MERCURY.
So far as we know at present, Mercury is the nearest planet to the Sun.
The circumstances under which it presents itself to us and a brief
general account of its movements have already been stated. In the
present chapter, therefore (and this remark applies in substance to each
of the succeeding chapters appropriated to particular planets), I shall
limit myself to such topics as seem to be of interest to an observer
armed with a telescope. Mercury, as already mentioned, exhibits from
time to time phases which may be said to be the same as those of the
moon; but as the only chance of seeing it is when it is at its greatest
distance east or west of the Sun, practically it can only be studied
when in, or rather near to, what may be called the half-moon phase; and
even then observations on its physical appearance can only be obtained
with difficulty. Perhaps its most definite feature is its colour. This,
undoubtedly, is more or less pink. Strange to say, in spite of the
multiplication of telescopes and observers, comparatively little
attention has been paid to this planet, and we really know very little
more about it than Schröter told us nearly a hundred years ago. He
obtained what he conceived to be satisfactory evidence of the existence
of at any rate one mountain, having a height of about 11 English miles—a
height which it will be noted, far exceeds, not only relatively but
absolutely, any mountain on the earth. What Schröter based this
conclusion upon was the fact that when the planet was near inferior
conjunction, the southern horn presented a truncated appearance, which
might be the result of a lofty projection arresting the Sun’s light.
Schröter also announced that Mercury rotated on its axis in 24 hours 5
minutes. Sir W. Herschel failed to satisfy himself that Schröter’s
conclusions were well-founded, but it must certainly be admitted that
some support for them is furnished by certain observations made within
the last few years. It is matter for regret, however, that most of these
were made with instruments of sizes which, for the most part, cannot be
said to have been equal to the task to which they were applied. The
truncature of the southern horn first spoken of by Schröter, was thought
by Denning, in 1882, to be obvious; and in the same year, by watching
the displacement of certain bright and dusky spaces on the disc, the
same observer concluded that a rotation period of about 25 hours was
indicated.
In 1882 Schiaparelli at Milan commenced a prolonged study of Mercury.
Believing that it was essential to observe through a good condition of
atmosphere, and that this was impossible if the planet were only looked
at in twilight, when it was necessarily at a low altitude, Schiaparelli
made all his observations with the Sun and planet high up in the
heavens. He considered, in effect, that the blaze of the Sun’s light was
a lesser evil than the tremors inseparable from observations of the
planet, clear it might be in some degree of inconvenient Sun-light, but
viewed through the vapours and atmospheric disturbances, which always
spoil all observations near the horizon. Schiaparelli’s observations
yielded various results, most of them novel, and one of them very
startling. He considers Mercury to be a much spotted globe and to be
enveloped in a tolerably dense atmosphere. He thought he noticed
brownish stripes and streaks (which might be regarded as permanent
markings), more clearly visible on some occasions than on others; and
that these systematically disappeared near the limb, owing to the
increased depth there of the atmosphere through which they had to be
looked at.
The foregoing observations may be regarded as not unreasonable; they may
even be accepted without further question. But what are we to say to
Schiaparelli’s conclusions that these markings are so nearly permanent,
taking one day with another, that Mercury’s rotation cannot be measured
in hours at all, but is a matter of days,—in point of fact, of 88 days;
and that in reality Mercury occupies in its rotation on its axis the
whole of the 88 days which constitute its sidereal year, or period of
revolution round the Sun. The counterpart of this for us would be that,
instead of the inhabitants of the earth having a day of 24 hours, they
would have only one day and night every 365 days. Astronomers are not at
present satisfied to accept this conclusion in regard to Mercury.
Some observers have thought that Mercury is more easy to observe than
Venus, and that, speaking generally, its surface, if we could only get
to see it constantly under favourable circumstances, might be considered
to resemble in most respects that of Mars. Mercury revolves round the
Sun at a mean distance of 36 millions of miles. Owing, however, to the
fact that the eccentricity of its orbit (or its departure from the
circular form) is greater than that of any of the other major planets,
it may approach to within 28½ millions of miles or recede to more than
43 millions of miles. Its apparent diameter varies between 4½″ in
superior conjunction to 13″ in inferior conjunction. The real diameter
may be taken at about 3000 miles.