Of the real movements of the planets, as described in the last chapter,
we get here on the earth only a very fragmentary view. Without the aid
of the telescope none of them is visible to us except the movements
in their orbits, and these, to our view, are somewhat different from
the simple, circling course apparent to an observer on the sun. The
difference is due to the fact that the earth itself is always in
movement in just the same way that the other planets are, and we, being
never at any time at the center of the orbits, do not see the movements
of the planets as they truly take place, but only as they are outlined
against the sky. So the appearances and disappearances and visible
travels among the stars by which we know the planets are only as we see
them. Some knowledge of the real movements is necessary to a proper
understanding of the apparent movements; but it is only with the latter
that, for ordinary observation, we need to be particularly acquainted.
The rotation of the earth on its axis, as we know, causes the familiar
daily apparent rising, passing, and setting of all the heavenly bodies.
In this apparent motion the planets share as well as the sun, moon, and
stars. But it is their movement _among_ the fixed stars, and not _with_
them, that distinguishes them as planets, and this it is necessary to
know in order to keep track of them and be able to recognize them in
their varying places and guises. For they sometimes shine in their
greatest glory in one season, and sometimes in another, and at the
recurrence of the same season they are sometimes in one part of the sky
and sometimes in another, so that their ways of coming and going border
almost on the mysterious, until one learns the manner of this apparent
vagrancy. Happily, this knowledge is easily attained, and then the
matter is simple enough.
The apparent motions of the inferior planets, Mercury and Venus, always
take place near the sun. Venus never wanders more than forty-eight
degrees from it, and Mercury never more than twenty-eight. Most of
the time they are much nearer than this. Since we cannot see either
of them except when the sun is below the horizon, the consequence of
their being always thus near to him is that they are in view for only
a short time after the sun has set or before he has risen. If they are
in the evening sky, and hence east of the sun, they soon follow him
when he sinks below the western horizon. If they are west of the sun,
and, consequently, are the first to rise in the morning, it is not long
before his brilliant rays flood with light the eastern sky and blot the
planets from our view. Venus can be seen sometimes for three hours at
a time, Mercury for never more than one. Within this limited region of
the sky they appear to journey evening by evening away from the sun,
somewhat obliquely, but toward the zenith, until they have reached
the end of their tether. Then they journey back and pass to the other
side of the sun. There they climb their path toward the zenith, moving
westward and, as we see them, obliquely upward. Morning by morning
they get farther from the sun until their westward limit of freedom
is reached, when they again draw in toward the sun, pass it, appear in
the evening sky, and pull off up the sky toward the east again. Thus
they swing from east to west of the sun, and back again, in unceasing
repetition.
As they pass the sun going from east to west--that is, from the evening
to the morning sky--the inferior planets go between us and the sun;
and when they swing back from west to east, or from the morning to the
evening sky, they pass on the side of the sun farthest away from us.
When they are in a direct line with the earth and the sun they are said
to be in conjunction. If at this point they are between us and the sun,
it is inferior conjunction. If they are on the other side of the sun,
they are said to be in superior conjunction. When the planet, as seen
in the evening, has traveled toward the east as far from the sun as
it will go during that particular revolution, it is said to be at its
greatest eastern elongation. Elongation means simply apparent distance
from the sun; hence, greatest eastern elongation is the greatest
distance possible east of the sun from our point of view. Greatest
western elongation, which we see in the morning before dawn, occurs
when the planet is at its greatest apparent distance west of the sun.
While apparently drawing near and then away from the sun, traveling
obliquely up and down the evening and the morning sky, the planet has
all the time been moving in one direction around the sun; but we could
see the motion only as it appeared on the background of the sky. The
planet is in reality just as far from the sun when it is in conjunction
as at elongation. The difference is that we see it at a different
angle, or from a different point of view. But it has not been at all
times equally near to the earth.
When an inferior planet is at greatest eastern elongation, it is, of
course, east of the sun, and can be seen above the sun in the evening
after sunset, and is an evening star. As it moves westward nearer and
nearer to the sun, it is above the horizon a proportionately shorter
time each evening, and is more and more obscured by the sun’s rays
until it reaches inferior conjunction, when it is exactly between us
and the sun, and hence at the point nearest to us. Here it becomes
invisible, largely because it has its dark side toward us, but partly
because the dazzling light of the sun entirely obscures it. Once in a
while our relative positions are such that we see it pass like a black
dot directly over the bright face of the sun. This is called a transit.
But a transit does not occur at every inferior conjunction. It would so
occur if the planet’s orbit and the earth’s were in exactly the same
plane. But the small tilt that they have is sufficient to throw the
planet, when it is passing the sun, into such an angle that it does not
pass directly between the disc of the sun and us, but a little above or
below. Thus transits are rather rare, though they occur periodically in
the case of both Venus and Mercury, and will be spoken of elsewhere.
When the planet has passed inferior conjunction, it is then west of
the sun, and rises in the morning before the sun is up, and is a
morning star. For a few days it can be seen either not at all or with
difficulty. Then, as it works its way out of the rays of the sun and
on toward the west, it rises earlier each morning until it reaches its
farthest point west.
As it starts back east again its distance from the earth increases
daily until it reaches its greatest distance from us at superior
conjunction. It is then the whole diameter of its orbit farther from us
than when it was at inferior conjunction, and it is again invisible.
The illuminated side of it is toward us; but it is at its smallest,
because it is at its greatest distance from us, and even when it is not
directly behind the sun the light of that luminary is too great for
successful competition. After it has passed superior conjunction it is
again in the evening sky, apparently moving farther from the sun each
day. It is at the same time actually coming nearer to us each day, and
these two facts cause a daily increase in its brightness.
But an inferior planet is not, like the superior planets and the stars,
brightest when it is nearest to us. It is, in fact, darkest when it is
nearest--that is, when it is at inferior conjunction--and we cannot
see it at all. This is because an inferior planet passes through
phases, like the moon, changing gradually during its rounds from full
to crescent, and back again. Its full face is toward us when it is on
the opposite side of the sun and farthest from us. The proportion of
the face that is illuminated grows smaller as the planet approaches
its eastern elongation. But the planet grows brighter because it is
coming nearer to us and is getting out of the dazzling rays of the
sun. One-half of its surface is illuminated when it is at greatest
elongation; but it is brightest a few days later, when less than half
of its face is illuminated, because it is enough nearer to compensate
for the slight diminution in the proportion of light on its disc. It is
brightest in the morning a short time before its western elongation,
for the same reason.
This in a general way describes the motion of an inferior planet, and
this is all that we need to know in order to understand its ordinary
visible movements. If we watch it carefully, however, we may detect
that shortly before inferior conjunction it pauses in its onward sweep
and seems for a time to be stationary, and then to retrace its way
among the stars until a short time after inferior conjunction, when it
again pauses and appears stationary, and finally starts off again in
its original direction on its way toward greatest western elongation.
During this capricious sort of progress the planet usually describes
more or less of a loop, sometimes almost a flourish, in its path. The
appearance is wholly due to the planet’s overtaking and passing us
in our journey around the sun. For a time it travels behind us, then
beside us, and then beyond us; and, since we are both in motion, the
effect is much the same as when one train passes another while they
are both traveling in the same direction. The orbits of the earth and
the planet are not exactly in the same plane, and, both bodies being
in motion, we are not in a position to see the planet at the same
angle more than once as it seems to pass back and forth, and so we get
the effect of its making a flourish or loop. But this effect, while
interesting, takes place only when the planet is so near the sun that
to the ordinary observer it itself does not count for much. We can see
but little of the inferior planets at that time, anyway, though it is
important for us to know where they are, in order to keep track of them
and to be ready for them when they are to be seen.