So far as we know, five of the planets--Mercury, Venus, Mars, Jupiter,
and Saturn--have been known from time immemorial. There are existing
records of them made thousands of years ago. There is no reason why
they should not have been thus known, since they have always been as
they are now, visible to the naked eye, and all of them save Mercury
are as easily seen as the sun or the moon. They do not, of course,
exact the instant attention that those great luminaries do, because,
being smaller, they are less isolated from the great body of the stars;
but they are in their seasons plainly visible, and can then always be
seen if one looks at them.
In ancient times, when people lived more out-of-doors than is the habit
now, they did look at them. The same primitive shepherds that, while
tending their flocks at night on the hills, named the constellations
according to the fanciful shapes that the unchanging stars seemed to
outline, watched also the five wandering stars, more wonderful to them
than any of the others. They observed how mysteriously these stars came
at certain seasons and silently threaded their way across the shining
heavens, and then as mysteriously disappeared. They saw them not only
differing from the other stars in glory, but changing in their own
brilliancy from one time to another, until, in some cases, they failed
to recognize them as the same stars under varying aspects. Venus, for
instance, they called Phosphorus, or Lucifer, when they saw her as a
morning star, and Hesperus, or Vesper, when she shone in the evening.
The sun and the moon, they noted, also moved from place to place among
the fixed stars, and they called all these errant bodies planets, which
means “wanderers.” These are the “seven planets” referred to in the
earlier literatures and in all early books on astronomy or astrology.
This is sometimes a little confusing, because, though the sun and the
moon are no longer called planets, we still (omitting the earth) have
seven. But Neptune and Uranus, not being visible to the naked eye,
were not known to the ancients. They were discovered by means of the
telescope, and that only within the last century and a half. So, owing
to these comparatively new-found members of the solar family, we have
yet the magic number of planets, seven.
These seven are the major planets and the ones with which mainly it
will be our endeavor here to promote and strengthen an acquaintance.
With Uranus and Neptune the acquaintance will necessarily be less
intimate than with the others, because we cannot see them in the same
free way; but they are not on this account much less interesting than
the others, and a little knowledge of them is pleasant family history.
They simply do not live within sight.
The planets that are nearer to the sun than we are, and hence lie
between us and the sun, are called the inferior, or sometimes interior,
planets. Those that lie outside the orbit of the earth are called the
superior, or the exterior, planets. In so grouping them the earth
is the dividing-point, and is not itself in either class. Mercury
and Venus are the inferior planets. The superior planets are Mars,
Jupiter, Saturn, Uranus, and Neptune. The distinction has importance,
especially when we are discussing the planets with relation to their
movements, as seen from the earth, because the planets with orbits
between us and the sun (the inferior planets) have very different
phases and apparent motions from those whose orbits are beyond us from
the sun (the superior planets).
When considered in regard to size, constitution, development, and
their likeness to each other, the planets are sometimes distinguished
as the terrestrial planets and the major planets. This need occasion
no confusion with the general division of them into major and minor
planets, because, as has been said, when simply “the planets” are
mentioned, these seven large planets are always the ones that are
meant, the others being usually called asteroids, or planetoids. The
terrestrial planets are Mercury, Venus, Earth, and Mars. As the name
implies, they are so called because they are in some respects similar
to the earth. The major planets are Jupiter, Saturn, Uranus, and
Neptune. They are all larger than the terrestrial planets, and, in
addition, have some other characteristics in common which the planets
of the other group do not have. The two classes represent different
stages of evolution.
The four planets forming the terrestrial group are sometimes called the
inner planets, and the four major planets are then known as the outer
planets. The point of division in mind then is the space between Mars
and Jupiter. This is so vast in comparison with the spaces between the
other planets from the sun out to Mars that it becomes a convenient
dividing-line, particularly as the groups divided by it are in some
respects essentially different from each other.
Of the four planets which have an especial interest to us because of
their being the ones most easily seen, two are terrestrial, or inner,
planets, Mars and Venus, and two are major, or outer, planets, Jupiter
and Saturn. The differences between the two classes are solely matters
of constitution and situation, and have nothing to do with their
appearance to us. Venus, the brightest of them all, belongs to one
group; Jupiter, the second in brilliancy, belongs to the other.
That there is at least one other planet beyond the present boundary of
our system (which is the orbit of Neptune) seems to be quite probable.
Some astronomers think there may be several others. There are certain
perturbations, or irregularities, in the movements of Neptune which the
influence of Uranus does not account for, and they seem to indicate
that there is some disturbing body even beyond the orbit of that
farthest known planet.
Several astronomers are working on the problem of locating this
undiscovered body. At various times it has been announced that such
a planet would probably be found in a certain position in the skies
at a specified date; but as yet no one has been able to get a view
of it. Recently the orbit of a far-off hypothetical planet has been
calculated, and its place predicted for 1914. Perhaps it may be found
then. Of course it could never be seen through any but the most
powerful telescopes. Its calculated distance from the sun is one
hundred and five times that of the earth. This would be more than nine
billions of miles, or more than three times farther than Neptune is
from the sun. It would require fourteen hours for light to pass from
the sun to a planet at that distance, and the sun would appear to it
smaller than Saturn or an ordinary first-magnitude star does to us.
A further reason for suspecting the existence of such a planet is
suggested by the orbits of certain comets. These erratic bodies,
when they chance to come within the bounds of the solar system, are
sometimes forced to remain because of the powerful influence of one of
the planets near which their path has taken them. Jupiter holds as many
as thirty of them in this way, Saturn and Uranus have two or three, and
Neptune has captured as many as six. But there are still others that
return to us in regular periods, but which go sufficiently far beyond
Neptune to escape entirely if there were not some still more distant
watch-dog to turn them back. So there seems good reason to believe that
Neptune is not really the outermost of the planets.
There has also been much said about the possibility of a planet nearer
to the sun than Mercury. When Mercury is at perihelion, or nearest to
the sun, there are certain irregularities in his movements which might
be explained by the presence of another planet between Mercury and the
sun. In 1859 it was thought that such a planet had been observed. Its
time of revolution and its distance from the sun were estimated, and
it was named Vulcan. In some of the books of astronomy published about
that time, and even in some published as many as fifteen years later,
Vulcan is mentioned as a reality. But now it is believed that the
observation was a mistake, and no such body is known to exist.
In 1878 it was again thought that two bodies nearer to the sun than
Mercury had been discovered during an eclipse. These observations
have never been explained or confirmed; but it is thought that the
objects seen were probably stars which were mistaken for planets by the
observers. If a body so situated does exist, it is so near the sun that
it probably can never be seen except during an eclipse, and the time of
observation is then so short and mistakes are so easily made that it
is difficult to verify the observation. The continued search for the
cause of the perturbations of Mercury may finally lead to the discovery
of something between it and the sun. But if it is a single body, this
seems a much less promising task than the search for a planet, or
planets, on the outer edge of the solar system.