Though the planets are called wanderers, they are not by any means the
vagrants that the name might imply. They have a fixed course among the
stars from which they never deviate, and the ways of all of them, and
also of the sun and the moon, are confined to a comparatively narrow
strip in the sky.
That strip is called the zodiac. It is only sixteen degrees wide, and
extends like a narrow band all the way around the heavens. It lies so
that it is always easy to observe; and, being so limited, very little
observation is necessary to become familiar with every part of it.
Within its limits all the movements of the sun, the moon, and the
planets take place. Through the center of it is the ecliptic, the great
circle that marks the annual apparent path of the sun through the
heavens. It is the standard circle from which we measure the paths of
the moon and the planets. Whatever degree their courses vary from the
ecliptic is what we call the inclination of their orbits. If the plane
of the orbit of a planet is tilted away from the ecliptic, the planet
will travel half the time on one side of it, and half the time on the
other.
The orbits are, in fact, very little inclined to the ecliptic, and all
but one of the planets may always be found within three degrees of it,
most of them nearer than this. The one exception is Mercury, which
is sometimes as much as seven degrees from this central line of the
zodiac, but ordinarily it is not so far as this. Uranus is so nearly on
the ecliptic that an ordinary observer would not notice the deviation,
and particularly as Uranus can rarely be detected with the naked eye,
and can never be thus followed. Of the four planets which are the ones
we ordinarily see, Mars and Jupiter are never as much as two degrees
from the ecliptic, Saturn never more than two and a half degrees, and
Venus never more than about three degrees. They are all usually nearer
than these outside limits. The greatest distance of the moon from the
ecliptic is about one and a half degrees.
Hence, with the exception of Mercury, all the planets and the sun and
the moon travel in a path six degrees wide, which is only one degree
wider than the distance between the pointers as we see them in the
Great Dipper. The fact that the zodiac is sixteen degrees wide, or
eight degrees on each side of the ecliptic, is due only to a very
generous allowance for the vagaries of Mercury, which he really does
not quite need. For Mercury is always as much as twice the breadth of
the moon, or one degree, inside of the zodiac, and usually more than
that.
Because the earth is tilted on its axis twenty-three and a half degrees
from the perpendicular, the ecliptic runs through the heavens in an
oblique circle, crossing the line of the equator at two points called
the vernal and autumnal equinoxes. The equator in the heavens is the
great circle extending around the celestial sphere half-way between the
north and south poles. It is always practically ninety degrees from
the north star, and the points at which the ecliptic intersects it are
called the equinoxes. These are the only two points on the ecliptic
that are just ninety degrees from the pole. The word equinox is derived
from _equus_ (equal) and _nox_ (night), and when the sun is at the
equinoxes the days and nights are of equal length.
From the vernal to the autumnal equinox the line of the ecliptic is
north of the equator, and hence high in the sky, reaching its highest
point midway between the equinoxes. It then crosses the equator again
and runs obliquely south to the lowest point in its path, and then
curves northerly back to the vernal equinox. The vernal equinox is the
point at which the sun arrives when spring begins. This results in the
sun’s being north of the equator from spring until autumn, and south of
it from autumn to spring.
As the part of the zodiac that we can see best at night is that
opposite where the sun is, so in summer, when the sun is high, we see
best the part of the zodiac which is low in the southern skies in the
evening; and in the winter, when the sun is in the southern half of his
journey, the part of the zodiac best seen by us is high in the heavens.
No part of it, however, is ever as high as the zenith, or directly
overhead, and no planet is ever seen as far north as the zenith in any
place whose latitude is more than twenty-three and one-half degrees
from the equator.
To know the paths of the planets it is necessary to know only twelve
constellations out of the seventy or more in the entire heavens; but
it is difficult to imagine any one’s learning these twelve without
becoming interested in and more or less acquainted with many of the
splendid stars and constellations that lie on each side of them.
The larger one’s acquaintance is with the appearance of the skies
as a whole, the easier, naturally, it will be to distinguish the
planets from the stars, and to follow their courses. But the planets
themselves may be intimately known quite apart from any but the twelve
constellations forming the zodiac. Happily, among them we shall find
some of the most beautiful constellations in the heavens, and some of
the most splendidly brilliant first-magnitude stars.[1]
[1] The reader will find fuller descriptions of the stars in the zodiac
in _The Friendly Stars_, by the author of this book.
The twelve constellations of the zodiac are as follows:
Pisces, the Fishes.
Aries, the Ram.
Taurus, the Bull.
Gemini, the Twins.
Cancer, the Crab.
Leo, the Lion.
Virgo, the Virgin.
Libra, the Scales or Balance.
Scorpio, the Scorpion.
Sagittarius, the Archer.
Capricornus, the Goat.
Aquarius, the Water-Carrier.
We shall begin at the point of the vernal equinox to trace the line of
the ecliptic through these constellations, and that line will mark for
us the path of the sun, the moon, and all the planets. It is convenient
to begin at this point, because it is where the sun crosses the equator
in the spring, and hence it is at the beginning of that part of the
ecliptic which lies north of the equator.
The point of the vernal equinox is now situated in the constellation
Pisces. It is not marked by any bright star, but is not very difficult
to find. It marks the point on the eastern horizon where the sun
rises about March 21st, and about the 21st of September it is on the
eastern horizon exactly opposite that point in the western sky where
the sun sets. It is always ninety degrees from the pole, and if
one chances to know the constellation Cassiopeia, which is shaped like
a chair and is on the opposite side of the pole from the Big Dipper,
one can locate the vernal equinox by drawing a line from the pole-star
through the star which marks the lower part of the front of the chair,
and extending it until it is ninety degrees long. The ninety degrees
can be estimated by using the distance between the pointers in the
Dipper (which is five degrees) as a measure. The star mentioned in
Cassiopeia is about thirty-two degrees from the north star.
[Illustration: MAP SHOWING THE CONSTELLATIONS OF THE ZODIAC AND THE
LINE OF THE ECLIPTIC RUNNING THROUGH THEM
The paths of all the planets, save one, lie always within three degrees
of the ecliptic.]
Having once learned the constellations of the zodiac and,
approximately, the line of the ecliptic, it is not necessary for the
ordinary observer to keep in mind the exact location of the vernal
equinox. It is, however, an important point for the student of
mathematical astronomy.
Beginning at this point, the ecliptic runs through Pisces in a
northeasterly direction for about thirty degrees to Aries, the second
constellation of the zodiac.