tudy the details of the observations, we are
immediately confronted with a multitude of technical and intricate
matters. Unfortunately, there are very great difficulties in making the
observations with the necessary precision. The moments when Venus enters
on and leaves the solar disc cannot be very accurately observed, partly
owing to a peculiar optical illusion known as "the black drop," whereby
Venus seems to cling to the sun's limb for many seconds, partly owing to
the influence of the planet's atmosphere, which helps to make the
observed time of contact uncertain. These circumstances make it
difficult to determine the distance of the sun from observations of
transits of Venus with the accuracy which modern science requires. It
seems therefore likely that the final determination of the sun's
distance will be obtained in quite a different manner. This will be
explained in Chapter XI., and hence we feel the less reluctance in
passing any from the consideration of the transit of Venus as a method
of celestial surveying.

We must now close our description of this lovely planet; but before
doing so, let us add--or in some cases repeat--a few statistical facts
as to the size and the dimensions of the planet and its orbit.

The diameter of Venus is about 7,660 miles, and the planet shows no
measurable departure from the globular form, though we can hardly doubt
that its polar diameter must really be somewhat shorter than the
equatorial diameter. This diameter is only about 258 miles less than
that of the earth. The mass of Venus is about three-quarters of the mass
of the earth; or if, as is more usual, we compare the mass of Venus with
the sun, it is to be represented by the fraction 1 divided by 425,000.
It is to be observed that the mass of Venus is not quite so great in
comparison with its bulk as might have been expected. The density of
this planet is about 0.850 of that of the earth. Venus would weigh 4.81
times as much as a globe of water of equal size. The gravitation at its
surface will, to a slight extent, be less than the gravitation at the
surface of the earth. A body here falls sixteen feet in a second; a body
let fall at the surface of Venus would fall about three feet less. It
seems not unlikely that the time of rotation of Venus may be equal to
the period of its revolution around the sun.

The orbit of Venus is remarkable for the close approach which it makes
to a circle. The greatest distance of this plan