[Illustration: CHART NO. 6.--FROM RIGHT ASCENSION 20 HOURS TO 24 HOURS
(0 II.); DECLINATION 10 deg. NORTH TO 30 deg. 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 deg. 36 min. Looking at the
sun's place for October 12th, I find it to be R.A. 13 h. 8 m. and Dec.
7 deg. 14 min. 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.
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