VENUS
Next in order to Mercury, proceeding outwards from the sun, comes the
planet Venus, the twin-sister, so to speak, of the earth, and familiar
more or less to everybody as the Morning and Evening Star. The
diameter of Venus, according to Barnard's measures with the 36-inch
telescope of the Lick Observatory, is 7,826 miles; she is therefore
a little smaller than our own world. Her distance from the sun is a
trifle more than 67,000,000 miles, and her orbit, in strong contrast
with that of Mercury, departs very slightly from the circular. Her
density is a little less than that of the earth.
There is no doubt that, to the unaided eye, Venus is by far the most
beautiful of all the planets, and that none of the fixed stars can
for a moment vie with her in brilliancy. In this respect she is
handicapped by her position as an inferior planet, for she never
travels further away from the sun than 48°, and, even under the most
favourable circumstances, cannot be seen for much more than four hours
after sunset. Thus we never have the opportunity of seeing her, as
Mars and Jupiter can be seen, high in the South at midnight, and far
above the mists of the horizon. Were it possible to see her under such
conditions, she would indeed be a most glorious object. Even as it
is, with all the disadvantages of a comparatively low position and a
denser stratum of atmosphere, her brilliancy is extremely striking,
having been estimated, when at its greatest, at about nine times that
of Sirius, which is the brightest of all the fixed stars, and five
times that of Jupiter when the giant planet is seen to the best
advantage. It is, in fact, so great that, when approaching its
maximum, the shadows cast by the planet's light are readily seen, more
especially if the object casting the shadow have a sharply defined
edge, and the shadow be received upon a white surface--of snow, for
instance. This extreme brilliance points to the fact that the surface
of Venus reflects a very large proportion of the sunlight which falls
upon it--a proportion estimated as being at least 65 per cent., or
not very much less than that reflected by newly fallen snow. Such
reflective power at once suggests an atmosphere very dense and heavily
cloud-laden; and other observations point in the same direction.
So that in the very first two planets of the system we are at once
confronted with that diversity in details which coexists throughout
with a broad general likeness as to figure, shape of orbit, and other
matters. Mercury's reflective power is very small, that of Venus is
exceedingly great; Mercury's atmosphere seems to be very attenuated,
that of Venus, to all appearance, is much denser than that of our own
earth.
Periodically, when Venus appears in all her splendour in the Western
sky, one meets with the suggestion that we are having a re-appearance
of the Star of Bethlehem; and it seems to be a perpetual puzzle to
some people to understand how the same body can be both the Morning
and the Evening Star. Those who have paid even the smallest attention
to the starry heavens are not, however, in the least likely to make
any mistake about the sparkling silver radiance of Venus; and it
would seem as though the smallest application of common-sense to the
question of the apparent motion of a body travelling round an almost
circular orbit which is viewed practically edgewise would solve for
ever the question of the planet's alternate appearance on either side
of the sun. Such an orbit must appear practically as a straight line,
with the sun at its middle point, and along this line the planet will
appear to travel like a bead on a wire, appearing now on one side of
the sun, now on another. If the reader will draw for himself a diagram
of a circle (sufficiently accurate in the circumstances), with the
sun in the centre, and divide it into two halves by a line supposed to
pass from his eye through the sun, he will see at once that when this
circle is viewed edgewise, and so becomes a straight line, a planet
travelling round it is bound to appear to move back and forward along
one half of it, and then to repeat the same movement along the other
half, passing the sun in the process.
Like Mercury, and for the same reason of a position interior to our
orbit, Venus exhibits phases to us, appearing as a fully illuminated
disc when she is furthest from the earth, as a half-moon at the
two intermediate points of her orbit, and as a new moon when she is
nearest to us. The actual proof of the existence of these phases was
one of the first-fruits which Galileo gathered by means of his
newly invented telescope. It is said that Copernicus predicted their
discovery, and they certainly formed one of the conclusive proofs
of the correctness of his theory of the celestial system. It was the
somewhat childish custom of the day for men of science to put forth
the statement of their discoveries in the form of an anagram, over
which their fellow-workers might rack their brains; probably this was
done somewhat for the same reason which nowadays makes an inventor
take out a patent, lest someone should rob the discoverer of the
credit of his discovery before he might find it convenient to make
it definitely public. Galileo's anagram, somewhat more poetically
conceived than the barbarous alphabetic jumble in which Huygens
announced his discovery of the nature of Saturn's ring, read as
follows: 'Hæc immatura a me jam frustra leguntur o. y.' This,
when transposed into its proper order, conveyed in poetic form the
substance of the discovery: 'Cynthiæ figuras æmulatur Mater Amorum'
(The Mother of the Loves [Venus] imitates the phases of Cynthia). It
is true that two letters hang over the end of the original sentence,
but too much is not to be expected of an anagram.
As a telescopic object, Venus is apt to be a little disappointing. Not
that her main features are difficult to see, or are not beautiful. A
2-inch telescope will reveal her phases with the greatest ease, and
there are few more exquisite sights than that presented by the silvery
crescent as she approaches inferior conjunction. It is a picture which
in its way is quite unique, and always attractive even to the most
hardened telescopist.
Still, what the observer wants is not merely confirmation of the
statement that Venus exhibits phases. The physical features of a
planet are always the most interesting, and here Venus disappoints.
That very brilliant lustre which makes her so beautiful an object to
the naked eye, and which is even so exquisite in the telescopic view,
is a bar to any great progress in the detection of the planet's actual
features. For it means that what we are seeing is not really the
surface of Venus, but only the sunward side of a dense atmosphere--the
'silver lining' of heavy clouds which interpose between us and the
true surface of the planet, and render it highly improbable that
anything like satisfactory knowledge of her features will ever be
attained. Newcomb, indeed, roundly asserts that all markings hitherto
seen have been only temporary clouds and not genuine surface markings
at all; though this seems a somewhat absolute verdict in view of the
number of skilled observers who have specially studied the planet and
assert the objective reality of the markings they have detected. The
blunting of the South horn of the planet, visible in Mr. MacEwen's
fine drawing (Plate X.), is a feature which has been noted by so many
observers that its reality must be conceded. On the other hand, some
of the earlier observations recording considerable irregularities of
the terminator (margin of the planet between light and darkness), and
detached points of light at one of the horns, must seemingly be given
up. Denning, one of the most careful of observers, gives the following
opinion: 'There is strong negative evidence among modern observations
as to the existence of abnormal features, so that the presence of
very elevated mountains must be regarded as extremely doubtful....
The detached point at the South horn shown in Schröter's telescope
was probably a false appearance due to atmospheric disturbances or
instrumental defects.' It will be seen, therefore, that the observer
should be very cautious in inferring the actual existence of any
abnormal features which may be shown by a small telescope; and
the more remarkable the features shown, the more sceptical he may
reasonably be as to their reality. The chances are somewhat heavily
in favour of their disappearance under more favourable conditions of
seeing.
[Illustrations (2):
PLATE X.
Venus. H. MacEwen. 5-inch Refractor.]
The same remark applies, with some modifications, to the dark markings
which have been detected on the planet by all sorts of observers
with all sorts of telescopes. There is no doubt that faint grey
markings, such as those shown in Plate X., are to be seen; the
observations of many skilled observers put this beyond all question.
Even Denning, who says that personally he has sometimes regarded the
very existence of these markings as doubtful, admits that 'the evidence
affirming their reality is too weighty and too numerously attested to
allow them to be set aside'; and Barnard, observing with the Lick
telescope, says that he has repeatedly seen markings, but always so
'vague and ill-defined that nothing definite could be made of them.'
The observations of Lowell and Douglass at Flagstaff, Arizona, record
quite a different class of markings, consisting of straight, dark,
well-defined lines; as yet, however, confirmation of these remarkable
features is scanty, and it will be well for the beginner who, with a
small telescope and in ordinary conditions of observing, imagines he
has detected such markings to be rather more than less doubtful about
their reality. The faint grey areas, which are real features, at least
of the atmospheric envelope, if not of the actual surface, are
beyond the reach of small instruments. Mr. MacEwen's drawings, which
accompany this chapter, were made with a 5-inch Wray refractor, and
represent very well the extreme delicacy of these markings. I have
suspected their existence when observing with an 8-1/2-inch With
reflector in good air, but could never satisfy myself that they were
really seen.
Up till the year 1890 the rotation period of Venus was usually stated
at twenty-three hours twenty-one minutes, or thereby, though this
figure was only accepted with some hesitation, as in order to
arrive at it there had to be some gentle squeezing of inconvenient
observations. But in that year Schiaparelli announced that his
observations were only consistent with a long period of rotation,
which could not be less than six months, and was not greater
than nine. The announcement naturally excited much discussion.
Schiaparelli's views were strongly controverted, and for a time the
astronomical world seemed to be almost equally divided in opinion.
Gradually, however, the conclusion has come to be more and more
accepted that Venus, like Mercury, rotates upon her axis in the same
time as she takes to make her journey round the sun--in other words,
that her day and her year are of the same length, amounting to about
225 of our days. In 1900 the controversy was to some extent reopened
by the statement of the Russian astronomer Bélopolsky that his
spectroscopic investigations pointed to a much more rapid rotation--to
a period, indeed, considerably shorter than twenty-four hours. It
is difficult, however, to reconcile this with the absence of polar
flattening in the globe of Venus. Lowell's spectroscopic observations
are stated by him to point to a period in accordance with his
telescopic results--namely, 225 days. The matter can scarcely be
regarded as settled in the meantime, but the balance of evidence seems
in favour of the longer period.
Another curious and unexplained feature in connection with the planet
is what is frequently termed the 'phosphorescence' of the dark side.
This is an appearance precisely similar to that seen in the case of
the moon, and known as 'the old moon in the young moon's arms.' The
rest of the disc appears within the bright crescent, shining with a
dull rusty light. In the case of Venus, however, an explanation is
not so easily arrived at as in that of the moon, where, of course,
earth-light accounts for the visibility of the dark portion. Had the
planet been possessed of a satellite, the explanation might have lain
there; but Venus has no moon, and therefore no moonlight to brighten
her unilluminated portion; and our world is too far distant for
earth-shine to afford an explanation. It has been suggested that
electrical discharges similar to the aurora may be at the bottom of
the mystery; but this seems a little far-fetched, as does also the
attribution of the phenomenon to real phosphorescence of the oceans
of Venus. Professor Newcomb cuts the Gordian knot by observing: 'It
is more likely due to an optical illusion.... To whatever we might
attribute the light, it ought to be seen far better after the end of
twilight in the evening than during the daytime. The fact that it is
not seen then seems to be conclusive against its reality.' But the
appearance cannot be disposed of quite so easily as this, for it is
not accurate to say that it is only seen in the daytime, and against
Professor Newcomb's dictum may be set the judgment of the great
majority of the observers who have made a special study of the planet.
We may, however, safely assign to the limbo of exploded ideas that of
the existence of a satellite of Venus. For long this object was one
of the most persistent of astronomical ghosts, and refused to be
laid. Observations of a companion to the planet, much smaller, and
exhibiting a similar phase, were frequent during the eighteenth
century; but no such object has presented itself to the far finer
instruments of modern times, and it may be concluded that the moon of
Venus has no real existence.
Venus, like Mercury, transits the sun's disc, but at much longer
intervals which render her transits among the rarest of astronomical
events. Formerly they were also among the most important, as they were
believed to furnish the most reliable means for determining the sun's
distance; and most of the estimates of that quantity, up to within the
last twenty-five years, were based on transit of Venus observations.
Now, however, other methods, more reliable and more readily
applicable, are coming into use, and the transit has lost somewhat of
its former importance. The interest and beauty of the spectacle still
remain; but it is a spectacle not likely to be seen by any reader of
these pages, for the next transit of Venus will not take place until
June, 2004.
As already indicated, Venus presents few opportunities for useful
observation to the amateur. The best time for observing, as in
the case of Mercury, is in broad daylight; and for this, unless in
exceptional circumstances, graduated circles and a fairly powerful
telescope are required. Practically the most that can be done by the
possessor of a small instrument is to convince himself of the reality
of the phases, and of the non-existence of a satellite of any size,
and to enjoy the exquisite and varying beauty of the spectacle
which the planet presents. Should his telescope be one of the small
instruments which show hard and definite markings on the surface,
he may also consider that he has learned a useful lesson as to the
possibility of optical illusion, and, incidentally, that he may be
well advised to procure a better glass when the opportunity of doing
so presents itself. The 'phosphorescence' of the dark side may be
looked for, and it may be noted whether it is not seen after dark,
or whether it persists and grows stronger. Generally speaking,
observations should be made as early in the evening as the planet can
be seen in order that the light of the sky may diminish as much as
possible the glare which is so evident when Venus is viewed against a
dark background.