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.
[Illustration: 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.