Venus
Venus was naturally--owing to its brightness--the first of the planets
known to the ancients. It is mentioned by Hesiod, Homer, Virgil, Martial,
and Pliny; and Isaiah's remark about "Lucifer, son of the morning" (Isaiah
xiv. 12) probably refers to Venus as a "morning star." An observation of
Venus is found on the Nineveh tablets of date B.C. 684. It was observed in
daylight by Halley in July, 1716.
In _very_ ancient times Venus, when a morning star, was called Phosphorus
or Lucifer, and when an evening star Hesperus; but, according to Sir G. C.
Lewis, the identity of the two objects was known so far back as 540 B.C.
When Venus is at its greatest brilliancy, and appears as a morning star
about Christmas time (which occurred in 1887, and again in 1889), it has
been mistaken by the public for a return of the "Star of Bethlehem."[29]
But whatever "the star of the Magi" was it certainly was _not_ Venus. It,
seems, indeed absurd to suppose that "the wise men" of the East should
have mistaken a familiar object like Venus for a strange apparition. There
seems to be nothing whatever in the Bible to lead us to expect that the
star of Bethlehem will reappear.
Mr. J. H. Stockwell has suggested that the "Star of Bethlehem" may perhaps
be explained by a conjunction of the planets Venus and Jupiter which
occurred on May 8, B.C. 6, which was two years before the death of Herod.
From this it would follow that the Crucifixion took place on April 3, A.D.
33. But it seems very doubtful that the phenomenon recorded in the Bible
refers to any conjunction of planets.
Chacornac found the intrinsic brightness of Venus to be ten times greater
than the most luminous parts of the moon.[30] But this estimate is
probably too high.
When at its brightest, the planet is visible in broad daylight to good
eyesight, if its exact position in the sky is known. In the clear air of
Cambridge (U.S.A.) it is said to be possible to see it in this way in all
parts of its orbit, except when the planet is within 10° of the sun.[31]
Mr. A. Cameron, of Yarmouth, Nova Scotia, has, however, seen Venus with
the naked eye three days before conjunction when the planet was only
6-1/4° from the sun.[32] This seems a remarkable observation, and shows
that the observer's eyesight must have been very keen. In a private letter
dated October 22, 1888, the late Rev. S. J. Johnson informed the present
writer that he saw Venus with the naked eye only four days before
conjunction with the sun in February, 1878, and February, 1886.
The crescent shape of Venus is said to have been seen with the naked eye
by Theodore Parker in America when he was only 12 years old. Other
observers have stated the same thing; but the possibility of such an
observation has been much disputed in recent years.
In the Chinese Annals some records are given of Venus having been seen in
the Pleiades. On March 16, A.D. 845, it is said that "Venus eclipsed the
Pleiades." This means, of course, that the cluster was apparently effaced
by the brilliant light of the planet. Computing backwards for the above
date, Hind found that on the evening of March 16, 845, Venus was situated
near the star Electra; and on the following evening the planet passed
close to Maia; thus showing the accuracy of the Chinese record. Another
"eclipse" of the Pleiades by Venus is recorded in the same annals as
having occurred on March 10, A.D. 1002.[33]
When Venus is in the crescent phase, that is near "Inferior conjunction"
with the sun, it will be noticed, even by a casual observer, that the
crescent is not of the same shape as that of the crescent moon. The horns
or "cusps" of the planetary crescent are more prolonged than in the case
of the moon, and extend beyond the hemisphere. This appearance is caused
by refraction of the sun's light through the planetary atmosphere, and is,
in fact, a certain proof that Venus has an atmosphere similar to that of
the earth. Observations further show that this atmosphere is denser than
ours.
Seen from Venus, the earth and moon, when in opposition, must present a
splendid spectacle. I find that the earth would shine as a star about half
as bright again as Venus at her brightest appears to us, and the moon
about equal in brightness to Sirius! the two forming a superb "naked eye
double star"--perhaps the finest sight of its kind in the solar
system.[34]
Some of the earlier observers, such as La Hire, Fontana, Cassini, and
Schröter, thought they saw evidence of mountains on Venus. Schröter
estimated some of these to be 27 or 28 miles in height! but this seems
very doubtful. Sir William Herschel severely attacked these supposed
discoveries. Schröter defended himself, and was supported by Beer and
Mädler, the famous lunar observers. Several modern observers seem to
confirm Schröter's conclusions; but very little is really known about the
topography of Venus.
The well-known French astronomer Trouvelot--a most excellent observer--saw
white spots on Venus similar to those on Mars. These were well seen and
quite brilliant in July and August, 1876, and in February and November,
1877. The observations seem to show that these spots do not (unlike Mars)
increase and decrease with the planet's seasons. These white spots had
been previously noticed by former observers, including Bianchini, Derham,
Gruithuisen, and La Hire; but these early observers do not seem to have
considered them as snow caps, like those of Mars. Trouvelot was led by his
own observations to conclude that the period of rotation of Venus is
short, and the best result he obtained was 23{h} 49{m} 28{s}. This does
not differ much from the results previously found by De Vico, Fritsch, and
Schröter.[35]
A white spot near the planet's south pole was seen on several occasions by
H. C. Russell in May and June, 1876.[36]
Photographs of Venus taken on March 18 and April 29, 1908, by M. Quénisset
at the Observatory of Juvissy, France, show a white polar spot. The spot
was also seen at the same observatory by M. A. Benoit on May 20, 1903.
The controversy on the period of rotation of Venus, or the length of its
day, is a very curious one and has not yet been decided. Many good
observers assert confidently that it is short (about 24 hours); while
others affirm with equal confidence that it is long (about 225 days, the
period of the planet's revolution round the sun). Among the observers who
favour the short period of rotation are: D. Cassini (1667), J. Cassini
(1730), Schröter (1788-93), Mädler (1836), De Vico (1840?) Trouvelot
(1871-79), Flammarion, Léo Brenner, Stanley Williams, and J. McHarg; and
among those who support the long period are: Bianchini (1727),
Schiaparelli, Cerulli, Tacchini, Mascari, and Lowell. Some recent
spectroscopic observations seem to favour the short period.
Flammarion thinks that "nothing certain can be descried upon the surface
of Venus, and that whatever has hitherto been written regarding its period
of rotation must be considered null and void"; and again he says, "Nothing
can be affirmed regarding the rotation of Venus, inasmuch as the
absorption of its immense atmosphere certainly prevents any detail on its
surface from being perceived."[37]
The eminent Swedish physicist Arrhenius thinks, however, that the dense
atmosphere and clouds of Venus are in favour of a rapid rotation on its
axis.[38] He thinks that the mean temperature of Venus may "not differ
much from the calculated temperature 104° F." "Under these circumstances
the assumption would appear plausible that a very considerable portion of
the surface of Venus, and particularly the districts about the poles,
would be favourable to organic life."[39]
The "secondary light of Venus," or the visibility of the dark side, seems
to have been first mentioned by Derham in his _Astro Theology_ published
in 1715. He speaks of the visibility of the dark part of the planet's disc
"by the aid of a light of a somewhat dull and ruddy colour." The date of
Derham's observation is not given, but it seems to have been previous to
the year 1714. The light seems to have been also seen by a friend of
Derham. We next find observations by Christfried Kirch, assistant
astronomer to the Berlin Academy of Sciences, on June 7, 1721, and March
8, 1726. These observations are found in his original papers, and were
printed in the _Astronomische Nachrichten_, No. 1586. On the first date
the telescopic image of the planet was "rather tremulous," but in 1726 he
noticed that the dark part of the circle seemed to belong to a smaller
circle than the illuminated portion of the disc.[40] The same effect was
also noted by Webb.[41] A similar illusion is seen in the case of the
crescent moon, and this has given rise to the saying, "the old moon in the
new moon's arms."
We next come, in order of date, to an observation made by Andreas Mayer,
Professor of Mathematics at Griefswald in Prussia. The observation was
made on October 20, 1759, and the dark part of Venus was seen distinctly
by Mayer. As the planet's altitude at the time was not more than 14° above
the horizon, and its apparent distance from the sun only 10°, the
phenomenon--as Professor Safarik has pointed out--"must have had a most
unusual intensity."
Sir William Herschel makes no mention of having ever seen the "secondary
light" of Venus, although he noticed the extension of the horns beyond a
semicircle.
In the spring and summer of the year 1793, Von Hahn of Remplin in
Mecklenburg, using excellent telescopes made by Dollond and Herschel, saw
the dark part of Venus on several occasions, and describes the light as
"grey verging upon brown."
Schröter of Lilienthal--the famous observer of the moon--saw the horns of
the crescent of Venus extended many degrees beyond the semicircle on
several occasions in 1784 and 1795, and the border of the dark part
faintly lit up by a dusky grey light. On February 14, 1806, at 7 P.M. he
saw the whole of the dark part visible with an ash-coloured light, and he
was satisfied that there was no illusion. On January 24 of the same year,
1806, Harding at Göttingen, using a reflector of 9 inches aperture and
power 84, saw the dark side of Venus "shining with a pale ash-coloured
light," and very visible against the dark background of the sky. The
appearance was seen with various magnifying powers, and he thought that
there could be no illusion. In fact the phenomenon was as evident as in
the case of the moon. Harding again saw it on February 28 of the same
year, the illumination being of a reddish grey colour, "like that of the
moon in a total eclipse."
The "secondary light" was also seen by Pastorff in 1822, and by
Gruithuisen in 1825. Since 1824 observations of the "light" were made by
Berry, Browning, Guthrie, Langdon, Noble, Prince, Webb, and others. Webb
saw it with powers of 90 and 212 on a 9·38-inch mirror, and found it
"equally visible when the bright crescent was hidden by a field bar."[42]
Captain Noble's observation was rather unique. He found that the dark side
was "always distinctly and positively _darker_ than the background upon
which it is projected."
The "light" was also seen by Lyman in America in 1867, and by Safarik at
Prague. In 1871 the whole disc of Venus was seen by Professor
Winnecke.[43] On the other hand, Winnecke stated that he only saw it twice
in 24 years; and the great observers Dawes and Mädler never saw it at
all![44]
Various attempts have been made to explain the visibility--at times--of
the "dark side" of Venus. The following may be mentioned[45]:--(1)
Reflected earth-light, analogous to the dark side of the crescent moon.
This explanation was advocated by Harding, Schröter, and others. But,
although the earth is undoubtedly a bright object in the sky of Venus, the
explanation is evidently quite inadequate. (2) Phosphorescence of the
planet's atmosphere. This has been suggested by some observers. (3)
Visibility by contrast, a theory advanced by the great French astronomer
Arago. (4) Illumination of the planet's surface by an aurora borealis.
This also seems rather inadequate, but would account for the light being
sometimes visible and sometimes not. (5) Luminosity of the oceans--if
there be any--on Venus. But this also seems inadequate. (6) A planetary
surface glowing with intense heat. But this seems improbable. (7) The
Kunstliche Feuer (artificial fire) of Gruithuisen, a very fanciful theory.
Flammarion thinks that the visibility of the dark side may perhaps be
explained by its projection on a somewhat lighter background, such as the
zodiacal light, or an extended solar envelope.[46]
It will be seen that none of these explanations are entirely satisfactory,
and the phenomenon, if real, remains a sort of astronomical enigma. The
fact that the "light" is visible on some occasions and not on others would
render some of the explanations improbable or even inadmissible. But the
condition of the earth's atmosphere at times might account for its
invisibility on many occasions.
A curious suggestion was made by Zöllner, namely, that if the secondary
light of Venus could be observed with the spectroscope it would show
bright lines! But such an observation would be one of extreme difficulty.
M. Hansky finds that the visibility of the "light" is greater during
periods of maximum solar activity--that is, at the maxima of sun spots.
This he explains by the theory of Arrhenius, in which electrified "ions
emitted by the sun cause the phenomena of terrestrial magnetic storms and
auroras." "In the same way the dense atmosphere of Venus is rendered more
phosphorescent, and therefore more easily visible by the increased solar
activity."[47] This seems a very plausible hypothesis.
On the whole the occasional illumination of the night side of Venus by a
very brilliant aurora (explanation (4) above) seems to the present writer
to be the most probable explanation. Gruithuisen's hypothesis (7) seems
utterly improbable.
There is a curious apparent anomaly about the motion of Venus in the sky.
Although the planet's period of revolution round the sun is 224·7 days, it
remains on the same side of the sun, as seen from the earth, for 290 days.
The reason of this is that the earth is going at the same time round the
sun in the same direction, though at a slower pace; and Venus must
continue to appear on the same side of the sun until the excess of her
daily motion above that of the earth amounts to 179°, and this at the
daily rate of 37' will be about 290 days.
Several observations have been recorded of a supposed satellite of Venus.
But the existence of such a body has never been verified. In the year
1887, M. Stroobant investigated the various accounts, and came to the
conclusion that in several at least of the recorded observations the
object seen was certainly a star. Thus, in the observation made by
Roedickoer and Boserup on August 4, 1761, a satellite and star are
recorded as having been seen near the planet. M. Stroobant finds that the
supposed "satellite" was the star [Greek: ch]_{4} Orionis, and the "star"
[Greek: ch]_{3} Orionis. A supposed observation of a satellite made by
Horrebow on January 3, 1768, was undoubtedly [Greek: th] Libræ. M.
Stroobant found that the supposed motion of the "satellite" as seen by
Horrebow is accurately represented by the motion of Venus itself during
the time of observation. In most of the other supposed observations of a
satellite a satisfactory identification has also been found. M. Stroobant
finds that with a telescope of 6 inches aperture, a star of the 8th or
even the 9th magnitude can be well seen when close to Venus.[49]
On the night of August 13, 1892, Professor Barnard, while examining Venus
with the great 36-inch telescope of the Lick Observatory, saw a star of
the 7th magnitude in the same field with the planet. He carefully
determined the exact position of this star, and found that it is not in
Argelander's great catalogue, the _Durchmusterung_. Prof. Barnard finds
that owing to elongation of Venus from the sun at the time of observation
the star could not possibly be an intra-Mercurial planet (that is, a
planet revolving round the sun inside the orbit of Mercury); but that
possibly it might be a planet revolving between the orbits of Venus and
Mercury. As the brightest of the minor planets--Ceres, Pallas, Juno, and
Vesta--were not at the time near the position of the observed object, the
observation remains unexplained. It might possibly have been a _nova_, or
temporary star.[50]
Scheuten is said to have seen a supposed satellite of Venus following the
planet across the sun at the end of the transit of June 6, 1761.[51]
Humboldt speaks of the supposed satellite of Venus as among "the
astronomical myths of an uncritical age."[52]
An occultation of Venus by the moon is mentioned in the Chinese Annals as
having occurred on March 19, 361 A.D., and Tycho Brahé observed another on
May 23, 1587.[53]
A close conjunction of Venus and Regulus ([Greek: a] Leonis) is recorded
by the Arabian astronomer, Ibn Yunis, as having occurred on September 9,
885 A.D. Calculations by Hind show that the planet and star were within 2'
of arc on that night, and consequently would have appeared as a single
star to the naked eye. The telescope had not then been invented.[54]
Seen from Venus, the maximum apparent distance between the earth and moon
would vary from about 5' to 31'.[55]
It is related by Arago that Buonaparte, when going to the Luxembourg in
Paris, where the Directory were giving a fête in his honour, was very
much surprised to find the crowd assembled in the Rue de Touracour "pay
more attention to a region of the heavens situated above the palace than
to his person or the brilliant staff that accompanied him. He inquired the
cause and learned that these curious persons were observing with
astonishment, although it was noon, a star, which they supposed to be that
of the conqueror of Italy--an allusion to which the illustrious general
did not seem indifferent, when he himself, with his piercing eyes,
remarked the radiant body." The "star" in question was Venus.[56]