ubject
of this chapter, The Planet of Romance.

It had often been thought that Mercury, long supposed to be the nearest
planet to the sun, was perhaps not really the body entitled to that
distinction. Mercury revolves round the sun at an average distance of
about 36,000,000 miles. In the interval between it and the sun there
might have been one or many other planets. There might have been one
revolving at ten million miles, another at fifteen, and so on. But did
such planets exist? Did even one planet revolve inside the orbit of
Mercury? There were certain reasons for believing in such a planet. In
the movements of Mercury indications were perceptible of an influence
that it was at one time thought might have been accounted for by the
supposition of an interior planet.[13] But there was necessarily a great
difficulty about seeing this object. It must always be close to the sun,
and even in the best telescope it is generally impossible to see a
star-like point in that position. Nor could such a planet be seen after
sunset, for under the most favourable conditions it would set almost
immediately after the sun, and a like difficulty would make it invisible
at sunrise.

Our ordinary means of observing a planet have therefore completely
failed. We are compelled to resort to extraordinary methods if we would
seek to settle the great question as to the existence of the
intra-Mercurial planets. There are at least two lines of observation
which might be expected to answer our purpose.

An opportunity for the first would arise when it happened that the
unknown planet came directly between the earth and the sun. In the
diagram (Fig. 40) we show the sun at the centre; the internal dotted
circle denotes the orbit of the unknown planet, which has received the
name of Vulcan before even its very existence has been at all
satisfactorily established. The outer circle denotes the orbit of the
earth. As Vulcan moves more rapidly than the earth, it will frequently
happen that the planet will overtake the earth, so that the three bodies
will have the positions represented in the diagram. It would not,
however, necessarily follow that Vulcan was exactly between the earth
and the luminary. The path of the planet may be tilted, so that, as seen
from the earth, Vulcan would be over or under the sun, according to
circumstances.

If, however, Vulcan really does exist, we might expect that sometimes
the three bodies will be directly in line,