In considering the movements of the planets, we have to regard their
actual motion in space and that motion as it appears to us. They all
have two principal motions in space. They revolve about the sun in
their orbits, and they rotate on their axes. The manner in which they
accomplish the rotation on their axes determines the length of their
days and nights, or whether, indeed, they shall have any such grateful
alternations of light and darkness. Those planets which, like the
earth, turn on their axes in less time than they make their journey
around the sun have one day and one night every time they make a
complete rotation. Those that turn on their axes in the same time that
they revolve around the sun, of which sort there seems to be at least
one, face always toward the sun, and have no alternations of day and
night. On one side it is always day; on the other it is always night.
The number of days a planet has during each revolution around the sun
depends upon how much time it requires to make a revolution, and how
fast it spins on its axis. In one year here on the earth we have three
hundred and sixty-five days and nights. Saturn, in its year, has more
than twenty-three thousand days and nights.
The manner in which the revolution of the planets in their orbits takes
place determines the length and character of their year; the nearer a
planet is to the sun, the shorter its orbit is, and the faster the rate
of speed at which the sun compels it to move, and hence the shorter
its year. The nearest of the planets, Mercury, makes more than five
hundred revolutions around the sun, while the farthest, Neptune, makes
one. Three times in a year--that is, a terrestrial year--the nearest
planet speeds around its orbit and back to the starting-place with
seventeen days to spare. One hundred and sixty-five terrestrial years
are necessary for the farthest planet to make one circuit of its orbit.
The first goes at the average rate of nearly thirty miles a second over
a path more than two hundred million miles long. The second travels a
path more than seventeen billion miles in length, at the average rate
of three and four-tenths miles a second. Between these two extremes
the other planets have orbits and rates of speed varying with their
distances from the sun. The farther they are from the sun, the larger
the orbit and the slower the speed.
To get something like a picture of the sun and the planets as they
actually lie and as they move in space, one should have in mind an
immense flat, circular disc five and a half billions of miles in
diameter passing through the sun, which is in the center of it. Around
the edge of the disc is the orbit through which Neptune moves. At
varying distances inside of it are the orbits of the other planets,
each growing smaller and smaller as one comes nearer and nearer to the
sun, until the orbit of Mercury, the planet nearest to the sun, is
reached.
Since it is not a hard metal disc that we are considering, but only
an imaginary one in space, there may be a little latitude allowed
for the orbits to tip somewhat out of the exact plane of the disc
without materially altering the figure in mind. And this they do,
very slightly--most of them to the extent only of from one to two
degrees, though one of them falls outside of the common plane about
seven degrees. In these orbits all the planets, as seen from the sun,
are going around from west to east. At the same time they are turning
on their axes in the same direction, some standing almost erect, as it
were, in their orbits and whirling like a dancing dervish as they skim
along, and others more or less inclined like a traveling top.
The time a planet requires to make one circuit of its orbit
constitutes, as with the earth, its year. But we who are on the earth
have, in our study of another planet, to regard it as having in a sense
two years. First, there is the time it takes, starting from a given
point in its orbit, to circle around the sun and return to that point.
This is known as its sidereal period, or year, and is so called from
_sidus_, meaning a star, because the only way to mark any point in
space is by a fixed star, and, as viewed from the sun, one revolution
of a planet would be from a given star back again to that star.
Then there is the time a planet takes, starting when it is in a
straight line with the earth and the sun in space, to return to the
place where the three bodies will be again in the same relative
position. This is known as its synodic period, or year. Synodic is from
our word synod, meaning a meeting or assembly, and the synodic year is
the time between two successive and similar meetings of these three
bodies. The sidereal year concerns the planet in its relation to the
sun; the synodic year, in its relation to the earth. The synodic year
is the only one that much concerns us while regarding the planets as
a part of the spectacle of the sky. It is the one that we know from
observation, while the sidereal year is mathematically computed.
The two periods, or years, are not of the same length, because the
sun with reference to the planet is always stationary, and the motion
resulting in the sidereal year is that of the planet only, while the
synodic year is the result of the movements of both the earth and the
planet, each, in its own orbit, being always in motion.
An inferior planet, situated as it is nearer to the sun than the earth
is, and so having a shorter orbit than the earth’s, will, when it
finishes its sidereal year and comes around to the point from which
it started, find the earth advanced from that position and will,
therefore, have to travel farther on in order to overtake it and come
into the same relative position from which they started, which makes
the time of its circuit with reference to the earth obviously longer
than with reference to the sun.
With the superior planets the case is just reversed. The earth is
the inside planet, or the one nearest the sun, and it must overtake
_them_. With one exception, they are all so far away from the sun and
move so slowly that it takes us but little more than one of our years
to overtake them and bring them into the same relative position with
us that they had when we started, while it requires many of our years
for any one of them to make a single circuit of the sun. Hence their
circuit with reference to the earth is shorter than with reference to
the sun.
With Mars, the exception referred to, we have a more hardly fought
race. That planet is not so far from us as are the other superior
planets. It makes its revolution around the sun in a little less than
two of our years. We travel eighteen miles a second, and it travels
fifteen miles in the same length of time. If we are in line with it at
the beginning of our journey, we glide off swiftly, and easily leave
it far behind. When, however, we come back to the starting-point, it
has not loitered, and is many millions of miles ahead of us, and it
remains ahead until more than seven weeks after we have returned to the
starting-point a second time. Fifty days after we have begun to make
our third round we overtake it, and are again in a direct line with the
planet and the sun. This makes its period with reference to the earth
ninety-three days longer than its own year, and fifty days longer than
two of ours. This is the longest synodic period among the planets.
The orbits in which the planets move all have the form of an
ellipse--that is, of a circle more or less flattened. This flattening,
or the extent to which an orbit departs from the form of a true circle,
is called its eccentricity. The sun is never at the exact center of an
orbit, but is always situated a little to one side of the center--that
is, it is at one of the foci of the ellipse. Consequently, the planet,
as it travels in its orbit, is not always at the same distance from
the sun, the amount of the variation in distance depending upon the
eccentricity of the orbit. The point in the orbit where the planet
is nearest to the sun is its perihelion, and the point at which it is
farthest is its aphelion. It is necessary to keep these elementary
facts in mind in order fully to understand the changes in the motions
and brightness of the planets.
The influence of one body over another that is circling around it is
to make it move faster or more slowly according to its distance from
the central body. Since a planet varies in its distance from the sun in
the different parts of its orbit, it is forced to move fastest when it
is in that part of the orbit which is nearest to the sun, and slowest
when it is in the part farthest away. In other words, the motion of a
planet is more rapid at perihelion than at aphelion. The earth is in
perihelion, or nearest to the sun, in winter--that is, winter in the
northern latitudes--and in consequence it moves faster in winter than
in summer, and the northern winters are, for this reason, a little
shorter than the summers.
These two simple movements of the planets--that around the sun and
that on their axes--are their principal real movements, and are such
as they would show to be if seen from the sun, which is the center
of them. There are also certain minor real movements arising from
various causes, one being the influence that the planets exercise on
one another; but for the ordinary observer these have no particular
significance. Then, the planets all share the one grand movement which
the sun itself is known to be making through limitless space to a
destination of which we are in utter ignorance, over even a path which
we know nothing of save that it leads toward the bright star Vega, in
the constellation of the Lyre. As the sun moves on in that direction
at the rate of eleven miles a second he takes with him all his family
of planets and planetoids, with their satellites, and whatever other
bodies have their abode in his domain. Thus they travel as a body,
each individual spinning on its axis, from the sun itself down to the
smallest planetoid, the satellites circling around the planets, and the
planets in their turn around the sun. And in all these movements the
earth takes part as one of the planets. The sun itself is following
a comparatively straight line in space, and, so far as we know, in
allegiance to no other body. It is, though, just possible that this
comparatively straight line may be the arc of a circle so vast that
we have not yet had time to discover its curvature, and that the sun
itself may be pursuing its own circuit around some still more powerful
body.