Myths and Marvels of Astronomy
either a direct or an inverted image, but it could not possibly be a
diminished image.
Single observations like Cassini's or Short's might be explained as
subjective phenomena, but this explanation will not avail in the case of
the Copenhagen observations.
I reject, as every student of astronomy will reject, the idea of wilful
deception. Occasionally an observer may pretend to see what he has not
seen, though I believe this very seldom happens. But even if Cassini and
the rest had been notoriously untrustworthy persons instead of being
some of them distinguished for the care and accuracy with which their
observations were made and recorded, these occasional views of a phantom
satellite are by no means such observations as they would have invented.
No distinction was to be gained by observations which could not be
confirmed by astronomers possessing more powerful telescopes. Cassini,
for example, knew well that nothing but his well-earned reputation could
have saved him from suspicion or ridicule when he announced that he had
seen Venus attended by a satellite.
It seems to me probable that the false satellite was an optical illusion
brought about in a different way from those referred to by Hell and
Brewster, though among the various circumstances which in an imperfect
instrument might cause such a result I do not undertake to make a
selection. It is certain that Venus's satellite has vanished with the
improvement of telescopes, while it is equally certain that even with
the best modern instruments illusions occasionally appear which deceive
even the scientific elect. Three years have passed since I heard the
eminent observer Otto Struve, of Pulkowa, give an elaborate account of a
companion to the star Procyon, describing the apparent brightness,
distance, and motions of this companion body, for the edification of the
Astronomer-Royal and many other observers. I had visited but a few
months before the Observatory at Washington, where, with a much more
powerful telescope, that companion to Procyon had been systematically
but fruitlessly sought for, and I entertained a very strong opinion,
notwithstanding the circumstantial nature of Struve's account and his
confidence (shared in unquestioningly by the observers present), that he
had been in some way deceived. But I could not then see, nor has any one
yet explained, how this could be. The fact, however, that he had been
deceived is now undoubted. Subsequent research has shown that the
Pulkowa telescope, though a very fine instrument, possesses the
undesirable quality of making a companion orb for all first-class stars
in the position where O. Struve and his assistant Lindenau saw the
supposed companion of Procyon.
I may as well point out, however, that theories so wild have recently
been broached respecting Venus, that far more interesting explanations
of the enigma than this optical one may be looked for presently. It has
been gravely suggested by Mr. Jos. Brett, the artist, that Venus has a
surface of metallic brilliancy, with a vitreous atmosphere,--which can
only be understood to signify a glass case. This stupendous theory has
had its origin in an observation of considerable interest which
astronomers (it is perhaps hardly necessary to say) explain somewhat
differently. When Venus has made her entry in part upon the sun's face
at the beginning of transit, there is seen all round the portion of her
disc which still remains outside the sun an arc of light so brilliant
that it records its photographic trace during the instantaneous exposure
required in solar photography. It is mathematically demonstrable that
this arc of light is precisely what _should_ be seen if Venus has an
atmosphere like our earth's. But mathematical demonstration is not
sufficient (or perhaps we may say it is too much) for some minds.
Therefore, to simplify matters, Venus has been provided with a mirror
surface and a glass case. (See preceding essay, on Astronomical
Paradoxes, for further details.)
The enigma next to be considered is of a more doubtful character than
the myth relating to the satellite of Venus. Astronomers are pretty well
agreed that Venus has no moon, but many, including some deservedly
eminent, retain full belief in the story of the planet Vulcan.
More than seventeen years ago the astronomical world was startled by the
announcement that a new planet had been discovered, under circumstances
unlike any which had heretofore attended the discovery of fresh members
of the solar system. At that time astronomers had already become
accustomed to the discovery, year after year, of several asteroids,
which are in reality planets, though small ones. In fact, no less than
fifty-six of these bodies were then known, whereof fifty-one had been
discovered during the years 1847-1858 inclusive, not one of these years
having passed without the detection of an asteroid. But all these
planets belonged to one family, and as there was every reason to believe
that thousands more travel in the same region of the solar system, the
detection of a few more among the number had no longer any special
interest for astronomers. The discovery of the first known member of the
family had indeed been full of interest, and had worthily inaugurated
the present century, on the first day of which it was made. For it had
been effected in pursuance of a set scheme, and astronomers had almost
given up all hopes of success in that scheme when Piazzi announced his
detection of little Ceres. Again the discovery of the next few members
of the family had been interesting as revealing the existence of a new
order of bodies in the solar system. No one had suspected the
possibility that besides the large bodies which travel round the sun,
either singly or attended by subordinate families of moons, there might
be a ring of many planets. This was what the discovery of Ceres, Pallas,
Juno, and Vesta seemed to suggest, unless--still stranger thought--these
were but fragments of a mighty planet which had been shattered in
long-past ages by some tremendous explosion. Since then, however, this
startling theory has been (itself) exploded. Year after year new members
of the ring of multitudinous planets are discovered, and that, not as
was recently predicted, in numbers gradually decreasing, but so rapidly
that more have been discovered during the last ten years than during the
preceding twenty.
The discovery of the giant planet Uranus, an orb exceeding our earth
twelve and a half times in mass and seventy-four times in volume, was a
matter of much greater importance, so far as the dignity of the
planetary system was concerned, for it is known that the whole ring of
asteroids together does not equal one-tenth part of the earth in mass,
while Uranus exceeds many times in volume the entire family of
terrestrial planets--Mercury, Venus, the Earth, and Mars. The detection
of Uranus, unlike that of Ceres, was effected by accident. Sir W.
Herschel was looking for double stars of a particular kind in the
constellation Gemini when by good fortune the stranger was observed.
The interest with which astronomers received the announcement of the
discovery of Uranus, though great, was not to be compared with that with
which they deservedly welcomed the discovery of Neptune, a larger and
more massive planet, revolving at a distance one-half greater even than
the mighty space which separates Uranus from the sun, a space so great
that by comparison with it the range of 184,000,000 of miles, which
forms the diameter of our earth's orbit, seems quite insignificant. It
was not, however, the vastness of Neptune's mass or volume, or the awful
remoteness of the path along which he pursues his gloomy course, which
attracted the interest of astronomers, but the strangeness of the
circumstances under which the planet had been detected. His influence
had been felt for many years before astronomers thought of looking for
him, and even when the idea had occurred to one or two, it was
considered, and that, too, by an astronomer as deservedly eminent as Sir
G. Airy, too chimerical to be reasonably entertained. All the world now
knows how Leverrier, the greatest living master of physical astronomy,
and Adams, then scarce known outside Cambridge, both conceived the idea
of finding the planet, not by the simple method of looking for it with a
telescope, but by the mathematical analysis of the planet's disturbing
influence upon known members of the solar system. All know, too, that
these mathematicians succeeded in their calculations, and that the
planet was found in the very region and close to the very point
indicated first by Adams, and later, but independently, and (fortunately
for him more publicly) by Leverrier.
None of these instances of the discovery of members of the solar system
resembled in method or details the discovery announced early in the year
1859. It was not amid the star-depths and in the darkness of night that
the new planet was looked for, but in broad day, and on the face of the
sun himself. It was not on the outskirts of the solar system that the
planet was supposed to be travelling, but within the orbit of Mercury,
hitherto regarded as of all planets the nearest to the sun. It was not
hoped that any calculation of the perturbations of other planets would
show the place of the stranger, though certain changes in the orbit of
Mercury seemed clearly enough to indicate the stranger's existence.
Early in 1860 Leverrier had announced that the position of Mercury's
path was not precisely in agreement with calculations based on the
adopted estimates of the masses of those planets which chiefly disturb
the motions of Mercury. The part of the path where Mercury is nearest to
the sun, and where, therefore, he travels fastest, had slightly shifted
from its calculated place. This part of the path was expected to move,
but it had moved more than was expected; and of course Mercury having
his region of swiftest motion somewhat differently placed than was
anticipated, himself moved somewhat differently.
Leverrier found that to explain this feature of Mercury's motion either
the mass of Venus must be regarded as one-tenth greater than had been
supposed, or some unknown cause must be regarded as affecting the motion
of Mercury. A planet as large as Mercury, about midway between Mercury
and the sun, would account for the observed disturbance; but Leverrier
rejected the belief that such a planet exists, simply because he could
not 'believe that it would be invisible during total eclipses of the
sun.' 'All difficulties disappear,' he added, 'if we admit, in place of
a single planet, small bodies circulating between Mercury and the sun.'
Considering their existence as not at all improbable, he advised
astronomers to watch for them.
It was on January 2, 1860, that Leverrier thus wrote. On December 22,
1859, a letter had been addressed by a M. Lescarbault of Orgeres to
Leverrier, through M. Vallee, hon. inspector-general of roads and
bridges, announcing that on March 26, 1859, about four in the afternoon,
Lescarbault had seen a round black spot on the face of the sun, and had
watched it as it passed across like a planet in transit--not with the
slow motion of an ordinary sun-spot. The actual time during which the
round spot was visible was one hour, seventeen minutes, nine seconds,
the rate of motion being such that, had the spot crossed the middle of
the sun's disc, at the same rate, the transit would have lasted more
than four hours. The spot thus merely skirted the sun's disc, being at
no time more than about one forty-sixth part of the sun's apparent
diameter from the edge of the sun. Lescarbault expressed his conviction
that on a future day, a black spot, perfectly round and very small, will