rom us,
during its revolution around the sun, will of course be productive of
great alterations in its apparent size. At superior conjunction it
ought, being then farthest away, to show the smallest disc; while at
inferior conjunction, being the nearest, it should look much larger.
When at greatest elongation, whether eastern or western, it should
naturally present an appearance midway in size between the two.

[Illustration: Various positions, and illumination by the Sun, of an
Inferior Planet in the course of its orbit.

Corresponding views of the same situations of an Inferior Planet as seen
from the Earth, showing consequent phases and alterations in apparent
size.

FIG. 11.--Orbit and Phases of an Inferior Planet.]

From the above considerations one would be inclined to assume that the
best time for studying the surface of an interior planet with the
telescope is when it is at inferior conjunction, or, nearest to us. But
that this is not the case will at once appear if we consider that the
sunlight is then falling upon the side away from us, leaving the side
which is towards us unillumined. In superior conjunction, on the other
hand, the light falls full upon the side of the planet facing us; but
the disc is then so small-looking, and our view besides is so dazzled by
the proximity of the sun, that observations are of little avail. In the
elongations, however, the sunlight comes from the side, and so we see
one half of the planet lit up; the right half at eastern elongation, and
the left half at western elongation. Piecing together the results given
us at these more favourable views, we are enabled, bit by bit, to gather
some small knowledge concerning the surface of an inferior planet.

From these considerations it will be seen at once that the inferior
planets show various phases comparable to the waxing and waning of our
moon in its monthly round. Superior conjunction is, in fact, similar to
full moon, and inferior conjunction to new moon; while the eastern and
western elongations may be compared respectively to the moon's first and
last quarters. It will be recollected how, when these phases were first
seen by the early telescopic observers, the Copernican theory was felt
to be immensely strengthened; for it had been pointed out that if this
system were the correct one, the planets Venus and Mercury, were it
possible to see them more distinctly, would of necessity present phases
like these when viewed from