William F. Denning.

Telescopic Work for Starlight Evenings online

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of his would-be pursuers; but there is every prospect that a patient
observer, careful to utilize all available opportunities, would soon
gather some profitable data relating to his appearance.

_Transits of Mercury._—One of the most interesting phenomena, albeit a
somewhat rare event, in connection with Mercury, is that of a transit
across the Sun. The planet then appears as a black circular spot.
Observers have noticed one or two very small luminous points on the
black disk, and an annulus has been visible round it. These features
are probably optical effects, and it will be worth while to remember
them on the occasion of future transits, of which the subjoined is a
list:—

1891, May 9.
1894, Nov. 10.
1907, Nov. 12.
1914, Nov. 6.
1924, May 7.
1927, Nov. 8.
1937, May 10.
1940, Nov. 12.
1953, Nov. 13.
1960, Nov. 6.
1970, May 9.
1973, Nov. 9.

The first observer of a transit of Mercury appears to have been
Gassendi, at Paris, on Nov. 7, 1631.

_Occultations of Mercury._—There was an occultation of Mercury by the
Moon on April 25, 1838. It occurred on the day of the planet’s greatest
elongation E., and at a time in the evening when it might have been
most suitably witnessed, but cloudy skies appear to have frustrated
the hopes of intending observers. There was a repetition of the event
on the morning of May 2, 1867, and it occurred, curiously enough, less
than 24 hours after an occultation of Venus.


FOOTNOTES:

[27] Chambers, in his ‘Descriptive Astronomy,’ 4th edition, 1889,
devotes a chapter to the discussion of facts having reference to
Vulcan; and the reader desiring full information will find it here.




CHAPTER VIII.

_VENUS._

Beauty of Venus.—Brilliancy.—Period &c.—Venus as a telescopic
object.—Surface-markings on the planet.—Rotation-period.—Faintness
of the markings.—Twilight on Venus.—Alleged Satellite.—Further
observations required.—Transits of Venus.—Occultations of Venus.

“Friend to mankind, she glitters from afar,—
Now the bright evening, now the morning star.”


_Beauty of Venus._—This planet has an expressive name, and it
naturally leads us to expect that the object to which it is applied
is a beautiful one. The observer will not be disappointed in this
anticipation: he will find Venus the most attractive planet of our
system. No such difficulties are encountered in finding Venus as in
detecting Mercury; for the former recedes to a distance of 47° from
the Sun, and sometimes remains visible 4½ hours after sunset, as in
February 1889. But Venus owes her beauty not so much to favourable
position as to surpassing lustre. None of the other planets can compare
with her in respect to brilliancy. The giant planet Jupiter is pale
beside her, and offers no parallel. Ruddy Mars looks faint in her
presence, and does not assume to rivalry.

This planet alternately adorns the morning and evening sky, as she
reaches her W. and E. elongations from the Sun. The ancients styled her
_Lucifer_ (“the harbinger of day”) when a morning star and _Hesperus_
when an evening star.

_Brilliancy._—Her brightness is such as to lead her to occasionally
become a conspicuous object to the naked eye in daytime, and at night
she casts a perceptible shadow. This is specially the case near the
epoch of her maximum brilliancy, which is attained when the planet
is in a crescent form, with an apparent diameter of about 40″, and
situated some 5 weeks from inferior conjunction. Though only a
fourth part of the disk is then illuminated, it emits more lustre
than a greater phase, because the latter occurs at a wider distance
from the Earth and when the diameter is much less. Her appearance is
sometimes so striking that it is not to be wondered at that people,
not well informed as to celestial events, have attributed it to causes
of unusual nature. When the planet was visible as a morning star in
the autumn of 1887, an idea became prevalent in the popular mind that
the “Star of Bethlehem” had returned, and there were many persons who
submitted to the inconvenience of rising before daylight to gaze upon a
spectacle of such phenomenal import. And they were not disappointed in
the expectancy of beholding a star of extreme beauty, though altogether
wrong in surrounding it with a halo of mystery and wonder.

At intervals of eight years the elongations of Venus are repeated on
nearly the same dates as before, and the planet is presented under very
similar conditions. This is because five synodical periods (nearly = 13
sidereal periods) of Venus are equal to eight terrestrial years. Thus
very favourable E. elongations occurred on May 9, 1860, May 7, 1868,
May 5, 1876, and May 2, 1884; and on April 30, 1892, there will be a
similar elongation.

_Period &c._—Venus moves round the Sun in an orbit of slight
eccentricity, and completes a revolution in 224^d 16^h 49^m 8^s. Her
mean distance from that luminary is 67,000,000 miles. The apparent
diameter of the planet varies from 9″·5 at superior to 65″ at
inferior conjunction, and it averages 25″ at elongations. Her real
diameter is 7500 miles. The polar compression is very slight—in fact,
not sufficiently decided for measurement; this is also true of Mercury.

_Venus as a Telescopic Object._—When the telescope is directed to Venus
it must be admitted that the result hardly justifies the anticipation.
Observers are led to believe, from the beauty of her aspect as viewed
with the unaided eye, that instrumental power will greatly enhance
the picture and reveal more striking appearances than are displayed
on less conspicuous planets. But the hope is illusive. The lustre
of Venus is so strong at night that her disk is rarely defined with
satisfactory clearness; there is generally a large amount of glare
surrounding it, and our instruments undergo a severe ordeal when
their capacities are tested upon this planet. Observations should be
undertaken in the daytime, or near the times of sunrise or sunset,
when the refulgence of this object does not exert itself in extreme
degree. But putting aside the question of definition for the moment,
there are other circumstances which conspire to render the view a
somewhat unattractive one. There are no dark spots, of bold outline,
such as we may plainly discern on Mars, visible on her surface. There
is no wonderful arrangement of luminous rings, such as encircle Saturn.
There are no signs of dark variegated belts, similar to those which
gird both Jupiter and Saturn; nor is there any system of attendant
satellites, such as accompany each of the superior planets. But though
Venus is wanting in these respects, she may yet boast an attraction
which the outer planets can never display to us, namely, the beautiful
crescented phase, which, tradition says, was predicted by Copernicus,
and, when afterwards observed in Galilei’s telescope, justly considered
a convincing fact in support of the Copernican system. The phases are
best seen in strong twilight, whenever Venus is favourably situated.
It has been asserted that the crescent of this planet has been
distinguished with the naked eye; but the statement is undoubtedly
erroneous. Any small glass will show it, however, as it is sometimes
well visible when subtending an angle of 50″ or 55″.

_Surface-markings._—In 1666 and the following year J. D. Cassini
observed several bright spots on Venus and also two obscure markings;
but the latter were extremely faint and of irregular extent, so that
little could be gleaned from them. He watched these forms closely and
remarked certain changes in their positions, which finally enabled him
to determine the period of the planet’s rotation. In 1726 and 1727
Bianchini, at Rome, repeatedly observed dark spots, and their outlines
seem to have been so consistent that he depicted them on a chart and
gave them names. But J. Cassini, at Paris, failed to confirm these
results, though he used telescopes of 82-and 114-feet focus; and it
was supposed the climate of Paris was not suitable for such delicate
observations. Schröter reviewed this planet in 1788 and later years,
and succeeded in detecting various markings and irregularities in the
terminator and cusps. He announced that he had seen the S. horn of the
crescent truncated, so that a bright point was apparently isolated at
its extremity. From this he concluded there must be mountains of great
altitude on the planet, and the perpendicular height of one of these
he computed at 22 miles, which is four times the height of the most
lofty mountain on the Earth. If the surface of Venus were uniformly
level, then her cusps would taper gradually away to points, and no such
deformation as that described by Schröter could possibly be produced.
And 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, if, indeed,
the theory has not to be entirely abandoned. The detached point at the
S. horn shown in Schröter’s telescope was probably a false appearance
due to atmospheric disturbances or instrumental defects. Whenever the
seeing is indifferent, this planet assumes some treacherous features
which are very apt to deceive the observer, especially if his telescope
is faulty. Spurious details are seen, which quite disappear from the
sharp images obtained in steadier air with a good glass. I have never
observed truncation in either of the horns of Venus; but on certain
occasions, when the planet has been ill-defined in passing vapours,
it was most easy to believe that a fragment became detached from the
extremity of the cusp, just in the manner described by Schröter.
But close attention has showed the effect to be false, and revealed
its cause. It was the rippling of the image that gave rise to the
apparently dissevered cusp, in the same way that passing air-waves
and resulting quivers in the image of Saturn’s ring will sometimes
produce displacements, so that the observer momentarily sees several
black divisions, and the edges are multiplied and superimposed one on
another. Refraction, exercised by heated vapours in crossing objects,
is obviously the source of all this.

Sir W. Herschel frequently examined this planet between 1777 and 1793,
but could not discern spots sufficiently definite and durable to enable
him to fix the time of rotation. He dissented from Schröter as to the
alleged mountains, and said, “No eye which is not considerably better
than mine, or assisted by much better telescopes, will ever get a sight
of them.”

Mädler effected some observations of this planet in 1833 and some
subsequent years. He detected spots on two occasions only, but noticed
irregularities in the terminator and cusps. Di Vico and others at Rome,
in 1840-1, devoted much attention to this object, and secured a large
number of observations. They appear to have recovered the spots charted
by Bianchini, and described them as of the last degree of faintness.
The observers who saw the spots most readily were those who had the
most difficulty in detecting the faint companion of a close double
star. In the spring of 1841 Di Vico saw a marking on the northern cusp
involved in an oval luminosity, and he likened it to a crater on the
Moon viewed obliquely. This spot had a diameter of at least 4½″, and
it was seen to advance even into the obscure part of the disk.

_Rotation-Period._—The following are the periods of rotation as given
by the different authorities whose observations we have mentioned:—

1666-7. Cassini 23 hrs. 21 min.
1726-8. Bianchini 24 days 8 hrs.[28]
1811. Schröter 23 hrs. 21 min. 8 sec.[29]
1840-1. Di Vico 23 hrs. 21 min. 22 sec.

Schiaparelli has recently discussed a large number of observations of
this planet, and concludes that, like Mercury, she rotates on her axis
in the same time that she completes a sidereal revolution round the
Sun, viz. in 224·7 days! I merely mention this remarkable deduction,
without quoting any facts in opposition to it.

From observations by Perrotin at Nice in 1890, including 74
observations, the rotation of this planet is very slow, and is made in
such a way that the relative positions of the spots and terminator do
not experience any notable change during many days.

[Illustration: Fig. 30.

1881, Mar. 22, 6^h. 1881, Mar. 26, 7^h. 1881, Mar. 28, 6½^h.

Venus as an evening star. (10-inch Reflector; power 212.)]

_Faintness of the Markings._—Several observers have noticed a slight
blunting of the S. horn of Venus, and in recent years dusky spots
have not unfrequently been seen, notably by Buffham, Langdon, and
others. The only markings distinguishable with my 10-inch reflector
are faint grey areas, without definite boundaries. These are sometimes
so delicate that it is difficult to assign exact form and position
to them, and occasionally I have regarded their very existence as of
doubtful character. They appear to be mere inequalities in brightness
of the surface, and may be due to different reflective power in parts
of the dense atmosphere of this planet. Certainly the spots are nothing
like those seen on the disks of Mars and Jupiter, many of which are
extremely distinct and show sharply terminated outlines. Dawes, an
observer endowed with very keen sight, could never succeed in finding
any markings on Venus, and many others have failed. But the evidence
affirming their reality is too weighty and too numerously attested to
allow them to be set aside. Occasionally the disk appears speckled with
minute shadings, and some observers have noticed crateriform objects
near the terminator; but these are uncertain. Brilliant spots have also
been recorded quite recently at the cusps.

Perhaps it may be advisable here to add a word of caution to observers
not to be hastily drawn to believe the spots are visible in very small
glasses. Accounts are sometimes published of very dark and definite
markings seen with only 2 or 3 inches aperture. Such assertions are
usually unreliable. Could the authors of such statements survey the
planet through a good 10-or 12-inch telescope, they would see at once
they had been deceived. Some years ago I made a number of observations
of Venus with 2-, 3-, and 4-1/4-inch refractors and 4-and 10-inch
reflectors, and could readily detect with the small instruments what
certainly appeared to be spots of a pronounced nature, but on appealing
to the 10-inch reflector, in which the view became immensely improved,
the spots quite disappeared, and there remained scarcely more than
a suspicion of the faint condensations which usually constitute the
only visible markings on the surface. I believe, also, the serrated
terminator is not a real feature of the object, but rather an effect
either of the rippling contour of the image or of an imperfect or
inadequate telescope.

An atmosphere of considerable density probably surrounds this planet,
for at the limb the brightness of the disk is much intensified. A
medium like this, that reflects and refracts light in extreme degree,
is brighter under oblique vision, as at the limb of Venus.

_Twilight on Venus._—When Venus is a slender crescent, near inferior
conjunction, a feeble luminosity pervades the dark part of the disk
similar to the “ashy light” or earthshine observed on the crescented
Moon. On such occasions the unilluminated surface appears to be
involved in a phosphorescence. Several observers have, however,
described the unilluminated limb of Venus as darker than the background
of sky. Zenger, at Prague, has noticed a brownish-red ring surrounding
the planet, and he attributes the appearance to much the same cause as
that which occasions the coppery colour of the Moon in a total eclipse.

_Alleged Satellite._—Cassini, Short, Montaigne, and others, in the 17th
and 18th centuries, observed small crescents near Venus and inferred
the existence of a satellite; but no such object has presented itself
in more recent times. It is extremely probable that the observers
were mistaken. In some cases the duplicate image may have been formed
by reflection in the eyepiece; in others a small star or planet
situated near Venus gave rise to the deception. M. Stroobant has
fully investigated this astronomical myth, and disposed of many of
the observations, without having recourse to the apocryphal satellite
named “Neith” by M. Niesten, who has discussed the question from an
affirmative point of view.

_Further Observations required._—From the foregoing summary amateurs
will notice that several difficult and more or less evanescent features
on this brilliant member of our system stand in need of confirmation.
Certain disputed forms require also to be looked for. The faint dusky
patches, the bright spots at the horns, and the inequalities in the
curve of the terminator will sure to be re-observed in future years;
and it is necessary that such details should be precisely noted in
regard to their positions and outlines as often as possible. A series
of reliable observations of this character might enable a fresh value
of the rotation-period to be deduced from them; and this is desirable,
for though Cassini, Schröter, and Di Vico give periods which are in
close harmony, there are elements of uncertainty attached to their
results. A new determination of the period would be valuable, and
especially so if based on really trustworthy data obtained by one of
the best modern telescopes. With the planet situated near inferior
conjunction, the crescent (reduced at such a time to a mere thread
of light) should be brought into the field, and the observer should
look for the extension of a faint glow over the interior parts of
the surface, and make comparisons between the relative brightness
of the planet’s dark limb and of the sky on which it is projected.
The telescopic images of Venus are often excellent in daylight, and
those who possess means of readily finding the planet at such times
will be very likely to gain some useful materials. As to the presumed
satellite, that may be relegated to the care of observers who have the
leisure and inclination to pursue an _ignis fatuus_; but should any
doubtful object appear in the field with Venus at any time, it ought to
be fully recorded and identified, if possible.

_Transits of Venus._—Those who were prevented by circumstances of
weather or otherwise from witnessing either of the transits of Venus
which occurred in 1874 and 1882 lost a spectacle of great rarity,
and one which they can never have another chance to behold. The next
transit occurs in the year 2004, and its phenomena will doubtless
be watched with avidity by the astronomers of a future generation.
The transit of 1882 was seen by many observers in England, though in
some parts of the country the Sun was obscured by clouds. The planet
was distinctly visible to the naked eye as a black circular spot in
gradual motion across the solar disk. The most important result of the
telescopic observations was of course the re-determination of the Sun’s
distance; but amongst the physical features noted, one of the most
interesting was the appearance of a silver arc of light on that portion
of the planet’s edge which was outside the Sun. This is assumed to have
been caused by the refraction of an atmosphere on Venus. The phenomenon
was seen by several observers, including Prof. Langley in America and
Messrs. Prince and Brodie in England.

_Occultations of Venus._—An occupation of this planet by the Moon
appears to have been recorded by the Chinese on March 19, 361 A.D.
Tycho Brahe witnessed a similar phenomenon on May 23, 1587. Mœstlin
observed Venus occult Regulus on Sept. 16, 1574; and on Oct. 2, 1590,
this planet appears to have passed over Mars. Visible occultations of
Venus are somewhat rare; they usually occur in daylight. A phenomenon
of this kind was witnessed on Dec. 8, 1877, over all the W. part of the
United States; and Prof. Pritchett, of Missouri, says:—“The interest
taken in it was shared alike by the educated and the illiterate, and
even by children.” The evening was cloudless, and many persons noted
the time of disappearance of Venus as seen by the unassisted eye. With
a 12½-inch refractor, power 275, Prof. Pritchett noted that “when the
bright limb of Venus was within 8″ or 10″ of the Moon’s dark limb, a
border of wavering light, several seconds in width, seemed to precede
the planet. Its general effect was such as to place in doubt the moment
of external contact.” A full description of this event, and of the
partial occultation of Venus on Oct. 12, 1879, is given in No. 1 of the
‘Publications’ of the Morrison Observatory, Missouri, U.S.A.

Venus is said, by the Arabian astronomer Ibn-Jounis, to have occulted
Regulus on Sept. 9, 885 A.D.; and Hind has examined the observations,
by means of Le Verrier’s tables of the Sun and planets. He finds
that on Sept. 9 in the year mentioned, at 16h 43m mean time, Venus
approached the star within 1′·7; so that to the naked eye the latter
would appear to be occulted, being overpowered in the glare of the
planet.


FOOTNOTES:

[28] This period was probably derived erroneously by Bianchini. It
includes 25 periods of 23^h 22^m, which corresponds with the times of
rotation by Cassini and others given in the table.

[29] Schröter’s final result. In 1788 he had derived a period of
23^h 28^m from observations of faint dark spots, and in 1789-91
irregularities in the S. horn of Venus gave him a period of 23^h 20^m
59^s.




CHAPTER IX.

_MARS._

Appearance of the planet.—Period &c.—Phase.—Surface
Configuration.—Charts and Nomenclature of Mars.—Discovery
of two Satellites and of Canal-shaped markings.—Summary of
Observations.—Rotation of Mars.—Further Observations required.—Changes
on Mars.—The two Satellites.—Occultations of Mars.


_Appearance of the Planet._—Mars is the fourth planet in the order of
distance from the Sun. He revolves in an orbit outside that of the
Earth, and is the smallest of the superior planets. His brilliancy is
sometimes considerable when he occupies a position near to the Earth,
and he emits an intense red light[30], which renders his appearance
all the more striking. Ordinarily his lustre does not equal that of
Jupiter, though when favourably placed he becomes a worthy rival of
that orb. In 1719 he shone so brightly and with such a fiery aspect as
to cause a panic. The superstitious notions and belief in astrological
influences prevailing at that time no doubt gave rise to the popular
apprehension that the ruddy star was an omen of disaster, and thus
it was regarded with feelings of terror. Fortunately the light of
science has long since removed such ideas from amongst us, and
celestial objects, in all their various forms, are contemplated without
misgiving. They are rather welcomed as affording the means of advancing
our knowledge of God’s wonderful works as displayed in the heavens.

_Period &c._—Mars revolves round the Sun in 686^d 23^h 30^m 41^s, and
his mean distance from that luminary is 141,500,000 miles. The orbit
is one of considerable eccentricity, the distance varying between
154,700,000 and 128,360,000 miles. The apparent diameter of the planet
when in conjunction with the Sun is only 4″; but this may augment
to 30″·4 at an opposition, when the Earth and Mars occupy the least
distant parts of their orbits. The real diameter of Mars is nearly 5000
miles.

_Phase._—At opposition the disk of Mars is round, but when in
quadrature he appears distinctly gibbous and resembles the Moon three
days from full. The phase is so palpable that Galilei glimpsed it at
the end of 1610. In delineations of Mars the disk is generally drawn
circular, the compression being very slight and the phase too trivial
to be regarded.

_Surface Configuration._—This planet being singularly variable in
his position relatively to the Earth, presents at times a diameter
so small that the most powerful instruments are ineffective to deal
with him. But at certain epochs he becomes an excellent object, with
a much expanded disk, on which are displayed a number of bright and
dark markings. This happens, however, with comparative rarity; for only
during two months or so near every opposition, occurring at intervals
of 780 days, can the planet be well seen. Generally the apparent size
of Mars is very inconsiderable, and the disk not sharply defined,



Online LibraryWilliam F. DenningTelescopic Work for Starlight Evenings → online text (page 13 of 32)