iew of Mercury, we
first turn to the Nautical Almanac, and find the position in which the
planet lies. If it happen to be above the horizon, we can at once direct
the telescope to the place, and even in broad daylight the planet will
very often be seen. The telescopic appearance of Mercury is, however,
disappointing. Though it is much larger than the moon, yet it is
sufficiently far off to seem insignificant. There is, however, one
feature in a view of this planet which would immediately attract
attention. Mercury is not usually observed to be a circular object, but
more or less crescent-shaped, like a miniature moon. The phases of the
planet are also to be accounted for on exactly the same principles as
the phases of the moon. Mercury is a globe composed, like our earth, of
materials possessing in themselves no source of illumination. One
hemisphere of the planet must necessarily be turned towards the sun, and
this side is accordingly lighted up brilliantly by the solar rays. When
we look at Mercury we see nothing of the non-illuminated side, and the
crescent is due to the foreshortened view which we obtain of the
illuminated part. The planet is such a small object that, in the glitter
of the naked-eye view, the _shape_ of the luminous body cannot be
defined. Indeed, even in the much larger crescent of Venus, the aid of
the telescope has to be invoked before the characteristic form can be
observed. Beyond, however, the fact that Mercury is a crescent, and that
it undergoes varying phases in correspondence with the changes in its
relative position to the earth and the sun, we cannot see much of the
planet. It is too small and too bright to admit of easy delineation of
details on its surface. No doubt attempts have been made, and
observations have been recorded, as to certain very faint and indistinct
markings on the planet, but such statements must be received with great
hesitation.

[Illustration: Fig. 41.--The Movement of Mercury, showing the Variations
in Phase and in apparent size.]

[Illustration: Fig. 42.--Mercury as a Crescent.]

The facts which have been thoroughly established with regard to Mercury
are mainly numerical statements as to the path it describes around the
sun. The time taken by the planet to complete one of its revolutions is
eighty-eight days nearly. The average distance from the sun is about
36,000,000 miles, and the mean velocity with which the body moves is
over twenty-nine miles a second. W