Mercury
As the elongation of Mercury from the sun seldom exceeds 18°, it is a
difficult object, at least in this country, to see without a telescope. As
the poet says, the planet--
"Can scarce be caught by philosophic eye
Lost in the near effulgence of its blaze."
Tycho Brahé, however, records several observations of Mercury with the
unaided vision in Denmark.
It can be occasionally caught with the naked eye in this country after
sunset, when it is favourably placed for observation, and I have so seen
it several times in Ireland. On February 19, 1888, I found it very visible
in strong twilight near the western horizon, and apparently brighter than
an average star of the first magnitude would be in the same position. In
the clear air of the Punjab sky I observed Mercury on November 24-29,
1872, near the western horizon after sunset. Its appearance was that of a
reddish star of the first magnitude. On November 29 I compared its
brilliancy with that of Saturn, which was some distance above it, and
making allowance for the glare near the horizon in which Mercury was
immersed, its brightness appeared to me to be quite equal to that of
Saturn. In June, 1874, I found it equal to Aldebaran, and of very much the
same colour. Mr. W. F. Denning, the famous observer of meteors, states
that he observed Mercury with the naked eye about 150 times during the
years 1868 to 1905.[14]
He found that the duration of visibility after sunset is about 1{h} 40{m}
when seen in March, 1{h} 30{m} in April, and 1{h} 20{m} in May. He thinks
that the planet is, at its brightest, "certainly much brighter than a
first magnitude star."[15] In February, 1868, he found that its brightness
rivalled that of Jupiter, then only 2° or 3° distant. In November, 1882,
it seemed brighter than Sirius. In 1876 it was more striking than Mars,
but the latter was then "faint and at a considerable distance from the
earth."
In 1878, when Mercury and Venus were in the same field of view of a
telescope, Nasmyth found that the surface brightness (or "intrinsic
brightness," as it is called) of Venus was at least twice as great as that
of Mercury; and Zöllner found that from a photometric point of view the
surface of Mercury is comparable with that of the moon.
With reference to the difficulty of seeing Mercury, owing to its proximity
to the sun, Admiral Smyth says, "Although Mercury is never in _opposition_
to the earth, he was, when in the house of Mars, always viewed by
astrologers as a most malignant planet, and one full of evil influences.
The sages stigmatized him as a false deceitful star (_sidus dolosum_), the
eternal torment of astronomers, eluding them as much as terrestrial
mercury did the alchemists; and Goad, who in 1686 published a whole folio
volume full of astro-meteorological aphorisms, unveiling the choicest
secrets of nature, contemptuously calls Mercury a 'squinting lacquey of
the sun, who seldom shows his head in these parts, as if he was in debt.'
His extreme mobility is so striking that chemists adopted his symbol to
denote quicksilver."[16]
Prof. W. H. Pickering thinks that the shortness of the cusps (or "horns")
of Mercury's disc indicates that the planet's atmosphere is of small
density--even rarer than that of Mars.
The diameter of Mercury is usually stated at about 3000 miles; but a long
series of measures made by Prof. See in the year 1901 make the real
diameter about 2702 miles. This would make the planet smaller than some of
the satellites of the large planets, probably smaller than satellites III.
and IV. of Jupiter, less than Saturn's satellite Titan, and possibly
inferior in size to the satellite of Neptune. Prof. Pickering thinks that
the density of Mercury is about 3 (water = 1). Dr. See's observations show
"no noticeable falling off in the brightness of Mercury near the limb."
There is therefore no evidence of any kind of atmospheric absorption in
Mercury, and the observer "gets the impression that the physical condition
of the planet is very similar to that of our moon."[17]
Schröter (1780-1815) observed markings on Mercury, from which he inferred
that the planet's surface was mountainous, and one of these mountains he
estimated at about 11 miles in height![18] But this seems very doubtful.
To account for the observed irregularities in the motion of Mercury in its
orbit, Prof. Newcomb thinks it possible that there may exist a ring or
zone of "asteroids" a little "outside the orbit of Mercury" and having a
combined mass of "one-fiftieth to one-three-hundredth of the mass of
Venus, according to its distance from Mercury." Prof. Newcomb, however,
considers that the existence of such a ring is extremely improbable, and
regards it "more as a curiosity than a reality."[19]
M. Léo Brenner thinks that he has seen the dark side of Mercury, in the
same way that the dark side of Venus has been seen by many observers. In
the case of Mercury the dark side appeared _darker_ than the background of
the sky. Perhaps this may be due to its being projected on the zodiacal
light, or outer envelope of the sun.[20]
Mercury is said to have been occulted by Venus in the year 1737.[21] But
whether this was an actual occultation, or merely a near approach does not
seem to be certain.
The first transit of Mercury across the sun's disc was observed by
Gassendi on November 6, 1631, and Halley observed one on November 7, 1677,
when in the island of St. Helena.
Seen from Mercury, Venus would appear brighter than even we see it, and as
it would be at its brightest when in opposition to the sun, and seen on a
dark sky with a full face, it must present a magnificent appearance in the
midnight sky of Mercury. The earth will also form a brilliant object, and
the moon would be distinctly visible. The other planets would appear very
much as they do to us, but with somewhat less brilliancy owing to their
greater distance.
As the existence of an intra-Mercurial planet (that is a planet revolving
round the sun within the orbit of Mercury) seems now to be very
improbable, Prof. Perrine suggests that possibly "the finely divided
matter which produces the zodiacal light when considered in the aggregate
may be sufficient to cause the perturbations in the orbit of Mercury."[22]
Prof. Newcomb, however, questions the exact accuracy of Newton's law, and
seems to adopt Hall's hypothesis that gravity does not act _exactly_ as
the inverse square of the distance, and that the exponent of the distance
is not 2, but 2·0000001574.[23]
Voltaire said, "If Newton had been in Portugal, and any Dominican had
discovered a heresy in his inverse ratio of the squares of the distances,
he would without hesitation have been clothed in a _san benito_, and burnt
as a sacrifice to God at an _auto da fé_."[24]
An occultation of Mercury by Venus was observed with a telescope on May
17, 1737.[25]
May transits of Mercury across the sun's disc will occur in the years
1924, 1957, and 1970; and November transits in the years 1914, 1927, and
1940.[26]
From measurements of the disc of Mercury during the last transit, M. R.
Jonckheere concludes that the _polar_ diameter of the planet is greater
than the _equatorial_! His result, which is very curious, if true, seems
to be supported by the observations of other observers.[27]
The rotation period of Mercury, or the length of its day, seems to be
still in doubt. From a series of observations made in the years 1896 to
1909, Mr. John McHarg finds a period of 1·0121162 day, or 1{d} 0{h} 17{m}
26{s}·8. He thinks that "the planet possesses a considerable atmosphere
not so clear as that of Mars"; that "its axis is very considerably
tilted"; and that it "has fairly large sheets of water."[28]