its
relation to its central body we are now in a position to consider the
application of Kepler's Second Law upon the sun's orbital motion, and
its resultant effect upon the orbit of our earth and all the other
planets.

From Kepler's Second Law we know that equal areas are described by the
radius vector in equal times, and if the first law of Kepler is at all
applicable to the sun, then it must follow that if the sun has an orbit,
and moreover an elliptic orbit as stated by Kepler himself, then, as a
natural result, the radius vector of the sun must move over equal areas
in equal times.

The physical explanation of Kepler's Second Law was given in Art. 103,
and there is no need to traverse the same ground again. It is,
therefore, true that the sun moves faster in certain parts of its orbit
than in others, being urged through space at its greatest velocity when
it is nearest its controlling centre, and slowest when farthest away
from that controlling centre.

Herschel, in his work on _Astronomy_, states: "The motion of the sun will
be such that equal areas are thus swept over by the revolving radius
vector in equal times in whatever part of the circumference of the ellipse
the sun may be moving." He, however, suggested that the earth forms a
focus of the sun's ellipse, a suggestion which is unphilosophical, it
seems to me, as we might equally suggest that the earth revolves round the
moon, which is contrary to all observation. Thus the sun is not carried
uniformly through space by the aetherial currents of its central body,
because it is nearer to that central body at certain times; its velocity
being regulated by its distance from that body, the same being increased
as the distance is decreased, and decreased as the distance increases.

Now if this reasoning be correct, and if the sun really moves round a
central body and is subject to Kepler's Second Law, then that increase
and decrease of distance will be made manifest in the increase and
decrease of the eccentricity of the earth's orbit.

So that if the eccentricity of the earth's orbit should vary from
century to century, then we have conclusive evidence that the sun obeys
the first and second of Kepler's Laws, and therefore that it revolves
around a controlling centre of its own. From observation we find that
this is exactly what is happening, and that at the present time the
eccentricity of the earth's orbit is gradually diminishing, and in about
24,000 yea