MERCURY
The planet nearest to the sun is not one which has proved itself
particularly attractive to observers in the past; and the reasons for
its comparative unattractiveness are sufficiently obvious. Owing to
the narrow limits of his orbit, he never departs further from the sun
either East or West than between 27° and 28°, and the longest period
for which he can be seen before sunrise or after sunset is two hours.
It follows that, when seen, he is never very far from the horizon,
and is therefore enveloped in the denser layers of our atmosphere, and
presents the appearance sadly familiar to astronomers under the name
of 'boiling,' the outlines of the planet being tremulous and confused.
Of course, observers who have powerful instruments provided with
graduated circles can find and follow him during the day, and it is in
daylight that nearly all the best observations have been secured. But
with humbler appliances observation is much restricted; and, in fact,
probably many observers have never seen the planet at all.
Views of Mercury, however, such as they are, are by no means so
difficult to secure as is sometimes supposed. Denning remarks that
he has seen the planet on about sixty-five occasions with the naked
eye--that in May, 1876, he saw it on thirteen different evenings, and
on ten occasions between April 22 and May 11, 1890; and he states
it as his opinion that anyone who will make it a practice to obtain
naked-eye views should succeed from about twelve to fifteen times in
the year. During the spring of 1905, to take a recent example, Mercury
was quite a conspicuous object for some time in the Western sky, close
to the horizon, and there was no difficulty whatever in obtaining
several views of him both with the telescope and with the naked eye,
though the disc was too much disturbed by atmospheric tremors
for anything to be made of it telescopically. In his little book,
'Half-hours with the Telescope,' Proctor gives a method of finding the
planet which would no doubt prove quite satisfactory in practice, but
is somewhat needlessly elaborate. Anyone who takes the pains to
note those dates when Mercury is most favourably placed for
observation--dates easily ascertained from Whitaker or any other good
almanac--and to carefully scan the sky near the horizon after sunset
either with the naked eye, or, better, with a good binocular, will
scarcely fail to detect the little planet which an old English writer
more graphically than gracefully calls 'a squinting lacquey of the
sun.'
Mercury is about 3,000 miles in diameter, and circles round the sun at
a mean distance of 36,000,000 miles. His orbit is very eccentric, so
that when nearest to the sun this distance is reduced to 28,500,000,
while when furthest away from him it rises to 43,500,000. The
proportion of sunlight which falls upon the planet must therefore vary
considerably at different points of his orbit. In fact, when he is
nearest to the sun he receives nine times as much light and heat
as would be received by an equal area of the earth; but when the
conditions are reversed, only four times the same amount. The bulk of
the planet is about one-nineteenth that of the earth, but its weight
is only one-thirtieth, so that its materials are proportionately less
dense than those of our own globe. It is about 3-1/2 times as dense as
water, the corresponding figure for the earth being rather more than
5-1/2.
Further, it is apparent that the materials of which Mercury's globe
is composed reflect light very feebly. It has been calculated that the
planet reflects only 17 per cent. of the light which falls upon it, 83
per cent. being absorbed; and this fact obviously carries with it the
conclusion that the atmosphere of this little world cannot be of any
great density. For clouds in full sunlight are almost as brilliantly
white as new-fallen snow, and if Mercury were surrounded with a
heavily cloud-laden atmosphere, he would reflect nearly five times the
amount of light which he at present sends out into space.
As his orbit falls entirely within that of our own earth, Mercury,
like his neighbour Venus, exhibits phases. When nearest to us the
planet is 'new,' when furthest from us it is 'full,' while at the
stages intermediate between these points it presents an aspect like
that of the moon at its first and third quarters. It may thus be
seen going through the complete series from a thin crescent up to a
completely rounded disc. The smallness of its apparent diameter,
and the poor conditions under which it is generally seen, make the
observation of these phases by no means so easy as in the case of
Venus; yet a small instrument will show them fairly well. Observers
seem generally to agree that the surface has a dull rosy tint, and a
few faint markings have, by patient observation, been detected upon
it (Fig. 20); but these are far beyond the power of small telescopes.
Careful attention to them and to the rate of their apparent motion
across the disc has led to the remarkable conclusion that Mercury
takes as long to rotate upon his axis as he does to complete his
annual revolution in his orbit; in other words, that his day and his
year are of the same length--namely, eighty-eight of our days. This
conclusion, when announced in 1882 by the well-known Italian observer
Schiaparelli, was received with considerable hesitation. It has,
however, been confirmed by many observers, notably by Lowell at
Flagstaff Observatory, Arizona, and is now generally received,
though some eminent astronomers still maintain that really nothing is
certainly known as to the period of rotation.
[Illustration: FIG. 20.--MERCURY AS A MORNING STAR. W. F. DENNING,
10-INCH REFLECTOR.]
If the long period be accepted, it follows that Mercury must always
turn the same face to the sun--that one of his hemispheres must always
be scorching under intense heat, and the other held in the grasp of an
unrelenting cold of which we can have no conception. 'The effects of
these arrangements upon climate,' says Miss Agnes Clerke, 'must be
exceedingly peculiar.... Except in a few favoured localities, the
existence of liquid water must be impossible in either hemisphere.
Mercurian oceans, could they ever have been formed, should long ago
have been boiled off from the hot side, and condensed in "thick-ribbed
ice" on the cold side.'
From what has been said it will be apparent that Mercury is scarcely
so interesting a telescopic object as some of the other planets. Small
instruments are practically ruled out of the field by the diminutive
size of the disc which has to be dealt with, and the average
observer is apt to be somewhat lacking in the patience without which
satisfactory observations of an object so elusive cannot be secured.
At the same time there is a certain amount of satisfaction and
interest in the mere detection of the little sparkling dot of light in
the Western sky after the sun has set, or in the Eastern before it has
risen; and the revelation of the planet's phase, should the telescope
prove competent to accomplish it, gives better demonstration than any
diagram can convey of the interior position of this little world. It
is consoling to think that even great telescopes have made very little
indeed of the surface of Mercury. Schiaparelli detected a number
of brownish stripes and streaks, which seemed to him sufficiently
permanent to be made the groundwork of a chart, and Lowell has made
a remarkable series of observations which reveal a globe seamed and
scarred with long narrow markings; but many observers question the
reality of these features altogether.
It is perhaps just within the range of possibility that, even with
a small instrument, there may be detected that blunting of the South
horn of the crescent planet which has been noticed by several reliable
observers. But caution should be exercised in concluding that such a
phenomenon has been seen, or that, if seen, it has been more than
an optical illusion. Those who have viewed Mercury under ordinary
conditions of observation will be well aware how extremely difficult
it is to affirm positively that any markings on the surface or any
deformations of the outline of the disc are real and actual facts, and
not due to the atmospheric tremors which affect the little image.
Interesting, though of somewhat rare occurrence, are the transits of
Mercury, when the planet comes between us and the sun, and passes as a
black circular dot across the bright solar surface. The first occasion
on which such a phenomenon was observed was November 7, 1631. The
occurrence of this transit was predicted by Kepler four years in
advance; and the transit itself was duly observed by Gassendi, though
five hours later than Kepler's predicted time. It gives some idea
of the uncertainty which attended astronomical calculations in those
early days to learn that Gassendi considered it necessary to begin
his observations two days in advance of the time fixed by Kepler.
If, however, the time of a transit can now be predicted with almost
absolute accuracy, it need not be forgotten that this result is
largely due to the labours of men who, like Kepler, by patient effort
and with most imperfect means, laid the foundations of the most
accurate of all sciences.
The next transit of Mercury available for observation will take place
on November 14, 1907. It may be noted that during transits certain
curious appearances have been observed. The planet, for example,
instead of appearing as a black circular dot, has been seen surrounded
with a luminous halo, and marked by a bright spot upon its dark
surface. No satisfactory explanation of these appearances has been
offered, and they are now regarded as being of the nature of optical
illusions, caused by defects in the instruments employed, or by
fatigue of the eye. It might, however, be worth the while of any who
have the opportunity of observing the transit of 1907 to take notice
whether these features do or do not present themselves. For their
convenience it may be noted that the transit will begin about eleven
o'clock on the forenoon of November 14, and end about 12.45.