Before making a brief statement of the several planets, the asteroids,
and the satellites, it will be well to consider in a general way the
motions of these bodies about their centres and about the sun. The
most characteristic and invariable of these movements is that by which
each of the planetary spheres, as well as the satellites, describes an
orbit around the gravitative centre which has the most influence upon
it--the sun. To conceive the nature of this movement, it will be well
to imagine a single planet revolving around the sun, each of these
bodies being perfect spheres, and the two the only members of the
solar system. In this condition the attraction of the two bodies would
cause them to circle around a common centre of gravity, which, if the
planet were not larger or the sun smaller than is the case in our
solar system, would lie within the mass of the sun. In proportion as
the two bodies might approach each other in size, the centre of
gravity would come the nearer to the middle point in a line connecting
the two spheres. In this condition of a sun with a single planet,
whatever were the relative size of sun and planet, the orbits which
they traverse would be circular. In this state of affairs it should be
noted that each of the two bodies would have its plane of rotation
permanently in the same position. Even if the spheres were more or
less flattened about the poles of their axes, as is the case with all
the planets which we have been able carefully to measure, as well as
with the sun, provided the axes of rotation were precisely parallel to
each other, the mutual attraction of the masses would cause no
disturbance of the spheres. The same would be the case if the polar
axis of one sphere stood precisely at right angles to that of the
other. If, however, the spheres were somewhat flattened at the poles,
and the axes inclined to each other, then the pull of one mass on the
other would cause the polar axes to keep up a constant movement which
is called nutation, or nodding.