now
the bulk of Jupiter, and the distances at which his satellites revolve;
we know the dimensions of the comets, and the number of miles to which
they recede in their wanderings; we know the velocity of the shooting
stars; and we learn the important lesson that our earth is but one of
the minor members of the sun's family.

As the path of Venus lies inside that of the earth, and as Venus moves
more quickly than the earth, it follows that the earth is frequently
passed by the planet, and just at the critical moment it will sometimes
happen that the earth, the planet, and the sun lie in the same straight
line. We can then see Venus on the face of the sun, and this is the
phenomenon which we call the _transit of Venus_. It is, indeed, quite
plain that if the three bodies were exactly in a line, an observer on
the earth, looking at the planet, would see it brought out vividly
against the brilliant background of the sun.

Considering that the earth is overtaken by Venus once every nineteen
months, it might seem that the transits of the planet should occur with
corresponding frequency. This is not the case; the transit of Venus is
an exceedingly rare occurrence, and a hundred years or more will often
elapse without a single one taking place. The rarity of these phenomena
arises from the fact that the path of the planet is inclined to the
plane of the earth's orbit; so that for half of its path Venus is above
the plane of the earth's orbit, and in the other half it is below. When
Venus overtakes the earth, the line from the earth to Venus will
therefore usually pass over or under the sun. If, however, it should
happen that Venus overtakes the earth at or near either of the points in
which the plane of the orbit of Venus passes through that of the earth,
then the three bodies will be in line, and a transit of Venus will be
the consequence. The rarity of the occurrence of a transit need no
longer be a mystery. The earth passes through one of the critical parts
every December, and through the other every June. If it happens that the
conjunction of Venus occurs on, or close to, June 6th or December 7th,
then a transit of Venus will occur at that conjunction, but in no other
circumstances.

The most remarkable law with reference to the repetition of the
phenomenon is the well-known eight-year interval. The transits may be
all grouped together into pairs, the two transits of any single pair
being separated by an interval of eigh