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.
[Ill.u.s.tration: 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 princ.i.p.al 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 pa.s.ses 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 roles 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. Ura.n.u.s and Neptune being invisible to the naked eye--Ura.n.u.s can occasionally be just glimpsed by a keen eye--are too faint to be found without the aid of more effective appliances.
THE END