The movements of the superior planets, Mars, Jupiter, Saturn, Uranus,
and Neptune, as they appear to us, are different from those of the
inferior planets in some important respects. Instead of swinging back
and forth east and west of the sun, and never appearing very far away
from it, as the inferior planets do, the superior planets make an
entire circuit of the heavens, and it is possible to see them at any
distance from the sun, and at any time during the night. Sometimes
they are, with relation to the earth, in that part of the sky exactly
opposite to the sun, and hence in line with it and the earth. At
such times they can be seen all night. They are then said to be in
opposition, and are in the best position for our observation. The
earth being, when in this situation, in a direct line between them and
the sun, we have the sun at our backs, as it were, shedding its full
rays on the disc of the planet under observation, which is then at its
nearest to us, and also at its brightest. For, since the orbits of all
the superior planets are outside of ours, the planets never get between
us and the sun, and, in consequence, never turn a dark side toward
us. Their entire discs are practically always illuminated, and their
changes in brightness depend largely upon their changes in distance,
which, as we have seen, is not the case with the inferior planets.
Mars, the nearest of them, is at times somewhat gibbous (that is, shows
a little less than a full face, as the moon does when just beginning
to wane), and, in less degree, Jupiter also. But in neither case
is this departure from fullness sufficient to have any appreciable
effect on the planet’s brightness, and, moreover, it does not occur
when the planet is in the most favorable position for us to see it.
At opposition, therefore, we always have the full face of the planet
presented to us; and being, as we then are, on the same side of the sun
with it, we are ninety-three millions of miles (our distance from the
sun) nearer to it than the sun is.
Being, when in opposition, exactly opposite the sun, the planet rises
just as the sun sets. After opposition it rises a little earlier each
evening, and is higher up in the sky at each succeeding sunset. When
we find it just half-way between the eastern and the western horizon
at sunset, it is at quadrature. After quadrature it appears nearer and
nearer the western horizon each evening at sunset, until it finally
is too near the sun to be visible. It is then traveling in that part
of its orbit which is beyond the sun from us. From opposition to this
situation it has been an evening star.
When a superior planet is in line with the sun and the earth, and is
on the far side of the sun from us, it is said to be in conjunction,
and we are then one hundred and eighty-six millions of miles, or twice
our distance from the sun, farther from it than we are when it is in
opposition. But besides being placed at so much greater distance from
it, we have in this situation the bright sun excluding the planet from
our view. It will be readily seen, therefore, why the superior planets
are in so much better position for us to see them in opposition than at
conjunction.
From conjunction to opposition the planet is west of the sun, and
will be below the horizon at sunset, and will rise some time during
the night. At first it will appear just before sunrise as a morning
star, but will gradually rise earlier each night until, when it reaches
opposition again, it will rise just as the sun sets. Half-way between
conjunction and opposition it is again at quadrature.
From opposition to conjunction the planet will be east of the sun and
above the horizon at sunset. When a planet is in conjunction with the
sun, it passes the meridian, or the point half-way between rising and
setting, about noon, and is above the horizon with the sun during the
day. When it is in opposition it passes the meridian about midnight,
and is above the horizon during the night. When it is at quadrature and
moving toward conjunction, it passes the meridian about six o’clock in
the evening, and may be seen in the western half of the sky during the
early evening, and will set before midnight. When it is at quadrature
and moving toward opposition, it will rise some time between midnight
and sunset, and will be in view in the east during a part of the first
half of the night. The nearer it is to opposition, the earlier in the
evening it rises and the longer it may be seen.
The main movement of the superior planets among the stars is from west
to east, and this is known as their direct motion. But not far from
opposition they seem to hesitate, then move more slowly, then finally
stop, remain stationary for a time, turn back on their tracks, and
start off in the opposite direction. This is their retrograde motion.
They do not continue in it as long as in the direct motion; but after
a comparatively short time they again hesitate, go more slowly, stop,
remain stationary, then turn back and swing off in the original
direction, and continue to move in this direction until they are again
approaching opposition. It is exactly in the middle of this sweep
toward the west that the planet is in opposition. Close observation
will show that the superior planets also make something of the same
sort of a loop in their path among the stars that the inferior planets
make, and for the same reason. The only difference is that when a
superior planet is retrograding we are passing it, and when an inferior
planet retrogrades it is passing us.
In giving this rather rough outline of the way the planets in general
move among the stars, reaching in their wanderings these various
positions with relation to the sun and the earth, the intention is only
to fix some definite situations from which to consider the movements
of the individual planets. When we come to know each planet as an
individual, and to follow it as it comes and goes in the heavens, and
to watch its ever-wonderful changes in brilliancy, these situations
will have a much more definite meaning to us and a relatively greater
interest and importance. The planets as they appear to us all move
along pretty much the same path; but each has its own way of gracing
this path, and each its particular manner of changing in aspect.