VENUS.
The planet Venus has two things in common with Mercury. One is, that
being an inferior planet, that is to say, a planet revolving round the
Sun in an orbit within that of the Earth, it is never very far distant
from the Sun, and therefore can never be seen on a distinctly dark sky.
The second point alluded to arises out of the first; Venus exhibits from
time to time a series of phases which are identical in character with
those of Mercury, and therefore with those of the Moon. Venus differs,
however, from Mercury in the very important point of size. Inasmuch as
its diameter is considerably more than double the diameter of Mercury it
has a surface more than six times as great, and therefore exhibits a far
larger area of illumination than Mercury does. The result of this
(coupled with another fact which will be stated presently) is that the
planet may often be easily seen in broad daylight, and sometimes casts a
sensible shadow at night. Under special circumstances, which recur every
8 years, this planet shines with very peculiar brilliancy. True, that
only about ¼th of the whole disc is then illuminated, but that fraction
transmits to us more light than phases of greater extent do, because
these latter coincide with epochs when the planet is more remote from
the Earth.
Spots and shadings have on various occasions been noticed on Venus, and
though it is not easy to harmonise the various accounts, there seems no
doubt of the reality of the facts, or that they must be ascribed to the
existence of mountains. Schröter found very much the same state of
things to exist on Venus that he found on Mercury, and putting together
what he saw he arrived at the conclusion that Venus possesses mountains
of considerable height, and that his observations must be taken to imply
that the planet revolved on its axis in rather more than 23 hours. This
conclusion as regards the planet’s axial rotation was not first arrived
at by Schröter, for the two Cassinis, one about 1666, and the other
about 1740, both ascribed to Venus a rotation period of about 23 hours,
an evaluation which was fully confirmed by Di Vico at Rome between 1839
and 1841, and by Flammarion in 1894.
What has been already said with respect to Mercury is true also of
Venus, namely that it has been much neglected by modern observers; and
accordingly an announcement made by Schiaparelli in 1890, that the
rotation period of Venus is to be measured not by hours but by months,
came upon the astronomical world as a startling revelation; but it is a
revelation which has been keenly contested, and certainly awaits legal
proof. Schiaparelli has not ventured to assert as he has done in the
case of Mercury, that Venus’s rotation period is identical with the
period of 7½ months in which it revolves round the Sun; he only claims
this as a strong probability arising out of what he says he is certain
of, namely that its period of rotation cannot be less than six months
and may be as much as nine months. His assumption is that previous
observers in endeavouring to ascertain Venus’s rotation period have used
and relied upon evanescent shadings which probably were of atmospheric
origin and scarcely recognisable from day to day, whereas he fixed his
attention upon round defined white spots, which, whatever their origin,
are so far permanent that their existence has been spoken of for two
centuries. Miss Clarke thus puts the matter:—“His steady watch over them
showed the invariability of their position with regard to the
terminator; and this is as much as to say that the regions of day and
night do not shift on the surface of the planet. In other words she
keeps the same face always turned towards the Sun.”
Various recent observations, some of them made with the express object
of throwing light upon Schiaparelli’s conclusions, are strangely
contradictory. Perrotin at Nice in 1890 thought his observations
confirmed Schiaparelli’s; on the other hand Niesten at Brussels
considered that numerous drawings of Venus made by himself and Stuyvaert
between 1881 and 1890 harmonised well with Di Vico’s rotation period of
23h. 21m. 22s.; which Trouvelot in 1892 only wished to increase to about
24 hours.
There is a general consensus of opinion that great irregularities exist
on the surface of Venus. These are made specially manifest to us in
connection with the terminator or visible edge of the planet seen as an
illuminated crescent. If the planet had a smooth surface this line would
at all times be a perfect and continuous curve, instead of which it is
frequently to be noticed as a jagged or broken line. Observations to
this effect go back as far as 1643, when Fontana at Naples observed this
to be the condition of the terminator. La Hire, Schröter, Mädler, Di
Vico and many others down to the present epoch have noted the same
thing. The fact that the southern horn of Venus is constantly to be seen
blunted is so well established as to admit of no doubt, and this
blunting is commonly ascribed to the existence of a lofty mountain, to
which Schröter ascribed a height of 27 miles. Whatever we may think as
to the precise accuracy of this figure, it seems impossible to doubt the
main fact on which it depends; whilst a Belgian observer, Van Ertborn,
in 1876 repeatedly saw a point of light in this locality which he
regarded as due to Sun-light impinging on a detached peak, adjacent
valleys remaining in shadow. This effect is common enough in the case of
the Moon, and is familiar to all who are in the habit of studying the
Moon.
[Illustration: Fig. 9.—Venus, Dec. 23, 1885.]
The existence on Venus of an atmosphere of considerable density and
extent is well established. Proof of this is to be found in the marked
diminution of the planet’s brilliancy towards the terminator; and in the
faint curved line of light which occasionally may be seen when the
planet is near inferior conjunction. When so situated, so much of the
planet itself as can be seen illuminated shows as a narrow radiant
crescent of light, ending off in two points called indifferently cusps
or horns. It sometimes happens, however, that from the point of each
cusp there runs round to the other cusp a faint continuation of the
crescent, resulting in the general appearance of the planet being that
of a nearly uniform ring of light. There is no known way in which the
Sun can illuminate so much more than the half of Venus so as to permit
of a perfect circle being visible except by supposing that an atmosphere
exists on the planet and refracts (or transmits by bending, as it were,
round the corner) a sufficient amount of Sun-light to give rise to the
appearance in question. Further proof of the existence of an atmosphere
on Venus is obtainable on those very rare occasions when the planet is
seen passing across the disc of the Sun—a phenomenon known as a “Transit
of Venus.” It then nearly always happens that a hazy nebulous ring of
feeble light may be detected encompassing the planet’s disc indicative
of course of the fact that the Sun’s rays are there slightly obstructed
in reaching the eye of an observer on the Earth. Some observers
scrutinising Venus when in transit have thought that they were able to
obtain, by means of the spectroscope, traces of aqueous vapour on the
planet, but the evidence of this does not appear to be altogether clear
or conclusive.
[Illustration: Fig. 10.—Venus near conjunction as a thin crescent, Sept.
21, 1887 (Flammarion).]
Everybody may be presumed to be acquainted with the spectacle popularly
known as “The Old Moon in the New Moon’s Arms” whereby when the Moon is
only about two or three days old and exhibits but a narrow crescent of
bright light, yet the whole outline of the disc is traceable on the sky.
A phenomenon analogous to this may often be seen in the case of Venus
when near its inferior conjunction. With the Moon the cause is due to
the reflection of Earth-light (so to speak) to the Moon, but that
explanation seems inadequate in respect of Venus, because it is
conceived that the amount of Earth-light available is altogether
insufficient for the purpose. Many other explanations have been put
forward including phosphorescence on the surface of Venus, electrical
displays in the nature of terrestrial auroræ, and what not, but it must
be frankly confessed that astronomers are all at sea on the subject.
The existence of snow at the poles of Venus has been suspected by
observers of tried skill and experience such as Phillips and Webb,
though the idea was first broached by Gruithuisen in 1813. Flammarion’s
observations during 1892 and the two following years are distinctly
confirmatory of this idea. He adds that as both polar caps are visible
at the same time the planet’s axis cannot be much inclined to the plane
of its orbit.
Compared with all the other planets the absolute brightness of Venus
stands very high. Of course it must be understood that by this phrase
“absolute brightness” no more is meant than its reflective power. Venus
is what it is by virtue of its power of reflecting Sun-light; presumably
it has no inherent brightness of its own. What its reflective power is
was probably never more effectively brought under the notice of a human
eye than on September 26, 1878, when Nasmyth enjoyed an opportunity of
seeing Venus and Mercury side by side for several hours in the same
field of view. He speaks of Venus as resembling clean silver and Mercury
as nothing better than lead or zinc. Seeing that owing to its greater
proximity to the Sun the light incident on Mercury must be some 3½ times
as strong as the light incident on Venus, it follows that the reflective
power of Venus must be very great. As a matter of fact it has been
calculated to be nearly equal to newly fallen snow; in other words to
reflect fully 70 per cent. of the light which impinges on it.
Venus has no satellite; this fact seems certain. Yet half a dozen or
more observers between 1645 and 1768 discovered such a satellite;
observed it; followed it! This startling mystery, as it really was,
attracted some years ago the attention of a very careful Belgian
observer, Stroobant, who examined in a most painstaking manner all the
recorded observations. His conclusions were that in almost all cases
particular stars (which he identified) were mistaken for a satellite.
Where the object seen was not capable of identification, possibly it was
a minor planet; whilst in one instance it was probable that it was
Uranus which had been seen and regarded as a satellite of Venus.
Venus is perhaps the planet which has most impressed the popular mind.
For the earliest illustration of this statement we must go as far back
as Homer who makes two references to it in the _Iliad_. These, in Pope’s
version, run as follows:—
“As radiant Hesper shines with keener light,
Far beaming o’er the silver host of night.”
—xxii. 399 [318].
“The morning planet told th’ approach of light;
And fast behind, Aurora’s warmer ray
O’er the broad ocean pour’d the golden day.”
—xxiii. 281 [226].
The phases of Venus were first discovered by Galileo and were made known
to the world, or rather to Kepler, in a mystic sentence which has often
been quoted:—
“_Hæc immatura, a me jam frustra leguntur—oy._”
“These things not ripe; at present [read] in vain [by others] are read
by me.”
The former sentence transposed becomes—
_Cynthiæ figuras æmulatur mater amorum._
The mother of loves [Venus] imitates the phases of Cynthia [the Moon].
Venus revolves round the Sun in 224½ days at a mean distance of about 67
millions of miles. Its apparent diameter varies between 9½″ in superior
conjunction, and 62″ in inferior conjunction. The real diameter is about
7500 miles; in other words Venus is nearly as large as the Earth.