Hours to 4 Hours; Declination 30° North to 10° South.

Having learned to recognize the constellations and their chief stars on sight, one other step, an extremely easy one, remains to be taken before beginning your search for the planets—buy the American Ephemeris and Nautical Almanac for the current year. It is published under the direction of the United States Naval Observatory at Washington, and can be purchased for one dollar.

This book, which may appear to you rather bulky and formidable for an almanac, contains hundreds of pages and scores of tables to which you need pay no attention. They are for navigators and astronomers, and are much more innocent than they look. The plain citizen, seeking only an introduction to the planets, can return their stare and pass by, without feeling in the least humiliated.

Chart No. 2.—From Right Ascension 4 Hours to 8 Hours; Declination 30° North to 10° South.

Chart No. 2.—From Right Ascension 4 Hours to 8 Hours; Declination 30° North to 10° South.

In the front part of the book, after the long calendar, and the tables relating to the sun and the moon, will be found about thirty pages of tables headed, in large black letters, with the names of the planets—Mercury, Venus, Mars, Jupiter, Saturn, etc. Two months are represented on each page, and opposite the number of each successive day of the month the position of the planet is given in hours, minutes, and seconds of right ascension, and degrees, minutes, and seconds of north and south declination, the sign + meaning north, and the sign south. Do not trouble yourself with the seconds in either column, and take the minutes only when the number is large. The hours of right ascension and the degrees of declination are the main things to be noticed.

Right ascension, by the way, expresses the distance of a celestial body, such as a star or a planet, east of the vernal equinox, or the first point of Aries, which is an arbitrary point on the equator of the heavens, which serves, like the meridian of Greenwich on the earth, as a starting-place for reckoning longitude. The entire circuit of the heavens along the equator is divided into twenty-four hours of right ascension, each hour covering 15° of space. If a planet then is in right ascension (usually printed for short R.A.) 0 h. 0 m. 0 s., it is on the meridian of the vernal equinox, or the celestial Greenwich; if it is in R.A. 1 h., it will be found 15° east of the vernal equinox, and so on.

Chart No. 3.—From Right Ascension 8 Hours to 12 Hours; Declination 30° North to 10° South.

Chart No. 3.—From Right Ascension 8 Hours to 12 Hours; Declination 30° North to 10° South.

Declination (printed D. or Dec.) expresses the distance of a celestial body north or south of the equator of the heavens.

With these explanations we may proceed to find a planet by the aid of the Nautical Almanac and our charts. I take, for example, the ephemeris for the year 1901, and I look under the heading "Jupiter" on page 239, for the month of July. Opposite the 15th day of the month I find the right ascension to be 18 h. 27 m., neglecting the seconds. Now 27 minutes are so near to half an hour that, for our purposes, we may say Jupiter is in R.A. 18 h. 30 m. I set this down on a slip of paper, and then examine the declination column, where I find that on July 15 Jupiter is in south declination (the sign meaning south, as before explained) 23° 17 52, which is almost 23° 18, and, for our purposes, we may call this 23° 20, which is what I set down on my slip.

Chart No. 4.—From Right Ascension 12 Hours to 16 Hours; Declination 10° North to 30° South.

Chart No. 4.—From Right Ascension 12 Hours to 16 Hours; Declination 10° North to 30° South.

Next, I turn to Chart No. 5, in this chapter, where I find the meridian line of R.A. 18 h. running through the center of the chart. I know that Jupiter is to be looked for about 30 m. east, or to the left, of that line. At the bottom and top of the chart, every twenty minutes of R.A. is indicated, so that it is easy, with the eye, or with the aid of a ruler, to place the vertical line at some point of which Jupiter is to be found.

Chart No. 5.—From Right Ascension 16 Hours to 20 Hours; Declination 10° North to 30° South.

Chart No. 5.—From Right Ascension 16 Hours to 20 Hours; Declination 10° North to 30° South.

Then I consult my note of the declination of the planet. It is south 23° 20. On the vertical borders of the chart I find the figures of the declination, and I observe that 0° Dec., which represents the equator of the heavens, is near the top of the chart, while each parallel horizontal line across the chart indicates 10° north or south of its next neighbor. Next to the bottom of the chart I find the parallel of 20°, and I see that every five degrees is indicated by the figures at the sides. By the eye, or with the aid of a ruler, I easily estimate where the horizontal line of 23° would fall, and since 20 is the third of a degree I perceive that it is, for the rough purpose of merely finding a conspicuous planet, negligible, although it, too, can be included in the estimate, if thought desirable.

Having already found the vertical line on which Jupiter is placed and having now found the horizontal line also, I have simply to regard their crossing point, which will be the situation of the planet among the stars. I note that it is in the constellation Sagittarius in a certain position with reference to a familiar group of stars in that constellation, and when I look at the heavens, there, in the place thus indicated, Jupiter stands revealed.

Chart No. 6.—From Right Ascension 20 Hours to 24 Hours (0 II.); Declination 10° North to 30° South.

Chart No. 6.—From Right Ascension 20 Hours to 24 Hours (0 II.); Declination 10° North to 30° South.

The reader will readily perceive that, in a precisely similar manner, any planet can be located, at any time of the year, and at any point in its course about the heavens. But it may turn out that the place occupied by the planet is too near the sun to render it easily, or at all, visible. Such a case can be recognized, either from a general knowledge of the location of the constellations at various seasons, or with the aid of the Nautical Almanac, where at the beginning of each set of monthly tables in the calendar the sun's right ascension and declination will be found. In locating the sun, if you find that its right ascension differs by less than an hour, one way or the other, from that of the planet sought, it is useless to look for the latter. If the planet is situated west of the sun—to the right on the chart—then it is to be looked for in the east before sunrise. But if it is east of the sun—to the left on the chart—then you must seek it in the west after sunset.

For instance, I look for the planet Mercury on October 12, 1901. I find its R.A. to be 14 h. 40 m. and its Dec. 18° 36. Looking at the sun's place for October 12th, I find it to be R.A. 13 h. 8 m. and Dec. 7° 14. Placing them both on Chart No. 4, I discover that Mercury is well to the east, or left hand of the sun, and will consequently be visible in the western sky after sundown.

Additional guidance will be found by noting the following facts about the charts:

The meridian (the north and south line) runs through the middle of Chart No. 1 between 11 and 12 o'clock p.m. on November 1st, between 9 and 10 o'clock p.m. on December 1st, and between 7 and 8 o'clock p.m. on January 1st.

The meridian runs through the middle of Chart No. 2 between 11 and 12 o'clock p.m. on January 1st, between 9 and 10 o'clock p.m. on February 1st, and between 7 and 8 o'clock p.m. on March 1st.

The meridian runs through the middle of Chart No. 3 between 11 and 12 o'clock p.m. on March 1st, between 9 and 10 o'clock p.m. on April 1st, and between 7 and 8 o'clock p.m. on May 1st.

The meridian runs through the middle of Chart No. 4 between 11 and 12 o'clock p.m. on May 1st, between 9 and 10 o'clock p.m. on June 1st, and between 7 and 8 o'clock p.m. on July 1st.

The meridian runs through the middle of Chart No. 5 between 11 and 12 o'clock p.m. on July 1st, between 9 and 10 o'clock p.m. on August 1st, and between 7 and 8 o'clock p.m. on September 1st.

The meridian runs through the middle of Chart No. 6 between 11 and 12 o'clock p.m. on September 1st, between 9 and 10 o'clock p.m. on October 1st, and between 7 and 8 o'clock p.m. on November 1st.

Note well, also, these particulars about the charts: Chart No. 1 includes the first four hours of right ascension, from 0 h. to 4 h. inclusive; Chart No. 2 includes 4 h. to 8 h.; Chart No. 3, 8 h. to 12 h.; Chart No. 4, 12 h. to 16 h.; Chart No. 5, 16 h. to 20 h.; and Chart No. 6, 20 h. to 24 h., which completes the circuit. In the first three charts the line of 0°, or the equator, is found near the bottom, and in the last three near the top. 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° 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° eastward among the stars every year. The zodiacal constellations are roughly about 30° in length, and as Jupiter was in Sagittarius in 1901, he will be in Capricornus in 1902. Saturn, requiring nearly thirty years for a revolution around the sun, moves only between 12° and 13° eastward every year, and, being in conjunction with Jupiter in Sagittarius in 1901, does not get beyond the border of that constellation in 1902.

Jupiter having been in opposition to the sun June 30, 1901, will be similarly placed early in August, 1902, the time from one opposition of Jupiter to the next being 399 days.

Saturn passes from one opposition to the next in 378 days, so that having been in that position July 5, 1901, it reaches it again about July 18, 1902.

Mars requires about 687 days to complete a revolution, and comes into conjunction with the earth, or opposition to the sun—the best position for observation—on the average once every 780 days. Mars was in opposition near the end of February, 1901, and some of its future oppositions will be in March, 1903; May, 1905; July, 1907; and September, 1909. The oppositions of 1907 and 1909 will be unusually favorable ones, for they will occur when the planet is comparatively near the earth. When a planet is in opposition to the sun it is on the meridian, the north and south line, at midnight.

Mercury and Venus being nearer the sun than the earth is, can never be seen very far from the place of the sun itself. Venus recedes much farther from the solar orb than Mercury does, but both are visible only in the sunset or the sunrise sky. All almanacs tell at what times these planets play their respective rÔles as morning or as evening stars. In the case of Mercury about 116 days on the average elapse between its reappearances; in the case of Venus, about 584 days. The latter, for instance, having become an evening star at the end of April, 1901, will become an evening star again in December, 1902.

With the aid of the Nautical Almanac and the charts the amateur will find no difficulty, after a little practise, in keeping track of any of the planets.

In the back part of the Nautical Almanac will be found two pages headed "Phenomena: Planetary Configurations." With the aid of these the student can determine the position of the planets with respect to the sun and the moon, and with respect to one another. The meaning of the various symbols used in the tables will be found explained on a page facing the calendar at the beginning of the book. From these tables, among other things, the times of greatest elongation from the sun of the planets Mercury and Venus can be found.

It may be added that only bright stars, and stars easily seen, are included in the charts, and there will be no danger of mistaking any of these stars for a planet, if the observer first carefully learns to recognize their configurations. Neither Mars, Jupiter, nor Saturn ever appears as faint as any of the stars, except those of the first magnitude, included in the charts. Uranus and Neptune being invisible to the naked eye—Uranus can occasionally be just glimpsed by a keen eye—are too faint to be found without the aid of more effective appliances.