The planets are dark, opaque bodies which revolve at varying distances
and at varying rates of speed in orbits more or less circular around
the sun as a center. They have no light of their own, as the stars
have, but shine wholly by reflected light received from the sun, which
itself is a star. The amount of light they show to us depends upon
their size, their distance, and their power of reflecting the light
they receive.
In comparison with the stars, the planets are very near to us. Our
sun, which is in constitution a star, but very widely separated from
any other star in the universe, holds all his family of planets by
the tether of gravitation, and so keeps them circling about him in a
very small space, as astronomical space is measured. To all of the
planets except Mercury, we ourselves are nearer than the sun is. To
be sure, this distance between us and the planets, as measured by any
terrestrial measure, is not exactly small. It is only by comparison
that we can be said to have anything like a cozy relation to them. For
merely earthly affairs we use terrestrial measures. In solar affairs we
measure by an astronomical unit, which is the sun’s distance from the
earth, ninety-three millions of miles. When we say a planet’s distance
from the sun is thirty astronomical units, we mean it is thirty times
farther than the earth is from the sun.
For matters outside of the solar system, the unit of measure is the
number of miles that light travels in a year. The speed of light is a
little more than 186,000 miles in a second. This is equal to about six
trillions of miles in a year, or about sixty-three thousand times the
distance of the sun from the earth, our family measuring-stick. From
the nearest star it takes light more than four years to come to us.
From the nearest planet light comes in less than three minutes, and
from the farthest one it makes the journey in a little more than four
hours.
As compared with other heavenly bodies, therefore, the sun and the
planets are very near together, occupying a very small space in the
immensity of the universe, immeasurably isolated from all the other
systems and, so far as we know, immeasurably smaller as a system than
most of them.
The whole body of the planets is divided according to size into two
classes, the major and the minor planets. When we refer generally to
the planets, the major planets only are meant. The minor planets are
usually called the asteroids, or planetoids. There are many hundreds
of them, and only one--and that barely--can be seen with the naked
eye. The other planets are eight in number, including the earth, which
is, after all, nothing but one of the smaller of the major planets.
They are, in the order of their distances from the sun: Mercury, the
nearest, Venus, the Earth, Mars, Jupiter, Saturn, Uranus, and Neptune.
Of these only five--Mercury, Venus, Mars, Jupiter, and Saturn--can be
seen from the earth without optical aid. Occasionally, when Uranus is
very favorably situated, a person with an exceptionally good eye, who
knows exactly where to look for the planet, can see it. Neptune is
about equal to an eighth-magnitude star in brightness, and can never be
seen without the aid of a telescope. Mercury, while quite bright enough
to be seen, is not often situated favorably for observation. It is very
near the sun, and is generally obscured either by the light of the sun
when the sun and the planet are above the horizon, or by the haziness
of the atmosphere when the sun is below the horizon and the planet a
little above it. In regions of considerable altitude with a clear, rare
atmosphere, Mercury is more often seen; but never for very long at a
time.
The only planets, therefore, that are a part of our evening spectacle
in the skies are Venus, Mars, Jupiter, and Saturn. These four happen
to be not only the ones we oftenest see, but also the most interesting
of all the planets from various points of view. Venus and Mars are
the nearest to the earth, and most resemble it, and hence are the
most inviting for speculations which have a human interest, such as
habitability, the presence of life, and kindred ideas. Jupiter and
Saturn are interesting above all the others in their splendor and
size, and in their importance as the centers of systems of their own.
As seen by us, the planets are similar to the stars, but with very
distinct differences in appearance, which, when once familiar, mark
them so unmistakably as planets, and not fixed stars, that we need
never get the two confused. The first and easiest distinguishing mark
to notice is that they do not twinkle, as the stars do, but shine with
a steady light similar to that of the moon. This is an invariable
difference between stars and planets, and one needs only to stop and
truly look at them in order to detect it. And once it has become
familiar, it discloses itself at a glance.
This difference between stars and planets is due almost solely to
difference of distance, though the twinkling is caused by our own
atmosphere. The stars are too far away to send us anything but a mere
point of light, and the unequal density of the waves of air sweeping
over this point of light keeps it dancing before our eyes, causing
the phenomenon that we call twinkling. But the planets, being nearer
to us, show a disc, from every point of which comes a line of light,
making the total light of some volume; and these inequalities of the
air are too small to interfere with it to any extent. Sometimes, when
the atmosphere is particularly unsteady, it happens that the light of
a planet is somewhat affected by it when the planet is just rising or
setting and is, consequently, near the horizon, and that it then seems
to twinkle a little. But this departure from the rule is always slight
and of short duration, in the case of the four planets most seen.
Mercury, never being seen anywhere except near the horizon, often seems
to twinkle; but then he is seldom seen at all, and, when visible, is in
other ways so well marked that one cannot fail to recognize him.
So the steady light may justly be said to be invariable, because the
unusual conditions are easily detected. When the atmosphere is such as
to cause even the planets to blink a little, it has an effect also on
the stars. At such a time they will appear to be fairly dancing. This
effect is apt to occur on the clear nights of winter, the atmosphere
being more unsteady then. Such nights, because of the extreme
liveliness and brilliancy that they lend to the stars, are attractive
times for amateur observations. For the astronomer, however, they are
not so favorable. For the seeing of small details such as he seeks, the
steadiest atmosphere is necessary.
Though the planets are near enough to show a disc, they are not
sufficiently near to show to the naked eye as sharp an outline as the
moon’s. Usually the edge is more or less rayed like that of a fixed
star, which adds somewhat to the difficulty of distinguishing them
from the stars until their aspect has become familiar to us. The fact
that we are looking at a disc is plainly shown when an occultation by
the moon occurs. When the moon occults a fixed star, it passes between
us and the star. At such times the star disappears behind the edge of
the moon instantly, as a mere point naturally would. When a planet is
occulted by the moon, it disappears gradually as the moon covers more
and more of its disc, thus showing unmistakably the nature of it.
After steadiness of shining, the next most obvious mark of difference
between a planet and a star, from our point of view, is the movement
of the planets. A star remains always in one place with relation
to the other stars, while the planets move about from constellation
to constellation, seeming to travel sometimes toward the east and
sometimes toward the west.
This difference also is due solely to a difference of distance. The
stars as well as the planets are constantly in motion. Most of them, in
truth, move at a rate which would make the rate of motion of a planet
a mere snail’s pace in comparison. Arcturus, for instance, is supposed
to be moving at the rate of two or three hundred miles a second, and
there are other fixed stars with an equally rapid motion. The swiftest
moving of the planets does not achieve much more than twenty-nine miles
a second, while the slowest swings along at a rate of but little more
than three miles in the same length of time.
These are the real rates of speed of the stars and planets; but they
are not at all what they seem to us. The difference in distance is so
great that for centuries and centuries the flying stars have seemed
to men to remain in the same place in the skies, and so we call them
fixed. The planets, so slow-journeying as they are in comparison, seem
to us to be moving among the constellations at rates varying from more
than a degree a day in the swiftest to between two and three degrees a
year in the slowest.
Hence, if through lack of practice in observation a person is not at
once able to distinguish the difference between the stars and the
planets in the character of their light--that is, whether they twinkle
or shine steadily--he can, by taking a little longer time, at most only
a few days, determine whether the object he sees is a star or a planet
by noticing whether it has any motion among the other stars. Venus and
Mars will show some movement in one evening. Jupiter and Saturn may
require a little more time to disclose their motion.