Venus is a planet of about the same size and the same weight as the
earth, revolving in an orbit which lies within that described by our
globe. Venus, consequently, takes less time than the earth to
accomplish one revolution round the sun, and it happens that the
relative movements of Venus and the earth are so proportioned that in
the time in which our earth accomplishes eight of her revolutions the
other planet will have accomplished almost exactly thirteen. It,
therefore, follows that if the earth and Venus are in line with the
sun at one date, then in eight years later both planets will again be
found at the same points in their orbits. In those eight years the
earth has gone round eight times, and has, therefore, regained its
original position, while in the same period Venus has accomplished
thirteen complete revolutions, and, therefore, this planet also has
reached the same spot where it was at first. Venus and the earth, of
course, attract each other, and in consequence of these mutual
attractions the earth is swayed from the elliptic track which it
would otherwise pursue. In like manner Venus is also forced by the
attraction of the earth to revolve in a track which deviates from
that which it would otherwise follow. Owing to the fact that the sun
is of such preponderating magnitude (being, in fact, upwards of
300,000 times as heavy as either Venus or the earth), the
disturbances induced in the motion of either planet, in consequence
of the attraction of the other, are relatively insignificant to the
main controlling agency by which each of the movements is governed.
It is, however, possible under certain circumstances that the
disturbing effects produced upon one planet by the other can become
so multiplied as to produce peculiar effects which attain measurable
dimensions. Suppose that the periodic times in which the earth and
Venus revolved had no simple relation to each other, then the points
of their tracks in which the two planets came into line with the sun